Ctenophore
Encyclopedia
The Ctenophora are a phylum
of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to 1.5 metres (59.1 in) in size. Like cnidaria
ns, their bodies consist of a mass of jelly with one layer of cells
on the outside and another lining the internal cavity. In ctenophores these layers are two cells deep while those in cnidarians are only one cell deep. Ctenophores also resemble cnidarians in having a decentralized nerve net
rather than a brain. Some authors combined ctenophores and cnidarians in one phylum, Coelenterata
, as both groups rely on water flow through the body cavity for both digestion and respiration. Increasing awareness of the differences persuaded more recent authors to classify
them in separate phyla.
Almost all ctenophores are predators, taking prey ranging from microscopic larva
e and rotifer
s to the adults of small crustaceans; the exceptions are juveniles of two species, which live as parasites on the salp
s on which adults of their species feed. In favorable circumstances ctenophores can eat ten times their own weight in a day. There are only 100–150 validated species and possibly another 25 that have not been fully described and named. The textbook examples are cydippids
with egg-shaped bodies and a pair of retractable tentacles fringed with tentilla ("little tentacles") that are covered with colloblast
s, sticky cells that capture prey. The phylum has a wide range of body forms, including the flattened deep-sea platyctenids, in which the adults of most species lack combs, and the coastal beroids, which lack tentacles and prey on other ctenophores by using huge mouths armed with groups of large, stiffened cilia that act as teeth. These variations enable different species to build huge populations in the same area, because they specialize in different types of prey, which they capture by as wide a range of methods as spider
s use.
Most species are hermaphrodites, in other words a single animal can produce both eggs and sperm; if they are both produced at the same time, the animal is a simultaneous hermaphrodite, and if the eggs and sperm mature at different times, the animal is a sequential hermaphrodite. Fertilization is generally external
, although platyctenids' eggs are fertilized inside their parents' bodies and kept there until they hatch. The young are generally plankton
ic and in most species look like miniature cydippids, gradually changing into the adult shape as they grow. The exceptions are the beroids, whose young are miniature beroids with large mouths and no tentacles, and the platyctenids, whose young live as cydippid-like plankton until they reach near-adult size, but then sink to the bottom and rapidly metamorphose
into the adult form. In at least some species, juveniles are capable of reproduction before reaching the adult size and shape. The combination of hermaphroditism and early reproduction enables small populations to grow at an explosive rate.
Ctenophores may be abundant during the summer months in some coastal locations, but in other places they are uncommon and difficult to find. In bays where they occur in very high numbers, predation by ctenophores may control the populations of small zooplanktonic organisms such as copepod
s, which might otherwise wipe out the phytoplankton
(planktonic plants), which are a vital part of marine food chain
s. One ctenophore, Mnemiopsis, has accidentally been introduced
into the Black Sea
, where it is blamed for causing fish stocks to collapse by eating both fish larvae and organisms that would otherwise have fed the fish. The situation was aggravated by other factors, such as over-fishing and long-term environmental changes that promoted the growth of the Mnemiopsis population. The later accidental introduction of Beroe helped to mitigate the problem, as Beroe preys on other ctenophores.
Despite their soft, gelatinous bodies, fossil
s thought to represent ctenophores, apparently with no tentacles but many more comb-rows than modern forms, have been found in lagerstätte
n as far back as the early Cambrian
, about . The position of the ctenophores in the evolutionary family tree of animals has long been debated, and the majority view at present, based on molecular phylogenetics, is that cnidarians and bilateria
ns are more closely related to each other than either is to ctenophores. A recent molecular phylogenetics analysis concluded that the common ancestor of all modern ctenophores was cydippid-like, and that all the modern groups appeared relatively recently, probably after the Cretaceous-Tertiary extinction . Evidence accumulating since the 1980s indicates that the "cydippids" are not monophyletic, in other words do not include all and only the descendants of a single common ancestor, because all the other traditional ctenophore groups are descendants of various cydippids.
phylum
that is more complex than sponges, about as complex as cnidaria
ns (jellyfish
, sea anemone
s, etc.), and less complex than bilateria
ns, which include almost all other animals. Unlike sponges, both ctenophores and cnidarians have: cells bound by inter-cell connections and carpet-like basement membrane
s; muscle
s; nervous system
s; and some have sensory
organs. Ctenophores are distinguished from all other animals by having colloblast
s that capture prey by squirting glue on them, although a few ctenophore species lack them.
Like sponges and cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea
in cnidarians and ctenophores; more complex animal
s have three main cell layers and no intermediate jelly-like layer. Hence ctenophores and cnidarians have traditionally been labelled diploblastic, along with sponges. Both ctenophores and cnidarians have a type of muscle
that, in more complex animals, arises from the middle cell layer
, and as a result some recent text books classify ctenophores as triploblastic, while others still regard them as diploblastic.
Ranging from about 1 millimetre (0.0393700787401575 in) to 1.5 metres (4.9 ft) in size, ctenophores are the largest non-colonial animals that use cilia ("hairs") as their main method of locomotion. Most species have eight strips, called comb rows, that run the length of their bodies and bear comb-like bands of cilia, called "ctenes," stacked along the comb rows so that when the cilia beat, those of each comb touch the comb below. The name "ctenophora" means "comb-bearing", from the Greek
κτείς (stem-form κτεν-) meaning "comb" and the Greek suffix -φορος meaning "carrying".
– Pleurobrachia
, Beroe and Mnemiopsis. At least two textbooks base their descriptions of ctenophores on the cydippid
Pleurobrachia.
Since the body of many species is almost radially symmetrical, the main axis is oral
to aboral
(from the mouth to the opposite end). However since only two of the canals near the statocyst
terminate in anal
pores, ctenophores have no mirror-symmetery, although many have rotational symmetry, in other words if the animal rotates in a half-circle it looks the same as when it started.
ns, (jellyfish
, sea anemone
s, etc.), ctenophores' bodies consist of a relatively thick, jelly-like mesoglea
sandwiched between two epithelia
, layers of cells
bound by inter-cell connections and by a fibrous basement membrane
that they secrete. The epithelia of ctenophores have two layers of cells rather than one, and some of the cells in the upper layer have several cilia per cell.
The outer layer of the epidermis
(outer skin) consists of: sensory cells; cells that secrete mucus
, which protects the body; and interstitial cells, which can transform into other types of cell. In specialized parts of the body the outer layer also contains colloblast
s, found along the surface of tentacles and used in capturing prey, or cells bearing multiple large cilia, for locomotion. The inner layer of the epidermis contains a nerve net
, and myoepithelial cells that act as muscle
s.
The internal cavity forms: a mouth that can usually be closed by muscles; a pharynx
("throat"); a wider area in the center that acts as a stomach
; and a system of internal canals. These branch through the mesoglea to the most active parts of the animal: the mouth and pharynx; the roots of the tentacles, if present; all along the underside of each comb row; and four branches round the sensory complex at the far end from the mouth – two of these four branches terminate in anal
pores. The inner surface of the cavity is lined with an epithelium
, the gastrodermis
. The mouth and pharynx have both cilia and well-developed muscles. In other parts of the canal system, the gastrodermis is different on the sides nearest to and furthest from the organ that it supplies. The nearer side is composed of tall nutritive cells that store nutrients in vacuole
s (internal compartments), germ cell
s that produce eggs or sperm, and photocytes
that produce bioluminescence
. The side furthest from the organ is covered with ciliated cells that circulate water through the canals, punctuated by ciliary rosettes, pores that are surrounded by double whorls of cilia and connect to the mesoglea.
by enzyme
s and by muscular contractions of the pharynx. The resulting slurry is wafted through the canal system by the beating of the cilia, and digested by the nutritive cells. The ciliary rosettes in the canals may help to transport nutrients to muscles in the mesoglea. The anal
pores may eject unwanted small particles, but most unwanted matter is regurgitated via the mouth.
Little is known about how ctenophores get rid of waste products produced by the cells. The ciliary rosettes in the gastrodermis
may help to remove wastes from the mesoglea, and may also help to adjust the animal's buoyancy
by pumping water into or out of the mesoglea.
"aboral pole"), and are spaced more or less evenly around the body, although spacing patterns vary by species and in most species the comb rows extend only part of the distance from the aboral pole towards the mouth. The "combs" (also called "ctenes" or "comb plates") run across each row, and each consists of thousands of unusually long cilia, up to 2 millimetre (0.078740157480315 in). These normally beat so that the propulsion stroke is away from the mouth, although they can also reverse direction. Hence ctenophores usually swim in the direction in which the mouth is pointing, unlike jellyfish
. When trying to escape predators, one species can accelerate to six times its normal speed; some other species reverse direction as part of their escape behavior, by reversing the power stroke of the comb plate cilia.
It is uncertain how ctenophores control their buoyancy, but experiments have shown that some species rely on osmotic pressure
to adapt to water of different densities. Their body fluids are normally as concentrated
as seawater. If they enter less dense brackish water, the ciliary rosettes in the body cavity may pump this into the mesoglea
to increase its bulk and decrease its density, to avoid sinking. Conversely if they move from brackish to full-strength seawater, the rosettes may pump water out of the mesoglea to reduce its volume and increase its density.
or central nervous system
, but instead have a nerve net
(rather like a cobweb) that forms a ring round the mouth and is densest near structures such as the comb rows, pharynx, tentacles (if present) and the sensory complex furthest from the mouth.
The largest single sensory feature is the aboral
organ (at the opposite end from the mouth). Its main component is a statocyst
, a balance sensor consisting of a statolith, a solid particle supported on four bundles of cilia, called "balancers", that sense its orientation. The statocyst is protected by a transparent dome made of long, immobile cilia. A ctenophore does not automatically try to keep the statolith resting equally on all the balancers. Instead its response is determined by the animal's "mood", in other words the overall state of the nervous system. For example if a ctenophore with trailing tentacles captures prey, it will often put some comb rows into reverse, spinning the mouth towards the prey.
, sometimes has an egg-shaped body with the mouth at the narrow end, although some individuals are more uniformly round. From opposite sides of the body extends a pair of long, slender tentacles, each housed in a sheath into which it can be withdrawn. Some species of cydippids have bodies that are flattened to various extents, so that they are wider in the plane of the tentacles.
The tentacles of cydippid ctenophores are typically fringed with tentilla ("little tentacles"), although a few genera have simple tentacles without these sidebranches. The tentacles and tentilla are densely covered with microscopic colloblast
s that capture prey by sticking to it. Colloblasts are specialized mushroom
-shaped cells in the outer layer of the epidermis, and have three main components: a domed head with vesicles
(chambers) that contain adhesive; a stalk that anchors the cell in the lower layer of the epidermis or in the mesoglea; and a spiral
thread that coils round the stalk and is attached to the head and to the root of the stalk. The function of the spiral thread is uncertain, but it may absorb stress when prey tries to escape, and thus prevent the collobast from being torn apart. In addition to colloblasts, members of the genus Haeckelia, which feed mainly on jellyfish
, incorporate their victims' stinging nematocytes into their own tentacles – some cnidaria-eating nudibranch
s similarly incorporate nematocytes into their bodies for defense. The tentilla of Euplokamis differ significantly from those of other cydippids: they contain striated muscle
, a cell type otherwise unknown in the phylum Ctenophora; and they are coiled when relaxed, while the tentilla of all other known ctenophores elongate when relaxed. Euplokamis tentilla have three types of movement that are used in capturing prey: they may flick out very quickly (in 40 to 60 millisecond
s); they can wriggle, which may lure prey by behaving like small planktonic worms; and they coil round prey. The unique flicking is an uncoiling movement powered by contraction of the striated muscle
. The wriggling motion is produced by smooth muscle
s, but of a highly specialized type. Coiling around prey is accomplished largely by the return of the tentilla to their inactive state, but the coils may be tightened by smooth muscle.
There are eight rows of combs that run from near the mouth to the opposite end, and are spaced evenly round the body. The "combs" beat in a metachronal rhythm
rather like that of a Mexican wave. From each balancer in the statocyst a ciliary groove runs out under the dome and then splits to connect with two adjacent comb rows, and in some species runs all the way along the comb rows. This forms a mechanical system for transmitting the beat rhythm from the combs to the balancers, via water disturbances created by the cilia.
The Lobata
have a pair of lobes, which are muscular, cuplike extensions of the body that project beyond the mouth. Their inconspicuous tentacles originate from the corners of the mouth, running in convoluted grooves and spreading out over the inner surface of the lobes (rather than trailing far behind, as in the Cydippida). Between the lobes on either side of the mouth, many species of lobates have four auricles, gelatinous projections edged with cilia that produce water currents that help direct microscopic prey toward the mouth. This combination of structures enables lobates to feed continuously on suspended plankton
ic prey.
Lobates have eight comb-rows, originating at the aboral pole and usually not extending beyond the body to the lobes; in species with (four) auricles, the cilia edging the auricles are extensions of cilia in four of the comb rows. Most lobates are quite passive when moving through the water, using the cilia on their comb rows for propulsion, although Leucothea has long and active auricles whose movements also contribute to propulsion. Members of the lobate genera
Bathocyroe
and Ocyropsis
can escape from danger by clapping their lobes, so that the jet of expelled water drives them backwards very quickly. Unlike cydippids, the movements of lobates' combs are coordinated by nerves rather than by via water disturbances created by the cilia, yet combs on the same row beat beat in the same Mexican wave style as the mechanically-coordinated comb rows of cydippids and beroids. This may have enabled lobates to grow larger than cydippids and to have shapes that are less egg-like.
An unusual species first described in 2000, Lobatolampea tetragona, has been classified as a lobate, although the lobes are "primitive" and the body is medusa
-like when floating and disk-like when resting on the sea-bed.
, have no feeding appendages, but their large pharynx
, just inside the large mouth and filling most of the saclike body, bears "macrocilia" at the oral end. These fused bundles of several thousand large cilia are able to "bite" off pieces of prey that is too large to swallow whole – almost always other ctenophores. In front of the field of macrocilia, on the mouth "lips" in some species of Beroe, is a pair of narrow strips of adhesive epithelial cells on the stomach wall that "zip" the mouth shut when the animal is not feeding, by forming intercellular connections with the opposite adhesive strip. This tight closure streamlines
the front of the animal when it is pursuing prey.
The Thalassocalycida, only discovered in 1978 and known from only one species, are medusa-like, with bodies that are shortened in the oral-aboral direction, and short comb-rows on the surface furthest from the mouth, originating from near the aboral pole. They capture prey by movements of the bell and possibly by using two short tentacles.
The Cestida
("belt animals") are ribbon-shaped planktonic animals, with the mouth and aboral organ aligned in the middle of opposite edges of the ribbon. There is a pair of comb-rows along each aboral edge, and tentilla emerging from a groove all along the oral edge, which stream back across most of the wing-like body surface. Cestids can swim by undulating their bodies as well as by the beating of their comb-rows. There are two known species, with worldwide distribution in warm, and warm-temperate waters: Cestum veneris ("Venus
' girdle") is among the largest ctenophores – up to 1.5 metres (4.9 ft) long, and can undulate slowly or quite rapidly. Velamen parallelum, which is typically less than 20 centimetre (0.656167979002625 ft) long, can move much faster in what has been described as a "darting motion".
Most Platyctenida
have oval bodies that are flattened in the oral-aboral direction, with a pair of tentilla-bearing tentacles on the aboral surface. They cling to and creep on surfaces by everting the pharynx and using it as a muscular "foot". All but one of the known platyctenid species lack comb-rows. Platyctenids are usually cryptically-colored, live on rocks, algae, or the body surfaces of other invertebrates, and are often revealed by their long tentacles with many sidebranches, seen streaming off the back of the ctenophore into the current.
Adults of most species can regenerate tissues that are damaged or removed, although only platyctenids reproduce by cloning
, splitting off from the edges of their flat bodies fragments that develop into new individuals.
Almost all species are hermaphrodites, in other words they function as both males and females at the same time – except that in two species of the genus
Ocryopsis individuals remain of the same single sex all their lives. The gonad
s are located in the parts of the internal canal network under the comb rows, and eggs and sperm are released via pores in the epidermis. Fertilization is external
in most species, but platyctenids use internal fertilization and keep the eggs in brood chambers until they hatch. Self-fertilization has occasionally been seen in species of the genus Mnemiopsis, and it is thought that most of the hermaphroditic species are self-fertile.
Development of the fertized eggs is direct, in other words there is no distinctive larval form, and juveniles of all groups generally resemble miniature cydippid adults. In the genus Beroe the juveniles, like the adults, lack tentacles and tentacle sheaths. In most species the juveniles gradually develop the body forms of their parents. In some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae, as they live among the plankton and thus occupy a different ecological niche
from their parents and attain the adult form by a more radical metamorphosis
, after dropping to the sea-floor.
At least in some species, juvenile ctenophores appear capable of producing small quantities of eggs and sperm while they are well below adult size, and adults produce eggs and sperm for as long as they have sufficient food. If they run short of food, they first stop producing eggs and sperm, and then shrink in size. When the food supply improves, they grow back to normal size and then resume reproduction. These features make ctenophores capable of increasing their populations very quickly.
Most ctenophores that live near the surface are mostly colorless and almost transparent. However some deeper-living species are strongly pigmented, for example the species known as "Tortugas red" (see illustration here), which has not yet been formally described. Platyctenids generally live attached to other sea-bottom organisms, and often have similar colors to these host organisms. The gut of the deep-sea genus Bathocyroe
is red, which hides the bioluminescence
of copepod
s it has swallowed.
The comb rows of most planktonic ctenophores produce a rainbow effect, which is not caused by bioluminescence
but by the scattering of light
as the combs move. Most species are also bioluminescent, but the light is usually blue or green and can only been seen in darkness. However some significant groups, including all known platyctenids and the cydippid genus
Pleurobrachia
, are incapable of bioluminescence.
When some species, including Bathyctena chuni, Euplokamis stationis and Eurhamphaea vexilligera, are disturbed, they produce secretions (ink) that luminesce at much the same wavelength
s as their bodies. Juveniles will luminesce more brightly in relation to their body size than adults, whose luminescence is diffused over their bodies. Detailed statistical investigation has not suggested the function of ctenophores' bioluminescence nor produced any correlation
between its exact color and any aspect of the animals' environments, such as depth or whether they live in coastal or mid-ocean waters.
Pleurobrachia, Beroe and Mnemiopsis, as these plankton
ic coastal forms are among the most likely to be collected near shore. No ctenophores have been found in fresh water.
(planktonic animals) ranging in size from the microscopic, including mollusc and fish larvae, to small adult crustaceans such as copepod
s, amphipods, and even krill
. Members of the genus Haeckelia prey on jellyfish
and incorporate their prey's nematocysts (stinging cells) into their own tentacles instead of colloblast
s. Ctenophores have been compared to spider
s in their wide range of techniques from capturing prey – some hang motionless in the water using their tentacles as "webs", some are ambush predators like Salticid jumping spider
s, and some dangle a sticky droplet at the end of a fine thread, as Bola spiders do. This variety explains the wide range of body forms in a phylum
with rather few species. The two-tentacled "cydippid" Lampea feeds exclusively on salp
s, close relatives of sea-squirts that form large chain-like foating colonies, and juveniles of Lampea attach themselves like parasites to salps that are too large for them to swallow. Members of the cydippid genus Pleurobrachia
and the lobate Bolinopsis often reach high population densities at the same place and time because they specialize in different types of prey: Pleurobrachia
s long tentacles mainly capture relatively strong swimmers such as adult copepods, while Bolinopsis generally feeds on smaller, weaker swimmers such as rotifer
s and mollusc and crustacean larvae
.
Ctenophores used to be regarded as "dead ends" in marine food chains because it was thought their low ratio of organic matter to salt and water made them a poor diet for other animals. It is also often difficult to identify the remains of ctenophores in the guts of possible predators, although the combs sometimes remain intact long enough to provide a clue. Detailed investigation of chum salmon
, Oncorhynchus keta, showed that these fish digest ctenophores 20 times as fast as an equal weight of shrimp
s, and that ctenophores can provide a good diet if there are enough of them around. Beroids prey mainly on other ctenophores. Some jellyfish
and turtle
s eat large quantities of ctenophores, and jellyfish may temporarily wipe out ctenophore populations. Since ctenophores and jellyfish often have large seasonal variations in population, most fish that prey on them are generalists, and may have a greater effect on populations than the specialist jelly-eaters. The larvae of some sea anemone
s are parasites on ctenophores, as are the larvae of some flatworm
s that parasitize fish when they reach adulthood.
(planktonic plants), which are the dominant marine producers of organic matter from non-organic ingredients.
On the other hand in the late 1980s the North Atlantic ctenophore Mnemiopsis leidyi
was accidentally introduced
into the Black Sea
and Sea of Azov
via the ballast tanks of ships, and has been blamed for causing sharp drops in fish catches by eating both fish larvae and small crustaceans that would otherwise feed the adult fish. Mnemiopsis is well-equipped to invade new territories, as it can breed very rapidly and tolerate a wide range of water temperatures and salinities
. The impact was increased by chronic overfishing, and by eutrophication
that gave the entire ecosystem a short-term boost, causing the Mnemiopsis population to increase even faster than normal – and above all by the absence of efficient predators on ctenophores. Mnemiopsis populations in those areas were eventually brought under control by the accidental introduction of the ctenophore-eating ctenophore Beroe ovata, and by a cooling of the local climate from 1991 to 1993, which significantly slowed the animal's metabolism. However the abundance of plankton in the area seems unlikely to reach pre-Mnemiopsis levels.
In the late 1990s Mnemiopsis appeared in the Caspian Sea
. Beroe ovata arrived shortly after, and is expected to reduce but not eliminate the impact of Mnemiopsis. Mnemiopsis also reached the eastern Mediterranean in the late 1990s and now appears to be thriving in the North Sea
and Baltic Sea
.
The traditional classification divides ctenophores into two classes
, those with tentacles (Tentaculata
) and those without (Nuda
). The Nuda contains only one order
(Beroida) and family
(Beroidae), and two genera
, Beroe (several species) and Neis (one species).
The Tentaculata
are divided into the following eight orders
:
n, places where the environment was exceptionally suited to preservation of soft tissue. Until the mid-1990s only two specimens good enough for analysis were known, both from the early Devonian
period. Three additional putative species were then found in the Burgess Shale
and other Canadian rocks of similar age, about in the mid-Cambrian
period. All three apparently lacked tentacles but had between 24 and 80 comb rows, far more than the 8 typical of living species. They also appear to have had internal organ-like structures unlike anything found in living ctenophores. One of the fossil species first reported in 1996 had a large mouth, apparently surrounded by a folded edge that may have been muscular. Evidence from China a year later suggests that such ctenophores were widespread in the Cambrian, but perhaps very different from modern species – for example one fossil's comb-rows were mounted on prominent vanes.
The early Cambrian sessile
frond
-like fossil Stromatoveris, from China's Chengjiang lagerstätte and dated to about , is very similar to Vendobionta of the preceding Ediacaran
period. De-Gan Shu, Simon Conway Morris
et al. found on its branches what they considered rows of cilia, used for filter feeding. They suggested that Stromatoveris was an evolutionary "aunt" of ctenophores, and that ctenophores originated from sessile animals whose descendants became swimmers and changed the cilia from a feeding mechanism to a propulsion system.
ary "family tree" of animals using only morphology
(their shapes and structures), because the large differences between Porifera (sponges), Cnidaria
(jellyfish
, sea anemone
s, etc.), Ctenophora, Placozoa and Bilateria
(all the more complex animals) make comparisons difficult. Hence reconstructions now rely largely or entirely on molecular phylogenetics, which groups organisms according to similarities and differences in their biochemistry
, usually in their DNA
or RNA
.
It is now generally thought that the Calcarea (sponges with calcium carbonate
spicule
s) are more closely related to Cnidaria, Ctenophora (comb jellies) and Bilateria
(all the more complex animals) than they are to the other groups of sponges. In 1866 it was proposed that Cnidaria and Ctenophora were more closely related to each other than to Bilateria and formed a group called Coelenterata
("hollow guts"), because Cnidaria and Ctenophora both rely on the flow of water in and out of a single cavity for feeding, excretion and respiration. In 1881 it was proposed that Ctenophora and Bilateria were more closely related to each other, since they shared features that Cnidaria lack, for example muscles in the middle layer (mesoglea
in Ctenophora, mesoderm
in Bilateria). More recent analyses indicate that these similarities are rather vague, and the current view, based on molecular phylogenetics, is that Cnidaria and Bilateria are more closely related to each other than either is to Ctenophora. This grouping of Cnidaria and Bilateria has been labelled "Planulozoa
" because it suggests that the earliest Bilateria were similar to the planula
larvae of Cnidaria.
In 2005 Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants of triploblastic animals, since ctenophores and the medusa stage of some cnidarians have striated muscle
, which in bilaterians arises from the mesoderm
. They did not commit themselves on whether bilaterians evolved from early cnidarians or from the hypothesized triploblastic ancestors of cnidarians. A study in 2008 of 150 genes in 21 genera proposed a third hypothesis, that sponges and cnidarians are more closely related to each other than either is to ctenophores.
Since all modern ctenophores except the beroids have cydippid-like larvae, it has widely been assumed that their last common ancestor also resembled cydippids, having an egg-shaped body and a pair of retractable tentacles. Richard Harbison's purely morphological analysis in 1985 concluded that the cydippids are not monophyletic, in other words do not contain all and only the descendants of a single common ancestor that was itself a cydippid. Instead he found that various cydippid families
were more similar to members of other ctenophore orders
than to other cydippids. He also suggested that the last common ancestor of modern ctenophores was either cydippid-like or beroid-like. A molecular phylogeny analysis in 2001, using 26 species including 4 recently-discovered ones, concluded that the cydippids are not monophyletic and that the last common ancestor of modern ctenophores was cydippid-like. It also found that the genetic differences between these species were very small – so small that the relationships between the Lobata, Cestida and Thalassocalycida remained uncertain. This suggests that the last common ancestor of modern ctenophores was relatively recent, and perhaps was lucky enough to survive the Cretaceous-Tertiary extinction while other lineages perished. When the analysis was broadened to include representatives of other phlya
, it concluded that cnidarians are probably more closely related to bilaterians than either group is to ctenophores but that this diagnosis is uncertain.
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to 1.5 metres (59.1 in) in size. Like cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
ns, their bodies consist of a mass of jelly with one layer of cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
on the outside and another lining the internal cavity. In ctenophores these layers are two cells deep while those in cnidarians are only one cell deep. Ctenophores also resemble cnidarians in having a decentralized nerve net
Nerve net
A nerve net is a type of simple nervous system that is found in members of the cnidaria, ctenophora, and echinodermata phyla. Nerve nets consist of interconnected neurons lacking a brain or any form of cephalization. This nervous system allows cnidarians to respond to physical contact. They may...
rather than a brain. Some authors combined ctenophores and cnidarians in one phylum, Coelenterata
Coelenterata
Coelenterata is an obsolete term encompassing two animal phyla, the Ctenophora and the Cnidaria . The name comes from the Greek "koilos" , referring to the hollow body cavity common to these two phyla...
, as both groups rely on water flow through the body cavity for both digestion and respiration. Increasing awareness of the differences persuaded more recent authors to classify
Biological classification
Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....
them in separate phyla.
Almost all ctenophores are predators, taking prey ranging from microscopic larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e and rotifer
Rotifer
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
s to the adults of small crustaceans; the exceptions are juveniles of two species, which live as parasites on the salp
Salp
A salp or salpa is a barrel-shaped, planktonic tunicate. It moves by contracting, thus pumping water through its gelatinous body...
s on which adults of their species feed. In favorable circumstances ctenophores can eat ten times their own weight in a day. There are only 100–150 validated species and possibly another 25 that have not been fully described and named. The textbook examples are cydippids
Cydippida
Cydippida is an order of comb jellies. They are distinguished from other comb jellies by their spherical or oval bodies, and the fact their tentacles are branched, and can be retracted into pouches on either side of the pharynx.-Anatomy:...
with egg-shaped bodies and a pair of retractable tentacles fringed with tentilla ("little tentacles") that are covered with colloblast
Colloblast
Colloblasts are a cell type found in ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. Colloblasts consist of a coiled spiral filament that is embedded in the epidermis and an axial filament with a granular dome. The apical surface of these cells...
s, sticky cells that capture prey. The phylum has a wide range of body forms, including the flattened deep-sea platyctenids, in which the adults of most species lack combs, and the coastal beroids, which lack tentacles and prey on other ctenophores by using huge mouths armed with groups of large, stiffened cilia that act as teeth. These variations enable different species to build huge populations in the same area, because they specialize in different types of prey, which they capture by as wide a range of methods as spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s use.
Most species are hermaphrodites, in other words a single animal can produce both eggs and sperm; if they are both produced at the same time, the animal is a simultaneous hermaphrodite, and if the eggs and sperm mature at different times, the animal is a sequential hermaphrodite. Fertilization is generally external
External fertilization
External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....
, although platyctenids' eggs are fertilized inside their parents' bodies and kept there until they hatch. The young are generally plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
ic and in most species look like miniature cydippids, gradually changing into the adult shape as they grow. The exceptions are the beroids, whose young are miniature beroids with large mouths and no tentacles, and the platyctenids, whose young live as cydippid-like plankton until they reach near-adult size, but then sink to the bottom and rapidly metamorphose
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
into the adult form. In at least some species, juveniles are capable of reproduction before reaching the adult size and shape. The combination of hermaphroditism and early reproduction enables small populations to grow at an explosive rate.
Ctenophores may be abundant during the summer months in some coastal locations, but in other places they are uncommon and difficult to find. In bays where they occur in very high numbers, predation by ctenophores may control the populations of small zooplanktonic organisms such as copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s, which might otherwise wipe out the phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
(planktonic plants), which are a vital part of marine food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
s. One ctenophore, Mnemiopsis, has accidentally been introduced
Introduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
into the Black Sea
Black Sea
The Black Sea is bounded by Europe, Anatolia and the Caucasus and is ultimately connected to the Atlantic Ocean via the Mediterranean and the Aegean seas and various straits. The Bosphorus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects that sea to the Aegean...
, where it is blamed for causing fish stocks to collapse by eating both fish larvae and organisms that would otherwise have fed the fish. The situation was aggravated by other factors, such as over-fishing and long-term environmental changes that promoted the growth of the Mnemiopsis population. The later accidental introduction of Beroe helped to mitigate the problem, as Beroe preys on other ctenophores.
Despite their soft, gelatinous bodies, fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s thought to represent ctenophores, apparently with no tentacles but many more comb-rows than modern forms, have been found in lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
n as far back as the early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
, about . The position of the ctenophores in the evolutionary family tree of animals has long been debated, and the majority view at present, based on molecular phylogenetics, is that cnidarians and bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
ns are more closely related to each other than either is to ctenophores. A recent molecular phylogenetics analysis concluded that the common ancestor of all modern ctenophores was cydippid-like, and that all the modern groups appeared relatively recently, probably after the Cretaceous-Tertiary extinction . Evidence accumulating since the 1980s indicates that the "cydippids" are not monophyletic, in other words do not include all and only the descendants of a single common ancestor, because all the other traditional ctenophore groups are descendants of various cydippids.
Distinguishing features
Ctenophores form an animalAnimal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
that is more complex than sponges, about as complex as cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
ns (jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
, sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s, etc.), and less complex than bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
ns, which include almost all other animals. Unlike sponges, both ctenophores and cnidarians have: cells bound by inter-cell connections and carpet-like basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
s; muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s; nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
s; and some have sensory
Sensory
Sensory may refer to:relating to senses or smellIn biology:* Sensory preference* Sensory system, part of the nervous system of organisms* Sensory neuron, nerve cell responsible for transmitting information about external stimuli...
organs. Ctenophores are distinguished from all other animals by having colloblast
Colloblast
Colloblasts are a cell type found in ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. Colloblasts consist of a coiled spiral filament that is embedded in the epidermis and an axial filament with a granular dome. The apical surface of these cells...
s that capture prey by squirting glue on them, although a few ctenophore species lack them.
Like sponges and cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea
Mesoglea
Mesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
in cnidarians and ctenophores; more complex animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s have three main cell layers and no intermediate jelly-like layer. Hence ctenophores and cnidarians have traditionally been labelled diploblastic, along with sponges. Both ctenophores and cnidarians have a type of muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
that, in more complex animals, arises from the middle cell layer
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
, and as a result some recent text books classify ctenophores as triploblastic, while others still regard them as diploblastic.
Ranging from about 1 millimetre (0.0393700787401575 in) to 1.5 metres (4.9 ft) in size, ctenophores are the largest non-colonial animals that use cilia ("hairs") as their main method of locomotion. Most species have eight strips, called comb rows, that run the length of their bodies and bear comb-like bands of cilia, called "ctenes," stacked along the comb rows so that when the cilia beat, those of each comb touch the comb below. The name "ctenophora" means "comb-bearing", from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
κτείς (stem-form κτεν-) meaning "comb" and the Greek suffix -φορος meaning "carrying".
| Sponges | Cnidarians | Ctenophores | Bilateria Bilateria The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back... |
|
|---|---|---|---|---|
| Cnidocyte Cnidocyte A cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on... s |
No | Yes | No | |
| Colloblast Colloblast Colloblasts are a cell type found in ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. Colloblasts consist of a coiled spiral filament that is embedded in the epidermis and an axial filament with a granular dome. The apical surface of these cells... s |
No | In most species | No | |
| Digestive and circulatory Circulatory system The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc... organ Organ (anatomy) In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are... s |
No | Yes | ||
| Number of main cell layers | Two, with jelly-like layer between them | Debate about whether two or three | Three | |
| Cells in each layer bound together | No, except that Homoscleromorpha Homoscleromorpha Homoscleromorpha is a subclass of marine demosponges containing a single order, Homosclerophorida and a single family, Plakinidae.-Taxonomy:This class has recently been recognised as the fourth major line of sponges.... have basement membranes. |
Yes: Inter-cell connections; basement membranes | ||
| Sensory Sensory Sensory may refer to:relating to senses or smellIn biology:* Sensory preference* Sensory system, part of the nervous system of organisms* Sensory neuron, nerve cell responsible for transmitting information about external stimuli... organs |
No | Yes | ||
| Number of cells in middle "jelly" layer | Many | Few | (Not applicable) | |
| Cells in outer layers can move inwards and change functions | Yes | No | (Not applicable) | |
| Nervous system | No | Yes, simple | Simple to complex | |
| Muscle Muscle Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to... s |
None | Mostly epitheliomuscular | Mostly myoepithelial | Mostly myocyte Myocyte A myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long, long chains of sarcomeres, the contractile units of the cell.... s |
Description
For a phylum with relatively few species, ctenophores have a wide range of body plans. Coastal species need to be tough enough to withstand waves and swirling sediment particles, while some oceanic species are so fragile that it is very difficult to capture them intact for study. In addition oceanic species do not preserve well, and are known mainly from photographs and from observers' notes. Hence most attention has until recently concentrated on three coastal generaGenus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
– Pleurobrachia
Pleurobrachia
Pleurobrachia is a common genus of Ctenophora, containing the following species:*Pleurobrachia arctica Wagner, 1885*Pleurobrachia australis *Pleurobrachia bachei L...
, Beroe and Mnemiopsis. At least two textbooks base their descriptions of ctenophores on the cydippid
Cydippida
Cydippida is an order of comb jellies. They are distinguished from other comb jellies by their spherical or oval bodies, and the fact their tentacles are branched, and can be retracted into pouches on either side of the pharynx.-Anatomy:...
Pleurobrachia.
Since the body of many species is almost radially symmetrical, the main axis is oral
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
to aboral
Aboral
In biology, aboral surfaces are surfaces away from or opposite the mouth. The term is a compound of the Latin preposition ā, a, abs, meaning from or away from and the noun ōs, ōris n., meaning mouth. It is also the opposite of oral which is the end containing the mouth of a bilaterally...
(from the mouth to the opposite end). However since only two of the canals near the statocyst
Statocyst
The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
terminate in anal
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
pores, ctenophores have no mirror-symmetery, although many have rotational symmetry, in other words if the animal rotates in a half-circle it looks the same as when it started.
Body layers
Like those of cnidariaCnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
ns, (jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
, sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s, etc.), ctenophores' bodies consist of a relatively thick, jelly-like mesoglea
Mesoglea
Mesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
sandwiched between two epithelia
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
, layers of cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
bound by inter-cell connections and by a fibrous basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
that they secrete. The epithelia of ctenophores have two layers of cells rather than one, and some of the cells in the upper layer have several cilia per cell.
The outer layer of the epidermis
Epidermis (zoology)
The Epidermis is an epithelium that covers the body of an eumetazoan . Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.Sponges have no epithelium, and therefore no epidermis or gastrodermis...
(outer skin) consists of: sensory cells; cells that secrete mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
, which protects the body; and interstitial cells, which can transform into other types of cell. In specialized parts of the body the outer layer also contains colloblast
Colloblast
Colloblasts are a cell type found in ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. Colloblasts consist of a coiled spiral filament that is embedded in the epidermis and an axial filament with a granular dome. The apical surface of these cells...
s, found along the surface of tentacles and used in capturing prey, or cells bearing multiple large cilia, for locomotion. The inner layer of the epidermis contains a nerve net
Nerve net
A nerve net is a type of simple nervous system that is found in members of the cnidaria, ctenophora, and echinodermata phyla. Nerve nets consist of interconnected neurons lacking a brain or any form of cephalization. This nervous system allows cnidarians to respond to physical contact. They may...
, and myoepithelial cells that act as muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s.
The internal cavity forms: a mouth that can usually be closed by muscles; a pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
("throat"); a wider area in the center that acts as a stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
; and a system of internal canals. These branch through the mesoglea to the most active parts of the animal: the mouth and pharynx; the roots of the tentacles, if present; all along the underside of each comb row; and four branches round the sensory complex at the far end from the mouth – two of these four branches terminate in anal
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
pores. The inner surface of the cavity is lined with an epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
, the gastrodermis
Gastrodermis
The gastrodermis is the inner layer of cells that lines a gastrovascular cavity of Cnidarians....
. The mouth and pharynx have both cilia and well-developed muscles. In other parts of the canal system, the gastrodermis is different on the sides nearest to and furthest from the organ that it supplies. The nearer side is composed of tall nutritive cells that store nutrients in vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
s (internal compartments), germ cell
Germ cell
A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate near the gut of an embryo and migrate to the developing gonads. There, they undergo cell division of two types, mitosis and meiosis, followed by...
s that produce eggs or sperm, and photocytes
Photocytes
A Photocyte is a cell that specializes in catalyzing enzymes to produce light . Photocytes typically occur in select layers of epithelial tissue, functioning singly or in a group, or as part of a larger apparatus ....
that produce bioluminescence
Bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
. The side furthest from the organ is covered with ciliated cells that circulate water through the canals, punctuated by ciliary rosettes, pores that are surrounded by double whorls of cilia and connect to the mesoglea.
Feeding, excretion and respiration
When prey is swallowed, it is liquefied in the pharynxPharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
by enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s and by muscular contractions of the pharynx. The resulting slurry is wafted through the canal system by the beating of the cilia, and digested by the nutritive cells. The ciliary rosettes in the canals may help to transport nutrients to muscles in the mesoglea. The anal
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
pores may eject unwanted small particles, but most unwanted matter is regurgitated via the mouth.
Little is known about how ctenophores get rid of waste products produced by the cells. The ciliary rosettes in the gastrodermis
Gastrodermis
The gastrodermis is the inner layer of cells that lines a gastrovascular cavity of Cnidarians....
may help to remove wastes from the mesoglea, and may also help to adjust the animal's buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
by pumping water into or out of the mesoglea.
Locomotion
The outer surface bears usually eight comb rows, which are used for swimming. The rows are oriented to run from near the mouth (the "oral pole") to the opposite end (the"aboral pole"), and are spaced more or less evenly around the body, although spacing patterns vary by species and in most species the comb rows extend only part of the distance from the aboral pole towards the mouth. The "combs" (also called "ctenes" or "comb plates") run across each row, and each consists of thousands of unusually long cilia, up to 2 millimetre (0.078740157480315 in). These normally beat so that the propulsion stroke is away from the mouth, although they can also reverse direction. Hence ctenophores usually swim in the direction in which the mouth is pointing, unlike jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
. When trying to escape predators, one species can accelerate to six times its normal speed; some other species reverse direction as part of their escape behavior, by reversing the power stroke of the comb plate cilia.
It is uncertain how ctenophores control their buoyancy, but experiments have shown that some species rely on osmotic pressure
Osmotic pressure
Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....
to adapt to water of different densities. Their body fluids are normally as concentrated
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
as seawater. If they enter less dense brackish water, the ciliary rosettes in the body cavity may pump this into the mesoglea
Mesoglea
Mesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
to increase its bulk and decrease its density, to avoid sinking. Conversely if they move from brackish to full-strength seawater, the rosettes may pump water out of the mesoglea to reduce its volume and increase its density.
Nervous system and senses
Ctenophores have no brainBrain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
or central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
, but instead have a nerve net
Nerve net
A nerve net is a type of simple nervous system that is found in members of the cnidaria, ctenophora, and echinodermata phyla. Nerve nets consist of interconnected neurons lacking a brain or any form of cephalization. This nervous system allows cnidarians to respond to physical contact. They may...
(rather like a cobweb) that forms a ring round the mouth and is densest near structures such as the comb rows, pharynx, tentacles (if present) and the sensory complex furthest from the mouth.
The largest single sensory feature is the aboral
Aboral
In biology, aboral surfaces are surfaces away from or opposite the mouth. The term is a compound of the Latin preposition ā, a, abs, meaning from or away from and the noun ōs, ōris n., meaning mouth. It is also the opposite of oral which is the end containing the mouth of a bilaterally...
organ (at the opposite end from the mouth). Its main component is a statocyst
Statocyst
The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
, a balance sensor consisting of a statolith, a solid particle supported on four bundles of cilia, called "balancers", that sense its orientation. The statocyst is protected by a transparent dome made of long, immobile cilia. A ctenophore does not automatically try to keep the statolith resting equally on all the balancers. Instead its response is determined by the animal's "mood", in other words the overall state of the nervous system. For example if a ctenophore with trailing tentacles captures prey, it will often put some comb rows into reverse, spinning the mouth towards the prey.
Cydippids
Cydippid ctenophores have bodies that are more or less rounded, sometimes nearly spherical and other times more cylindrical or egg-shaped; the common coastal "sea gooseberry," PleurobrachiaPleurobrachia
Pleurobrachia is a common genus of Ctenophora, containing the following species:*Pleurobrachia arctica Wagner, 1885*Pleurobrachia australis *Pleurobrachia bachei L...
, sometimes has an egg-shaped body with the mouth at the narrow end, although some individuals are more uniformly round. From opposite sides of the body extends a pair of long, slender tentacles, each housed in a sheath into which it can be withdrawn. Some species of cydippids have bodies that are flattened to various extents, so that they are wider in the plane of the tentacles.
The tentacles of cydippid ctenophores are typically fringed with tentilla ("little tentacles"), although a few genera have simple tentacles without these sidebranches. The tentacles and tentilla are densely covered with microscopic colloblast
Colloblast
Colloblasts are a cell type found in ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. Colloblasts consist of a coiled spiral filament that is embedded in the epidermis and an axial filament with a granular dome. The apical surface of these cells...
s that capture prey by sticking to it. Colloblasts are specialized mushroom
Mushroom
A mushroom is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground on soil or on its food source. The standard for the name "mushroom" is the cultivated white button mushroom, Agaricus bisporus; hence the word "mushroom" is most often applied to those fungi that...
-shaped cells in the outer layer of the epidermis, and have three main components: a domed head with vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
(chambers) that contain adhesive; a stalk that anchors the cell in the lower layer of the epidermis or in the mesoglea; and a spiral
Spiral
In mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point.-Spiral or helix:...
thread that coils round the stalk and is attached to the head and to the root of the stalk. The function of the spiral thread is uncertain, but it may absorb stress when prey tries to escape, and thus prevent the collobast from being torn apart. In addition to colloblasts, members of the genus Haeckelia, which feed mainly on jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
, incorporate their victims' stinging nematocytes into their own tentacles – some cnidaria-eating nudibranch
Nudibranch
A nudibranch is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms...
s similarly incorporate nematocytes into their bodies for defense. The tentilla of Euplokamis differ significantly from those of other cydippids: they contain striated muscle
Striated muscle
Striated muscle tissue is a form of fibers that are combined into parallel fibers. More specifically, it can refer to:* Cardiac muscle .* Skeletal muscle* Branchiomeric muscles...
, a cell type otherwise unknown in the phylum Ctenophora; and they are coiled when relaxed, while the tentilla of all other known ctenophores elongate when relaxed. Euplokamis tentilla have three types of movement that are used in capturing prey: they may flick out very quickly (in 40 to 60 millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s); they can wriggle, which may lure prey by behaving like small planktonic worms; and they coil round prey. The unique flicking is an uncoiling movement powered by contraction of the striated muscle
Striated muscle
Striated muscle tissue is a form of fibers that are combined into parallel fibers. More specifically, it can refer to:* Cardiac muscle .* Skeletal muscle* Branchiomeric muscles...
. The wriggling motion is produced by smooth muscle
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...
s, but of a highly specialized type. Coiling around prey is accomplished largely by the return of the tentilla to their inactive state, but the coils may be tightened by smooth muscle.
There are eight rows of combs that run from near the mouth to the opposite end, and are spaced evenly round the body. The "combs" beat in a metachronal rhythm
Metachronal rhythm
A metachronal rhythm or metachronal wave refers to wavy movements produced by the sequential action of structures such as cilia, segments of worms or legs. These movements produce the appearance of a travelling wave. A Mexican wave is a large scale example of a metachronal wave...
rather like that of a Mexican wave. From each balancer in the statocyst a ciliary groove runs out under the dome and then splits to connect with two adjacent comb rows, and in some species runs all the way along the comb rows. This forms a mechanical system for transmitting the beat rhythm from the combs to the balancers, via water disturbances created by the cilia.
Lobates
Lobata
Lobata is an order of Ctenophora in the class Tentaculata with smaller tentacles than other ctenophores, and distinctive flattened lobes extending outwards from their bodies.They are about long.-Anatomy:...
have a pair of lobes, which are muscular, cuplike extensions of the body that project beyond the mouth. Their inconspicuous tentacles originate from the corners of the mouth, running in convoluted grooves and spreading out over the inner surface of the lobes (rather than trailing far behind, as in the Cydippida). Between the lobes on either side of the mouth, many species of lobates have four auricles, gelatinous projections edged with cilia that produce water currents that help direct microscopic prey toward the mouth. This combination of structures enables lobates to feed continuously on suspended plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
ic prey.
Lobates have eight comb-rows, originating at the aboral pole and usually not extending beyond the body to the lobes; in species with (four) auricles, the cilia edging the auricles are extensions of cilia in four of the comb rows. Most lobates are quite passive when moving through the water, using the cilia on their comb rows for propulsion, although Leucothea has long and active auricles whose movements also contribute to propulsion. Members of the lobate genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Bathocyroe
Bathocyroe
Bathocyroe is a genus of ctenophores, the only genus in the family Bathocyroidae....
and Ocyropsis
Ocyropsis
Ocyropsis is a genus of oceanic lobate ctenophores. Species include:* Ocyropsis crystallina * Ocyropsis fusca * Ocyropsis maculata * Ocyropsis pteroessa Bigelow, 1904...
can escape from danger by clapping their lobes, so that the jet of expelled water drives them backwards very quickly. Unlike cydippids, the movements of lobates' combs are coordinated by nerves rather than by via water disturbances created by the cilia, yet combs on the same row beat beat in the same Mexican wave style as the mechanically-coordinated comb rows of cydippids and beroids. This may have enabled lobates to grow larger than cydippids and to have shapes that are less egg-like.
An unusual species first described in 2000, Lobatolampea tetragona, has been classified as a lobate, although the lobes are "primitive" and the body is medusa
Medusa (biology)
In biology, a medusa is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below...
-like when floating and disk-like when resting on the sea-bed.
Beroids
The Beroida, also known as NudaNuda
Nuda is a class of comb jellies. As their name implies, they are distinguished from other comb jellies by the complete absence of tentacles. Nudans are found in all the world's oceans and seas, where they swim freely in the plankton.-Anatomy:...
, have no feeding appendages, but their large pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
, just inside the large mouth and filling most of the saclike body, bears "macrocilia" at the oral end. These fused bundles of several thousand large cilia are able to "bite" off pieces of prey that is too large to swallow whole – almost always other ctenophores. In front of the field of macrocilia, on the mouth "lips" in some species of Beroe, is a pair of narrow strips of adhesive epithelial cells on the stomach wall that "zip" the mouth shut when the animal is not feeding, by forming intercellular connections with the opposite adhesive strip. This tight closure streamlines
Streamliner
A streamliner is a vehicle incorporating streamlining in a shape providing reduced air resistance. The term is applied to high-speed railway trainsets of the 1930s to 1950s, and to their successor "bullet trains". Less commonly, the term is applied to fully faired recumbent bicycles...
the front of the animal when it is pursuing prey.
Other body forms
The Ganeshida have a pair of small oral lobes and a pair of tentacles. The body is circular rather than oval in cross-section, and the pharynx extends over the inner surfaces of the lobes.The Thalassocalycida, only discovered in 1978 and known from only one species, are medusa-like, with bodies that are shortened in the oral-aboral direction, and short comb-rows on the surface furthest from the mouth, originating from near the aboral pole. They capture prey by movements of the bell and possibly by using two short tentacles.
The Cestida
Cestida
Cestida is an order of comb jellies. It has one family, Cestidae, with two genera: Cestum and Velamen, each containing one species.* Cestum veneris Lesueur, 1813* Velamen parallelum...
("belt animals") are ribbon-shaped planktonic animals, with the mouth and aboral organ aligned in the middle of opposite edges of the ribbon. There is a pair of comb-rows along each aboral edge, and tentilla emerging from a groove all along the oral edge, which stream back across most of the wing-like body surface. Cestids can swim by undulating their bodies as well as by the beating of their comb-rows. There are two known species, with worldwide distribution in warm, and warm-temperate waters: Cestum veneris ("Venus
Venus (mythology)
Venus is a Roman goddess principally associated with love, beauty, sex,sexual seduction and fertility, who played a key role in many Roman religious festivals and myths...
' girdle") is among the largest ctenophores – up to 1.5 metres (4.9 ft) long, and can undulate slowly or quite rapidly. Velamen parallelum, which is typically less than 20 centimetre (0.656167979002625 ft) long, can move much faster in what has been described as a "darting motion".
Most Platyctenida
Platyctenida
Platyctenida is an order of comb jellies.Platyctenids have flattened oval bodies, with a pair of tentilla-bearing tentacles on the surface opposite to the mouth. They cling to and creep on surfaces by everting the pharynx and using it as a muscular "foot"...
have oval bodies that are flattened in the oral-aboral direction, with a pair of tentilla-bearing tentacles on the aboral surface. They cling to and creep on surfaces by everting the pharynx and using it as a muscular "foot". All but one of the known platyctenid species lack comb-rows. Platyctenids are usually cryptically-colored, live on rocks, algae, or the body surfaces of other invertebrates, and are often revealed by their long tentacles with many sidebranches, seen streaming off the back of the ctenophore into the current.
Reproduction and development
Cloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...
, splitting off from the edges of their flat bodies fragments that develop into new individuals.
Almost all species are hermaphrodites, in other words they function as both males and females at the same time – except that in two species of the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Ocryopsis individuals remain of the same single sex all their lives. The gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s are located in the parts of the internal canal network under the comb rows, and eggs and sperm are released via pores in the epidermis. Fertilization is external
External fertilization
External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....
in most species, but platyctenids use internal fertilization and keep the eggs in brood chambers until they hatch. Self-fertilization has occasionally been seen in species of the genus Mnemiopsis, and it is thought that most of the hermaphroditic species are self-fertile.
Development of the fertized eggs is direct, in other words there is no distinctive larval form, and juveniles of all groups generally resemble miniature cydippid adults. In the genus Beroe the juveniles, like the adults, lack tentacles and tentacle sheaths. In most species the juveniles gradually develop the body forms of their parents. In some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae, as they live among the plankton and thus occupy a different ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
from their parents and attain the adult form by a more radical metamorphosis
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
, after dropping to the sea-floor.
At least in some species, juvenile ctenophores appear capable of producing small quantities of eggs and sperm while they are well below adult size, and adults produce eggs and sperm for as long as they have sufficient food. If they run short of food, they first stop producing eggs and sperm, and then shrink in size. When the food supply improves, they grow back to normal size and then resume reproduction. These features make ctenophores capable of increasing their populations very quickly.
Colors and bioluminescence
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Most ctenophores that live near the surface are mostly colorless and almost transparent. However some deeper-living species are strongly pigmented, for example the species known as "Tortugas red" (see illustration here), which has not yet been formally described. Platyctenids generally live attached to other sea-bottom organisms, and often have similar colors to these host organisms. The gut of the deep-sea genus Bathocyroe
Bathocyroe
Bathocyroe is a genus of ctenophores, the only genus in the family Bathocyroidae....
is red, which hides the bioluminescence
Bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
of copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s it has swallowed.
The comb rows of most planktonic ctenophores produce a rainbow effect, which is not caused by bioluminescence
Bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
but by the scattering of light
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
as the combs move. Most species are also bioluminescent, but the light is usually blue or green and can only been seen in darkness. However some significant groups, including all known platyctenids and the cydippid genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Pleurobrachia
Pleurobrachia
Pleurobrachia is a common genus of Ctenophora, containing the following species:*Pleurobrachia arctica Wagner, 1885*Pleurobrachia australis *Pleurobrachia bachei L...
, are incapable of bioluminescence.
When some species, including Bathyctena chuni, Euplokamis stationis and Eurhamphaea vexilligera, are disturbed, they produce secretions (ink) that luminesce at much the same wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s as their bodies. Juveniles will luminesce more brightly in relation to their body size than adults, whose luminescence is diffused over their bodies. Detailed statistical investigation has not suggested the function of ctenophores' bioluminescence nor produced any correlation
Correlation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
between its exact color and any aspect of the animals' environments, such as depth or whether they live in coastal or mid-ocean waters.
Distribution
Ctenophores are found in most marine environments: from polar waters to the tropics; near coasts and in mid-ocean; from the surface waters to the ocean depths. The best-understood are the generaGenus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Pleurobrachia, Beroe and Mnemiopsis, as these plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
ic coastal forms are among the most likely to be collected near shore. No ctenophores have been found in fresh water.
Prey and predators
Almost all ctenophores are predators – there are no vegetarians and only one genus that is partly parasitic. If food is plentiful, they can eat 10 times their own weight per day. While Beroe preys mainly on other ctenophores, other surface-water species prey on zooplanktonZooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
(planktonic animals) ranging in size from the microscopic, including mollusc and fish larvae, to small adult crustaceans such as copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s, amphipods, and even krill
Krill
Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...
. Members of the genus Haeckelia prey on jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
and incorporate their prey's nematocysts (stinging cells) into their own tentacles instead of colloblast
Colloblast
Colloblasts are a cell type found in ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. Colloblasts consist of a coiled spiral filament that is embedded in the epidermis and an axial filament with a granular dome. The apical surface of these cells...
s. Ctenophores have been compared to spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s in their wide range of techniques from capturing prey – some hang motionless in the water using their tentacles as "webs", some are ambush predators like Salticid jumping spider
Jumping spider
The jumping spider family contains more than 500 described genera and about 5,000 described species, making it the largest family of spiders with about 13% of all species. Jumping spiders have some of the best vision among invertebrates and use it in courtship, hunting and navigation...
s, and some dangle a sticky droplet at the end of a fine thread, as Bola spiders do. This variety explains the wide range of body forms in a phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
with rather few species. The two-tentacled "cydippid" Lampea feeds exclusively on salp
Salp
A salp or salpa is a barrel-shaped, planktonic tunicate. It moves by contracting, thus pumping water through its gelatinous body...
s, close relatives of sea-squirts that form large chain-like foating colonies, and juveniles of Lampea attach themselves like parasites to salps that are too large for them to swallow. Members of the cydippid genus Pleurobrachia
Pleurobrachia
Pleurobrachia is a common genus of Ctenophora, containing the following species:*Pleurobrachia arctica Wagner, 1885*Pleurobrachia australis *Pleurobrachia bachei L...
and the lobate Bolinopsis often reach high population densities at the same place and time because they specialize in different types of prey: Pleurobrachia
Pleurobrachia
Pleurobrachia is a common genus of Ctenophora, containing the following species:*Pleurobrachia arctica Wagner, 1885*Pleurobrachia australis *Pleurobrachia bachei L...
s long tentacles mainly capture relatively strong swimmers such as adult copepods, while Bolinopsis generally feeds on smaller, weaker swimmers such as rotifer
Rotifer
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
s and mollusc and crustacean larvae
Crustacean larvae
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow...
.
Ctenophores used to be regarded as "dead ends" in marine food chains because it was thought their low ratio of organic matter to salt and water made them a poor diet for other animals. It is also often difficult to identify the remains of ctenophores in the guts of possible predators, although the combs sometimes remain intact long enough to provide a clue. Detailed investigation of chum salmon
Chum salmon
The chum salmon, Oncorhynchus keta, is a species of anadromous fish in the salmon family. It is a Pacific salmon, and may also be known as dog salmon or Keta salmon, and is often marketed under the name Silverbrite salmon...
, Oncorhynchus keta, showed that these fish digest ctenophores 20 times as fast as an equal weight of shrimp
Shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
s, and that ctenophores can provide a good diet if there are enough of them around. Beroids prey mainly on other ctenophores. Some jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
and turtle
Turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
s eat large quantities of ctenophores, and jellyfish may temporarily wipe out ctenophore populations. Since ctenophores and jellyfish often have large seasonal variations in population, most fish that prey on them are generalists, and may have a greater effect on populations than the specialist jelly-eaters. The larvae of some sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s are parasites on ctenophores, as are the larvae of some flatworm
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s that parasitize fish when they reach adulthood.
Ecological impacts
Ctenophores may balance marine ecosystems by preventing an over-abundance of copepods from eating all the phytoplanktonPhytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
(planktonic plants), which are the dominant marine producers of organic matter from non-organic ingredients.
On the other hand in the late 1980s the North Atlantic ctenophore Mnemiopsis leidyi
Mnemiopsis leidyi
The warty comb jelly or sea walnut is a species of tentaculate ctenophore , originally native to the western Atlantic coastal waters. Three species have been named in the genus Mnemiopsis, but they are now believed to be different ecological forms of a single species M...
was accidentally introduced
Introduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
into the Black Sea
Black Sea
The Black Sea is bounded by Europe, Anatolia and the Caucasus and is ultimately connected to the Atlantic Ocean via the Mediterranean and the Aegean seas and various straits. The Bosphorus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects that sea to the Aegean...
and Sea of Azov
Sea of Azov
The Sea of Azov , known in Classical Antiquity as Lake Maeotis, is a sea on the south of Eastern Europe. It is linked by the narrow Strait of Kerch to the Black Sea to the south and is bounded on the north by Ukraine mainland, on the east by Russia, and on the west by the Ukraine's Crimean...
via the ballast tanks of ships, and has been blamed for causing sharp drops in fish catches by eating both fish larvae and small crustaceans that would otherwise feed the adult fish. Mnemiopsis is well-equipped to invade new territories, as it can breed very rapidly and tolerate a wide range of water temperatures and salinities
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
. The impact was increased by chronic overfishing, and by eutrophication
Eutrophication
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...
that gave the entire ecosystem a short-term boost, causing the Mnemiopsis population to increase even faster than normal – and above all by the absence of efficient predators on ctenophores. Mnemiopsis populations in those areas were eventually brought under control by the accidental introduction of the ctenophore-eating ctenophore Beroe ovata, and by a cooling of the local climate from 1991 to 1993, which significantly slowed the animal's metabolism. However the abundance of plankton in the area seems unlikely to reach pre-Mnemiopsis levels.
In the late 1990s Mnemiopsis appeared in the Caspian Sea
Caspian Sea
The Caspian Sea is the largest enclosed body of water on Earth by area, variously classed as the world's largest lake or a full-fledged sea. The sea has a surface area of and a volume of...
. Beroe ovata arrived shortly after, and is expected to reduce but not eliminate the impact of Mnemiopsis. Mnemiopsis also reached the eastern Mediterranean in the late 1990s and now appears to be thriving in the North Sea
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
and Baltic Sea
Baltic Sea
The Baltic Sea is a brackish mediterranean sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...
.
Classification
The number of known living ctenophore species is uncertain, since many of those named and formally described have turned out to be identical to species known under other scientific names. Claudia Mills estimates that there about 100 to 150 valid species that are not duplicates, and that at least another 25, mostly deep-sea forms, have been recognized as distinct but not yet analyzed in enough detail to support a formal description and naming.The traditional classification divides ctenophores into two classes
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
, those with tentacles (Tentaculata
Tentaculata
Tentaculata is a class of comb jellies. The common feature of this class is a pair of long, feathery, contractile tentacles, which can be retracted into specialised ciliated sheaths. In some species, the primary tentacles are reduced and they have smaller, secondary tentacles...
) and those without (Nuda
Nuda
Nuda is a class of comb jellies. As their name implies, they are distinguished from other comb jellies by the complete absence of tentacles. Nudans are found in all the world's oceans and seas, where they swim freely in the plankton.-Anatomy:...
). The Nuda contains only one order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
(Beroida) and family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
(Beroidae), and two genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
, Beroe (several species) and Neis (one species).
The Tentaculata
Tentaculata
Tentaculata is a class of comb jellies. The common feature of this class is a pair of long, feathery, contractile tentacles, which can be retracted into specialised ciliated sheaths. In some species, the primary tentacles are reduced and they have smaller, secondary tentacles...
are divided into the following eight orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
:
- CydippidaCydippidaCydippida is an order of comb jellies. They are distinguished from other comb jellies by their spherical or oval bodies, and the fact their tentacles are branched, and can be retracted into pouches on either side of the pharynx.-Anatomy:...
, egg-shaped animals with long tentacles - LobataLobataLobata is an order of Ctenophora in the class Tentaculata with smaller tentacles than other ctenophores, and distinctive flattened lobes extending outwards from their bodies.They are about long.-Anatomy:...
, with paired thick lobes - PlatyctenidaPlatyctenidaPlatyctenida is an order of comb jellies.Platyctenids have flattened oval bodies, with a pair of tentilla-bearing tentacles on the surface opposite to the mouth. They cling to and creep on surfaces by everting the pharynx and using it as a muscular "foot"...
, flattened animals that live on or near the sea-bed; most lack combs as adults, and use their pharyngesPharynxThe human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
as suckers to attach themselves to surfaces - Ganeshida, with a pair of small lobes round the mouth, but an extended pharynxPharynxThe human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
like that of platyctenids - Cambojiida
- Cryptolobiferida
- Thalassocalycida, with short tentacles and a jellyfish-like "umbrella"
- CestidaCestidaCestida is an order of comb jellies. It has one family, Cestidae, with two genera: Cestum and Velamen, each containing one species.* Cestum veneris Lesueur, 1813* Velamen parallelum...
, ribbon-shaped and the largest ctenophores
Fossil record
Because of their soft, gelatinous bodies, ctenophores are extremely rare as fossils, and fossils that have been interpreted as ctenophores have been found only in lagerstätteLagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
n, places where the environment was exceptionally suited to preservation of soft tissue. Until the mid-1990s only two specimens good enough for analysis were known, both from the early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period. Three additional putative species were then found in the Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
and other Canadian rocks of similar age, about in the mid-Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period. All three apparently lacked tentacles but had between 24 and 80 comb rows, far more than the 8 typical of living species. They also appear to have had internal organ-like structures unlike anything found in living ctenophores. One of the fossil species first reported in 1996 had a large mouth, apparently surrounded by a folded edge that may have been muscular. Evidence from China a year later suggests that such ctenophores were widespread in the Cambrian, but perhaps very different from modern species – for example one fossil's comb-rows were mounted on prominent vanes.
The early Cambrian sessile
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
frond
Frond
The term frond refers to a large, divided leaf. In both common usage and botanical nomenclature, the leaves of ferns are referred to as fronds and some botanists restrict the term to this group...
-like fossil Stromatoveris, from China's Chengjiang lagerstätte and dated to about , is very similar to Vendobionta of the preceding Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
period. De-Gan Shu, Simon Conway Morris
Simon Conway Morris
Simon Conway Morris FRS is an English paleontologist made known by his detailed and careful study of the Burgess Shale fossils, an exploit celebrated in Wonderful Life by Stephen Jay Gould...
et al. found on its branches what they considered rows of cilia, used for filter feeding. They suggested that Stromatoveris was an evolutionary "aunt" of ctenophores, and that ctenophores originated from sessile animals whose descendants became swimmers and changed the cilia from a feeding mechanism to a propulsion system.
Family tree
It is difficult to reconstruct the early stages in the evolutionEvolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary "family tree" of animals using only morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
(their shapes and structures), because the large differences between Porifera (sponges), Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
(jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
, sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s, etc.), Ctenophora, Placozoa and Bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
(all the more complex animals) make comparisons difficult. Hence reconstructions now rely largely or entirely on molecular phylogenetics, which groups organisms according to similarities and differences in their biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
, usually in their DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
or RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
.
It is now generally thought that the Calcarea (sponges with calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
spicule
Spicule
Spicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....
s) are more closely related to Cnidaria, Ctenophora (comb jellies) and Bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
(all the more complex animals) than they are to the other groups of sponges. In 1866 it was proposed that Cnidaria and Ctenophora were more closely related to each other than to Bilateria and formed a group called Coelenterata
Coelenterata
Coelenterata is an obsolete term encompassing two animal phyla, the Ctenophora and the Cnidaria . The name comes from the Greek "koilos" , referring to the hollow body cavity common to these two phyla...
("hollow guts"), because Cnidaria and Ctenophora both rely on the flow of water in and out of a single cavity for feeding, excretion and respiration. In 1881 it was proposed that Ctenophora and Bilateria were more closely related to each other, since they shared features that Cnidaria lack, for example muscles in the middle layer (mesoglea
Mesoglea
Mesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
in Ctenophora, mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
in Bilateria). More recent analyses indicate that these similarities are rather vague, and the current view, based on molecular phylogenetics, is that Cnidaria and Bilateria are more closely related to each other than either is to Ctenophora. This grouping of Cnidaria and Bilateria has been labelled "Planulozoa
Planulozoa
Planulozoa is a proposed clade including the Cnidaria and the Bilateria . The clade excludes the Ctenophora ....
" because it suggests that the earliest Bilateria were similar to the planula
Planula
A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. The planula forms from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...
larvae of Cnidaria.
In 2005 Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants of triploblastic animals, since ctenophores and the medusa stage of some cnidarians have striated muscle
Striated muscle
Striated muscle tissue is a form of fibers that are combined into parallel fibers. More specifically, it can refer to:* Cardiac muscle .* Skeletal muscle* Branchiomeric muscles...
, which in bilaterians arises from the mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
. They did not commit themselves on whether bilaterians evolved from early cnidarians or from the hypothesized triploblastic ancestors of cnidarians. A study in 2008 of 150 genes in 21 genera proposed a third hypothesis, that sponges and cnidarians are more closely related to each other than either is to ctenophores.
Since all modern ctenophores except the beroids have cydippid-like larvae, it has widely been assumed that their last common ancestor also resembled cydippids, having an egg-shaped body and a pair of retractable tentacles. Richard Harbison's purely morphological analysis in 1985 concluded that the cydippids are not monophyletic, in other words do not contain all and only the descendants of a single common ancestor that was itself a cydippid. Instead he found that various cydippid families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
were more similar to members of other ctenophore orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
than to other cydippids. He also suggested that the last common ancestor of modern ctenophores was either cydippid-like or beroid-like. A molecular phylogeny analysis in 2001, using 26 species including 4 recently-discovered ones, concluded that the cydippids are not monophyletic and that the last common ancestor of modern ctenophores was cydippid-like. It also found that the genetic differences between these species were very small – so small that the relationships between the Lobata, Cestida and Thalassocalycida remained uncertain. This suggests that the last common ancestor of modern ctenophores was relatively recent, and perhaps was lucky enough to survive the Cretaceous-Tertiary extinction while other lineages perished. When the analysis was broadened to include representatives of other phlya
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
, it concluded that cnidarians are probably more closely related to bilaterians than either group is to ctenophores but that this diagnosis is uncertain.
Further reading
- R. S. K. Barnes, P. Calow, P. J. W. Olive, D. W. Golding, J. I. Spicer, The invertebrates – a synthesis, 3rd ed, Blackwell, 2001, ch. 3.4.3, p. 63, ISBN 0-632-04761-5
- R. C. Brusca, G. J. Brusca, Invertebrates, 2nd Ed, Sinauer Associates, 2003, ch. 9, p. 269, ISBN 0-87893-097-3
- J. Moore, An Introduction to the Invertebrates, Cambridge Univ. Press, 2001, ch. 5.4, p. 65, ISBN 0-521-77914-6
- W. Schäfer, Ctenophora, Rippenquallen, in W. Westheide and R. Rieger: Spezielle Zoologie Band 1, Gustav Fischer Verlag, Stuttgart 1996
- Bruno Wenzel, Glastiere des Meeres. Rippenquallen (Acnidaria), 1958, ISBN 3-7403-0189-9
- Mark Shasha, Night of the Moonjellies, 1992, Simon & Schuster, ISBN 0-6717-7565-0
External links
- Ctenophores from the São Sebastião Channel, Brazil
- Video of ctenophores at the National Zoo in Washington DC
- Tree Of Animal Life Has Branches Rearranged, By Evolutionary Biologists
- Australian Ctenophora Fact Sheet
- The Jelly Connection – striking images, including a Beroe specimen attacking another ctenophore