Sacoglossa
Sacoglossa Temporal range:
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Placobranchidae
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Oxynoe viridis, a shelled sacoglossan in the family Oxynoidae | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Mollusca |
Class: | Gastropoda |
Subclass: | Heterobranchia |
Infraclass: | Euthyneura |
Superorder: | Sacoglossa H. von Ihering, 1876[2] |
Diversity[3] | |
284 species | |
Synonyms | |
Ascoglossa Bergh, 1876 |
Sacoglossa, commonly known as the sacoglossans or the "solar-powered sea slugs", are a superorder of small
Some sacoglossans simply digest the fluid which they suck from the algae, but in some other species, the slugs sequester and use within their own tissues living
The Sacoglossa are divided into two clades - the shelled families (Oxynoacea) and the shell-less families (Plakobranchacea).[7] The four families of shelled species are
Kathe R. Jensen (2007)[3] recognized 284 valid species within the Sacoglossa.
Appearance
Many of these gastropods (e.g. Elysia spp.) resemble winged slugs with a pair of cephalic tentacles. In photosynthetic members of the group, the wings, or
Distribution
Sacoglossa species are found worldwide in tropical and
Use of ingested cellular material
The sacoglossans can use 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;
Sacoglossans can also use antiherbivory compounds produced by their algal foodstuffs to deter their own would-be predators, in a process termed kleptochemistry.[10] This may be achieved by converting algal metabolites to toxins,[15] or by using algal pigments for camouflage in a process termed nutritional homochromy.[9][16]
Oxynoacea
Around 20% of sacoglossan species bear a shell. The Oxynoacea contains three shelled families, and all feed solely on algae of the genus Caulerpa.[1] None of these organisms benefits from the photosynthesis of the ingested chloroplasts, but the chloroplasts may have been retained to perform a camouflaging function.[9] 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 have been known since 1959,[citation needed] and had previously only been known to science as fossils (which had been interpreted as bivalves).[citation needed]
Plakobranchoidea
The majority of sacoglossans are shell-less, consequently, the Plakobranchoidea are commonly described using the vernacular term "sea slugs", which can lead to their confusion with the only very distantly related nudibranchs. However, the plakobranchoid Elysia (and undoubtedly others) do develop a shell before hatching from their egg.[17] Indeed, at least the Elysiidae, Limapontiidae, and Hermaeidae all bear larval shells, which are spiral, and possess between three-quarters and one complete whorl.[18]
The plakobranchoids have a more diverse feeding range than the Oxynoacea, feeding on a wider range of green (and sometimes red)[9] algae, and even, in three cases, being carnivorous.[1]
Evolution
The ancestor of the Sacoglossa is presumed to have fed on a now-extinct calcifying green alga in the Udoteaceae.[1] The first fossil evidence of the group comes from bivalved shells dating to the Eocene, and further bivalved shells are known from later geological periods, although the thin nature of the shells and their high-erosion habitat usually make for poor preservation.[1] The corresponding fossil record of algae points to an origin of the group deeper in time, perhaps as early as the Jurassic or Cretaceous.[1]
The loss of the shell, which was 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 used as functioning chloroplasts, which could generate energy by photosynthesis.[1]
Taxonomy
2004 taxonomy
This taxonomy follows Marin 2004.[19]
- Cylindrobulloidea
- Cylindrobullidae
- Oxynoacea(Shelled sacoglossans)
- Plakobranchacea (Non-shelled sacoglossans)
- Superfamily Plakobranchoidea (= Plakobranchacea; = Elysioidea)
- Plakobranchidae (=Elysiidae)
- Boselliidae
- Platyhedylidae
- Superfamily Limapontioidea (= Polybranchioidea; = Stiligeroidea)
- Limapontiidae (Stillergeridae)
- Polybranchiidae (= Caliphyllidae)
- Hermaeidae
2005 taxonomy
In the taxonomy of Bouchet & Rocroi (2005),[20] the clade Sacoglossa is arranged as follows:
- Subclade Oxynoacea
- Superfamily Oxynooidea: family Oxynoidae, family Juliidae, family Volvatellidae
- Subclade Plakobranchacea
- Superfamily Plakobranchoidea: family Boselliidae, family Platyhedylidae,
- Superfamily Limapontioidea: family Caliphyllidae, family Hermaeidae
- Superfamily Plakobranchoidea: family
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, 1926and Stiligeridae Iredale & O'Donoghue, 1923 are synonyms of the family Limapontiidae Gray, 1847.
The family
2010 taxonomy
Jörger et al. (2010)[22] moved Sacoglossa into the Panpulmonata.
A
2017 taxonomy
Bouchet et al. (2017) moved Sacoglossa from Panpulmonata to the subterclass Tectipleura.[24][25]
Autotomy
Extreme autotomy has been observed on two species, Elysia marginata and E. atroviridis, studied in vitro.[26][27] Over the course of the study, some individuals decapitated themselves, a behavior known as autotomy. The neck wound usually closed within one day, and the heads, especially in younger specimens, began to feed on algae within hours. Twenty days later, an entirely new body had regrown, while the discarded bodies never regrew heads. In E. atroviridis, three of 82 studied individuals autotomized, and two of the three eventually grew new bodies. All of these animals were infected with small crustaceans known as copepods. In another group of 64 without parasites, none self-decapitated, leading the researchers to hypothesize that animals cast off their bodies as a means to get rid of parasites. Another possibility is that the slugs autotomized to escape predators, but when the researchers tried to mimic an enemy's attack by pinching and cutting the creatures, none cast off their bodies. The process itself takes several hours, which the scientists say would make it ineffective as means of escape.
How the slugs survive without a heart and other vital organs for nearly a month remains a mystery. Mitoh and her colleagues suspect it may be tied to their ability to survive using the photosynthetic algae in their diet while other energy sources are unavailable.
References
- ^ S2CID 30138345.
- ^ Ihering H. v. (1876). "Versuch eines natürlichen Systemes der Mollusken". Jahrbücher der Deutschen Malakozoologischen Gesellschaft 3: 97-148. Sacoglossa is on the page 146.
- ^ Bonner zoologische Beiträge55(2006)(3-4): 255–281.
- ^ Anonymous. (Last changed: December 2008). "Sap-sucking slugs". Accessed 12 January 2010.
- PMID 24767983.
- PMID 19951407.
- PMID 19951407.
- ISBN 0-03-056747-5.
- ^ ISBN 978-1-4020-4060-3.
- ^ .
- PMID 19004808.
- PMID 20829345.
- PMID 23645554.
- PMID 21799783.
- .
- ^ Clark, K. B.; Jensen, K. R.; Stirts, H. M. (2009). "Survey for Functional Kleptoplasty among West Atlantic Ascoglossa (= Sacoglossa) (Mollusca, Opisthobranchia)". Veliger. 33 (4): 339–345.
- ^ Thompson, T. E.; Salghetti-Drioli, U. (1984). "Unusual features of the development of the sacoglossan Elysia hopei in the Mediterranean Sea". J. Molluscan Stud. 50 (1): 61–63. Archived from the original on 2005-07-10.
- .
- .
- ISSN 0076-2997.
- ^ Discussion in the Seaslug Forum : Ascobulla, Cylindrobulla[permanent dead link].
- PMID 20973994.
- ^ S2CID 27608931.
- ISSN 0076-2997.
- ^ Bouchet P, Gofas S (2018-02-02). Bieler R, Bouchet P, Gofas S, Marshall B, Rosenberg G, La Perna R, Neubauer TA, Sartori AF, Schneider S, Vos C, ter Poorten JJ, Taylor J, Dijkstra H, Finn J, Bank R, Neubert E, Moretzsohn F, Faber M, Houart R, Picton B, Garcia-Alvarez O (eds.). "Sacoglossa". MolluscaBase. World Register of Marine Species. Retrieved 2018-05-15.
- S2CID 232145105.
- ^ Shultz, David (8 March 2021). "This sea slug cut off its own head—and lived to tell the tale". Sciencemag. Retrieved 22 April 2021.
Further reading
- Marcus E. d. B.-R. (1982). "Systematics of the genera of the order Ascoglossa (Gastropoda)". Journal of Molluscan Studies. 48 (supp10): 1–31. doi:10.1093/oxfordjournals.mollus.a065666. Archived from the originalon 2013-04-15.