Isopoda
Encyclopedia
Isopods are an order
of peracarid
crustacean
s, including familiar animals such as woodlice
and pill bugs. The name Isopoda derives from the Greek
roots
(, meaning "same") and (, meaning "foot"). The fossil record of isopods dates back to the Carboniferous period (in the US Pennsylvanian epoch
), at least 300 million years ago.
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Isopods are relatively small crustaceans with seven pairs of legs of similar size and form, ranging in size from 300 um to nearly 50 centimetres (20 in) in the case of Bathynomus giganteus. They are typically flattened dorso-ventrally, although many species deviate from this plan, particularly those from the deep sea or from ground water
. Isopods lack an obvious carapace
, which is reduced to a "cephalic shield" covering only the head. Gas exchange
is carried out by specialised gill-like pleopods towards the rear of the animal's body. In terrestrial isopods, these are often adapted into structures which resemble lung
s, and these "lungs" are readily visible on the underside of a woodlouse. Eyes, when present, are always sessile, never on stalks. They share with the Tanaidacea
the fusion of the last abdominal
body segment with the telson
, forming a "pleotelson", and the first body segment of the thorax
is fused to the head. The pereiopods are uniramous, but the pleopods are biramous.
Around 4,500 species of isopods are found in marine environments, mostly on the sea floor. Some 500 species are found in fresh water
; and another 5,000 species are the woodlice
in the suborder Oniscidea, which are thus by far the most successful group of terrestrial
crustaceans. In the deep sea, members of the suborder Asellota
predominate, to the near exclusion of all other isopods, having undergone a large adaptive radiation
in that environment.
A number of isopod groups have evolved a parasitic lifestyle. The suborder Cymothoida
is exclusively parasitic, while the polyphyletic
suborder Flabellifera
is partly parasitic. Cymothoa exigua
, for example, is a parasite of the spotted rose snapper fish Lutjanus guttatus in the Gulf of California
; it eats the tongue of the fish, and takes its place, in the only known instance of a parasite functionally replacing a host structure.
In marine
and reef aquarium
s, parasitic isopods can become a pest, endangering both the fish and the aquarium keepers.
, which are united by the presence of a special brood pouch
for brooding eggs. There are around 10,215 described species
of isopod, classified into eleven suborders.
e hatch as mancae, which resemble adults except for the lack of the last pair of pereiopods (thoracic legs). The lack of a swimming phase in the life cycle
is a limiting factor
in isopod dispersal
, and may be responsible for the high levels of endemism in the order. As adults, isopods differ from other crustaceans in that they replace their exoskeleton
(in the process called ecdysis
) in two phases; this is known as "biphasic moulting".
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...
of peracarid
Peracarida
The superorder Peracarida is a large group of malacostracan crustaceans, having members in marine, freshwater, and terrestrial habitats. They are chiefly defined by the presence of a brood pouch, or marsupium, formed from thin flattened plates borne on the basalmost segments of the legs Other...
crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s, including familiar animals such as woodlice
Woodlouse
A woodlouse is a crustacean with a rigid, segmented, long exoskeleton and fourteen jointed limbs...
and pill bugs. The name Isopoda derives 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;...
roots
Root (linguistics)
The root word is the primary lexical unit of a word, and of a word family , which carries the most significant aspects of semantic content and cannot be reduced into smaller constituents....
(, meaning "same") and (, meaning "foot"). The fossil record of isopods dates back to the Carboniferous period (in the US Pennsylvanian epoch
Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
), at least 300 million years ago.
Description
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
. Isopods lack an obvious carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...
, which is reduced to a "cephalic shield" covering only the head. Gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
is carried out by specialised gill-like pleopods towards the rear of the animal's body. In terrestrial isopods, these are often adapted into structures which resemble lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s, and these "lungs" are readily visible on the underside of a woodlouse. Eyes, when present, are always sessile, never on stalks. They share with the Tanaidacea
Tanaidacea
The crustacean order Tanaidacea make up a minor group within the class Malacostraca. There are about 940 species in this order.-Description:...
the fusion of the last abdominal
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
body segment with the telson
Telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...
, forming a "pleotelson", and the first body segment of the thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
is fused to the head. The pereiopods are uniramous, but the pleopods are biramous.
Ecology
Fresh Water
Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
; and another 5,000 species are the woodlice
Woodlouse
A woodlouse is a crustacean with a rigid, segmented, long exoskeleton and fourteen jointed limbs...
in the suborder Oniscidea, which are thus by far the most successful group of terrestrial
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
crustaceans. In the deep sea, members of the suborder Asellota
Asellota
Asellota is a suborder of isopod crustaceans found in marine and freshwater environments. Roughly one quarter of all marine isopods belong to this suborder. Members of this suborder are readily distinguished from other isopods by their complex copulatory apparatus...
predominate, to the near exclusion of all other isopods, having undergone a large adaptive radiation
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...
in that environment.
A number of isopod groups have evolved a parasitic lifestyle. The suborder Cymothoida
Cymothoida
Cymothoida is a suborder of isopod crustaceans. It contains the following superfamilies and families:*Superfamily Anthuroidea**Antheluridae**Anthuridae**Expanathuridae**Hyssuridae**Leptanthuridae**Paranthuridae*Superfamily Cymothooidea**Aegidae...
is exclusively parasitic, while the polyphyletic
Polyphyly
A polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded...
suborder Flabellifera
Flabellifera
Flabellifera is a former suborder of isopod crustaceans. It is a polyphyletic or paraphyletic group, and contained over 3000 species. Its members are now placed in the Sphaeromatidea and Cymothoida....
is partly parasitic. Cymothoa exigua
Cymothoa exigua
Cymothoa exigua, or the tongue-eating louse, is a parasitic crustacean of the family Cymothoidae. It tends to be long. This parasite enters through the gills, and then attaches itself at the base of the spotted rose snapper's tongue. It extracts blood through the claws on its front, causing the...
, for example, is a parasite of the spotted rose snapper fish Lutjanus guttatus in the Gulf of California
Gulf of California
The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
; it eats the tongue of the fish, and takes its place, in the only known instance of a parasite functionally replacing a host structure.
In marine
Marine aquarium
A marine aquarium is an aquarium that keeps marine plants and animals in a contained environment. Marine aquaria are further subdivided by hobbyists into fish only , fish only with live rock , and reef aquaria. Fish only tanks often showcase large or aggressive marine fish species and generally...
and reef aquarium
Reef aquarium
A reef aquarium or reef tank is an marine aquarium that prominently displays live corals and other marine invertebrates as well as fish that play a role in maintaining the coral reef environment...
s, parasitic isopods can become a pest, endangering both the fish and the aquarium keepers.
Diversity and classification
Isopods belong to the larger group PeracaridaPeracarida
The superorder Peracarida is a large group of malacostracan crustaceans, having members in marine, freshwater, and terrestrial habitats. They are chiefly defined by the presence of a brood pouch, or marsupium, formed from thin flattened plates borne on the basalmost segments of the legs Other...
, which are united by the presence of a special brood pouch
Brood pouch (Peracarida)
The marsupium or brood pouch, is a characteristic feature of Peracarida, including the orders Amphipoda, Isopoda and Cumacea. It is an egg chamber formed by oostegites, which are appendices which are attached to the coxae of the first pereiopods...
for brooding eggs. There are around 10,215 described species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of isopod, classified into eleven suborders.
Development
Isopod larvaLarva
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 hatch as mancae, which resemble adults except for the lack of the last pair of pereiopods (thoracic legs). The lack of a swimming phase in the life cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
is a limiting factor
Limiting factor
A limiting factor or limiting resource is a factor that controls a process, such as organism growth or species population, size, or distribution. The availability of food, predation pressure, or availability of shelter are examples of factors that could be limiting for an organism...
in isopod dispersal
Biological dispersal
Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...
, and may be responsible for the high levels of endemism in the order. As adults, isopods differ from other crustaceans in that they replace their exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
(in the process called ecdysis
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
) in two phases; this is known as "biphasic moulting".