Sacoglossa
Encyclopedia
Sacoglossa, commonly known as the sacoglossans or the "sap-sucking sea slugs", are a clade
of small sea slug
s and sea snail
s, marine
gastropod mollusks that belong to the clade Heterobranchia
. Sacoglossans live by ingesting the internal contents of algae
, hence the adjective "sap-sucking".
Some sacoglossans simply digest the sap which they suck from the algae, but in some other species the slugs sequester and utilize within their own tissues, living chloroplasts from the algae they eat, a very unusual phenomenon known as kleptoplasty
. This earns them the title of the "solar-powered sea slugs", and makes them unique among animals.
Sacoglossa can be divided into shelled (Oxynoacea) families and non-shelled (Plakobranchacea) families.
There are however four families of shelled species: Cylindrobullidae
, Volvatellidae
, Oxynoidae
and Juliidae
. Saccoglossans are distinguished from related groups by the presence of a single row of teeth on the radula
. The teeth are adapted for the suctorial
feeding habits of the group.
Kathe R. Jensen (2007) recognized 284 valid species within the Sacoglossa.
, where they frequent the shorelines of tropical islands; diverse tracts of species are also known in the Caribbean and Indomalaya. These three provinces have distinct ranges of species, indicating a high degree of biogeographic separation. Where saccoglossans are present further from the equator, in places such as Australia or Japan, diversity is lower, and the species present are typically tropical species that have a higher tolerance for temperature variation. Their temperate distribution closely maps to the distribution of their important food source Caulerpa. They typically live at very low population densities, making scientific study of the group difficult.
However it is achieved, kleptoplasty is an important strategy for many genera of Placobranchacaea. One species of Elysia feeds on a seasonally-calcifying alga. Because it is unable to penetrate the calcified cell walls, the animal can only feed for part of the year, relying on the ingested chloroplasts to survive whilst the foodstuff is calcified, until later in the season when the calcification is lost and the grazing can continue.
Sacoglossans can also utilize anti-herbivory compounds produced by their algal foodstuffs to deter their own would-be predators, in a process termed kleptochemistry. This may be achieved by converting algal metabolites to toxins, or by using algal pigments for camouflage in a process termed nutritional homochromy.
. None of these organisms benefit from the photosynthesis of the ingested chloroplasts, but there is some suggestion that the chloroplasts may have been retained in order to perform a camouflaging function. The shells of the Volvatellidae
and Oxynoidae
somewhat resemble those of the Cephalaspid bubble snails. The Juliidae
are extraordinary in that they are shelled, bivalved, gastropods. They have a shell in two pieces which resemble the valves of a minute clam. Living members of this family has been known since 1959, and had previously only been known to science as fossils (which had been interpreted as bivalves).
s. However, the placobranchoid Elysia
(and undoubtedly others) does develop a shell before hatching from its egg. Indeed, at least the Elysiidae, Limapontiidae and Hermaeidae all bear larval shells, which are spiral and possess between three-quarters and a single whorl.
The placobranchoids have a more diverse feeding range than the Oxynoacea, feeding on a wider range of green (and sometimes red) algae and even, in three cases, being carnivorous.
The loss of a shell, apparently a single evolutionary event, opened up a new ecological avenue for the clade, as the chloroplasts of the green algae on which they fed could now be retained and utilised as functioning chloroplasts to generate energy by photosynthesis.
(2005), the clade
Sacoglossa is arranged as follows:
In this taxonomy the family Elysiidae Forbes & Hanley, 1851 is considered a synonym of the family Placobranchidae Gray, 1840 and the families Oleidae O'Donoghue, 1926 and Stiligeridae Iredale & O'Donoghue, 1923 synonyms of the family Limapontiidae Gray, 1847
The family Cylindrobullidae
belongs to the superfamily Cylindrobulloidea in the sister "group" Cylindrobullida.
.
Molecular phylogeny
analysis by Maeda et al. (2010) have confirmed placement of Cylindrobulla within Sacoglossa.
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of small sea slug
Sea slug
Sea slug is a common name used for several different groups of saltwater snails that either lack a shell or have only an internal shell, in other words this name is used for various lineages of marine gastropod mollusks that are either not conchiferous or appear not to be.The phrase "sea slug" is...
s and sea snail
Sea snail
Sea snail is a common name for those snails that normally live in saltwater, marine gastropod molluscs....
s, marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
gastropod mollusks that belong to the clade Heterobranchia
Heterobranchia
Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....
. Sacoglossans live by ingesting the internal contents of algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, hence the adjective "sap-sucking".
Some sacoglossans simply digest the sap which they suck from the algae, but in some other species the slugs sequester and utilize within their own tissues, living chloroplasts from the algae they eat, a very unusual phenomenon known as kleptoplasty
Kleptoplasty
Kleptoplasty or kleptoplastidy is a symbiotic phenomenon whereby plastids from algae are sequestered by host organisms. The alga is eaten normally and partially digested, leaving the plastid intact. The plastids are maintained within the host, temporarily retaining functional photosynthesis for use...
. This earns them the title of the "solar-powered sea slugs", and makes them unique among animals.
Sacoglossa can be divided into shelled (Oxynoacea) families and non-shelled (Plakobranchacea) families.
There are however four families of shelled species: Cylindrobullidae
Cylindrobullidae
Cylindrobulla is a genus of sea snails or bubble snails, marine gastropod mollusks in the clade Sacoglossa.Cylindrobulla is the type genus of the family Cylindrobullidae and it is the only genus in the family.- Taxonomy :...
, Volvatellidae
Volvatellidae
Volvatellidae is a family of sea snails, marine gastropod mollusks in the superfamily Oxynooidea.This family has no subfamilies.-Genera:There are two genera in the family Volvatellidae:* Ascobulla Marcus, 1972* Volvatella Pease, 1860...
, Oxynoidae
Oxynoidae
Oxynoidae is a family of sea snails, bubble snails, marine gastropod mollusks in the superfamily Oxynooidea, an opisthobranch group.This family has no subfamilies.-Genera:The following three genera or four are included in the family Oxynoidae:...
and Juliidae
Juliidae
Juliidae, common name the bivalved gastropods, is a family of minute sea snails, marine gastropod mollusks or micromollusks in the superfamily Oxynooidea, an opisthobranch group....
. Saccoglossans are distinguished from related groups by the presence of a single row of teeth on the radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...
. The teeth are adapted for the suctorial
Suctorial
Suctorial pertains to the adaptation for sucking or suction, as possessed by marine parasites such as the Cookiecutter shark, specifically in a specialised lip organ enabling attachment to the host....
feeding habits of the group.
Kathe R. Jensen (2007) recognized 284 valid species within the Sacoglossa.
Appearance
Many of the organisms (e.g. Elysia spp.) resemble winged slugs with a pair of cephalic tentacles. In photosynthetic members of the group, the wings, or parapodia, can be unfurled to maximise the area of the organism that is struck by sunlight. Others (e.g. Placida spp.) have cylindrical cerata extending from the dorsal surface. The majority of the organisms are between one and three centimetres in length; they are typically uniform in colour by virtue of adapting ingested chloroplasts into their own cells.Distribution
The species mainly live in the central Pacific OceanPacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
, where they frequent the shorelines of tropical islands; diverse tracts of species are also known in the Caribbean and Indomalaya. These three provinces have distinct ranges of species, indicating a high degree of biogeographic separation. Where saccoglossans are present further from the equator, in places such as Australia or Japan, diversity is lower, and the species present are typically tropical species that have a higher tolerance for temperature variation. Their temperate distribution closely maps to the distribution of their important food source Caulerpa. They typically live at very low population densities, making scientific study of the group difficult.
Use of ingested cellular material
The sacoglossans can utilise the chloroplasts of the algae on which they feed, which they keep alive for hours to months after their ingestion. They maintain the cells and metabolise the photosynthetic products; this process is termed kleptoplasty, and the sacoglossans are the only animals to employ it; some ciliates and foramanifera (protists) also employ the strategy. Sacoglossans have been known to survive for months living solely on the photosynthetic products of their acquired plastids. This process is somewhat mystifying, as the upkeep of chloroplasts usually requires interaction with genes encoded in the plant cell nucleus. This almost seems to suggest that the genes have been laterally transferred from algae to the animals. DNA amplification experiments on E. chloritica adults and eggs using V.litora derived primers revealed the presence of psbO, an algal nuclear gene. These results support the inter-domain horizontal transfer of an algal gene to a mollusk host, its expression in that host, and its insertion into the germline even though the plastids are not transmitted vertically in the sea slugs. Another unclear step in the process is how the chloroplasts are protected from digestion, and how they adapt to their new position in animal cells without the membranes that would control their environment in the algae.However it is achieved, kleptoplasty is an important strategy for many genera of Placobranchacaea. One species of Elysia feeds on a seasonally-calcifying alga. Because it is unable to penetrate the calcified cell walls, the animal can only feed for part of the year, relying on the ingested chloroplasts to survive whilst the foodstuff is calcified, until later in the season when the calcification is lost and the grazing can continue.
Sacoglossans can also utilize anti-herbivory compounds produced by their algal foodstuffs to deter their own would-be predators, in a process termed kleptochemistry. This may be achieved by converting algal metabolites to toxins, or by using algal pigments for camouflage in a process termed nutritional homochromy.
Oxynoacea
Around 20% of sacoglossan species bear a shell. The Oxynoacea contains three shelled families, and all feed solely on algae of the genus CaulerpaCaulerpa
Caulerpa is a genus of seaweeds in the family Caulerpaceae . They are unusual because they consist of only one cell with many nuclei, making them among the biggest single cells in the world...
. None of these organisms benefit from the photosynthesis of the ingested chloroplasts, but there is some suggestion that the chloroplasts may have been retained in order to perform a camouflaging function. The shells of the Volvatellidae
Volvatellidae
Volvatellidae is a family of sea snails, marine gastropod mollusks in the superfamily Oxynooidea.This family has no subfamilies.-Genera:There are two genera in the family Volvatellidae:* Ascobulla Marcus, 1972* Volvatella Pease, 1860...
and Oxynoidae
Oxynoidae
Oxynoidae is a family of sea snails, bubble snails, marine gastropod mollusks in the superfamily Oxynooidea, an opisthobranch group.This family has no subfamilies.-Genera:The following three genera or four are included in the family Oxynoidae:...
somewhat resemble those of the Cephalaspid bubble snails. The Juliidae
Juliidae
Juliidae, common name the bivalved gastropods, is a family of minute sea snails, marine gastropod mollusks or micromollusks in the superfamily Oxynooidea, an opisthobranch group....
are extraordinary in that they are shelled, bivalved, gastropods. They have a shell in two pieces which resemble the valves of a minute clam. Living members of this family has been known since 1959, and had previously only been known to science as fossils (which had been interpreted as bivalves).
Placobranchoidea
No other sacoglossans bear a shell; consequently the placobranchoidea are grouped into the vernacular term "sea slugs", which can lead to their confusion with the unrelated nudibranchNudibranch
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. However, the placobranchoid Elysia
Elysia
Elysia may signify:*Elysia , a deathcore band*Elysia , a genus of gastropods*Elysium, a section of the underworld containing the Elysian Fields*Elysia, pure heart.*Elysia, a gas giant in Metroid Prime 3: Corruption....
(and undoubtedly others) does develop a shell before hatching from its egg. Indeed, at least the Elysiidae, Limapontiidae and Hermaeidae all bear larval shells, which are spiral and possess between three-quarters and a single whorl.
The placobranchoids have a more diverse feeding range than the Oxynoacea, feeding on a wider range of green (and sometimes red) algae and even, in three cases, being carnivorous.
Evolution
The ancestor of the Sacoglossa is presumed to have fed on a now-extinct calcifying green alga in the Odoteaceae. The first fossil evidence of the group comes from bivalved shells dating to the Eocene, and further bivalved shells are known from later, although the thin nature of the shells and their high-erosion habitat make for poor preservation. The corresponding fossil record of algae points to an origin of the group deeper in time, perhaps as early as the Jurassic or Cretaceous.The loss of a shell, apparently a single evolutionary event, opened up a new ecological avenue for the clade, as the chloroplasts of the green algae on which they fed could now be retained and utilised as functioning chloroplasts to generate energy by photosynthesis.
2005 taxonomy
In the taxonomy of Bouchet & RocroiTaxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
The taxonomy of the Gastropoda as it was revised by Philippe Bouchet and Jean-Pierre Rocroi is currently the most up-to-date overall system for classifying gastropod mollusks...
(2005), the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
Sacoglossa is arranged as follows:
- Subclade Oxynoacea
- Superfamily Oxynooidea: family OxynoidaeOxynoidaeOxynoidae is a family of sea snails, bubble snails, marine gastropod mollusks in the superfamily Oxynooidea, an opisthobranch group.This family has no subfamilies.-Genera:The following three genera or four are included in the family Oxynoidae:...
, family JuliidaeJuliidaeJuliidae, common name the bivalved gastropods, is a family of minute sea snails, marine gastropod mollusks or micromollusks in the superfamily Oxynooidea, an opisthobranch group....
, family VolvatellidaeVolvatellidaeVolvatellidae is a family of sea snails, marine gastropod mollusks in the superfamily Oxynooidea.This family has no subfamilies.-Genera:There are two genera in the family Volvatellidae:* Ascobulla Marcus, 1972* Volvatella Pease, 1860...
- Superfamily Oxynooidea: family Oxynoidae
- Subclade Placobranchacea
- Superfamily Placobranchoidea: family Placobranchidae, family BoselliidaeBoselliidaeBosellia is a genus of sea slugs, marine gastropod mollusks within the superfamily Placobranchoidea.Bosellia is the only genus in the family Boselliidae. Such families are monotypic families and this family has no subfamilies....
, family PlatyhedylidaePlatyhedylidaePlatyhedylidae is a family of sea slugs, marine gastropod mollusks.This family has no subfamilies.- Genera :There are two genera in the family Platyhedylidae:* Gascoignella Jensen, 1985* Platyhedyle Salwini-Plawen, 1973 - type genus...
, - Superfamily Limapontioidea: family Limapontiidae, family CaliphyllidaeCaliphyllidaeCaliphyllidae is a taxonomic family of sacoglossan sea slugs. These are marine opisthobranch gastropod mollusks in the clade Sacoglossa.This family has no subfamilies.-Genera:There are five genera in the family Caliphyllidae:* Caliphylla A...
, family HermaeidaeHermaeidaeHermaeidae is a taxonomic family of sacoglossan sea slugs. These are marine opisthobranch gastropod mollusks in the superfamily Limapontioidea.This family has no subfamilies.-Genera:...
- Superfamily Placobranchoidea: family Placobranchidae, family Boselliidae
In this taxonomy the family Elysiidae Forbes & Hanley, 1851 is considered a synonym of the family Placobranchidae Gray, 1840 and the families Oleidae O'Donoghue, 1926 and Stiligeridae Iredale & O'Donoghue, 1923 synonyms of the family Limapontiidae Gray, 1847
The family Cylindrobullidae
Cylindrobullidae
Cylindrobulla is a genus of sea snails or bubble snails, marine gastropod mollusks in the clade Sacoglossa.Cylindrobulla is the type genus of the family Cylindrobullidae and it is the only genus in the family.- Taxonomy :...
belongs to the superfamily Cylindrobulloidea in the sister "group" Cylindrobullida.
2010 taxonomy
Jörger et al. (2010) have moved Sacoglossa to PanpulmonataPanpulmonata
Panpulmonata is a taxonomic clade of snails and slugs in the clade Heterobranchia within the clade Euthyneura.Panpulmonata was established as a new taxon by Jörger et al...
.
Molecular phylogeny
Molecular phylogeny
Molecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree...
analysis by Maeda et al. (2010) have confirmed placement of Cylindrobulla within Sacoglossa.