Sea slug

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The nudibranch Nembrotha aurea is a gastropod. Nembrotha aurea B.jpg
The nudibranch Nembrotha aurea is a gastropod.
A sea cucumber also looks slug-like and is sometimes loosely called a "sea slug", but it is not a gastropod. Fish3923 - Flickr - NOAA Photo Library.jpg
A sea cucumber also looks slug-like and is sometimes loosely called a "sea slug", but it is not a gastropod.

Sea slug is a common name for some marine invertebrates with varying levels of resemblance to terrestrial slugs. Most creatures known as sea slugs are gastropods, i.e. they are sea snails (marine gastropod mollusks) that, over evolutionary time, have either entirely lost their shells or have seemingly lost their shells due to having a significantly reduced or internal shell. [1] The name "sea slug" is often applied to nudibranchs and a paraphyletic set of other marine gastropods without apparent shells. [2]

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Sea slugs have an enormous variation in body shape, color, and size. Most are partially translucent. The often bright colors of reef-dwelling species imply that these animals are under constant threat of predators. Still, the color can warn other animals of the sea slug's toxic stinging cells (nematocysts) or offensive taste. Like all gastropods, they have small, razor-sharp teeth called radulas. Most sea slugs have a pair of rhinophores—sensory tentacles used primarily for the sense of smell—on their head, with a small eye at the base of each rhinophore. Many have feathery structures (cerata) on the back, often in a contrasting color, which act as gills. All species of genuine sea slugs have a selected prey animal on which they depend for food, including certain jellyfish, bryozoans, sea anemones, plankton, and other species of sea slugs. [3] [4]

Sea slugs have brains. For example, Aplysia californica has a brain of about 20,000 nerve cells. [5]

Shell-less marine gastropods

The name "sea slug" is often applied to numerous different evolutionary lineages of marine gastropod molluscs or sea snails, specifically those gastropods that are either not conchiferous (shell-bearing) or appear not to be. [6] In evolutionary terms, losing the shell altogether, having a small internal shell, or having a shell so small that the soft parts of the animal cannot retract into it, are all features that have evolved many times independently within the class Gastropoda, on land and in the sea; these features often cause a gastropod to be labeled with the common name "slug".

The nudibranch Glossodoris atromarginata Glossodoris atromarginata.jpg
The nudibranch Glossodoris atromarginata

Nudibranchs (clade Nudibranchia ) are a large group of marine gastropods that have no shell at all. These may be the most familiar sort of sea slug. Although most nudibranchs are not large, they are often very eye-catching because so many species have brilliant coloration. In addition to nudibranchs, a number of other taxa of marine gastropods (some easily mistaken for nudibranchs) are also often called "sea slugs". [7]

Gastropod groups

The sacoglossan Elysia crispata Lettuce Sea Slug 11-03-2006.jpg
The sacoglossan Elysia crispata
The cephalaspidean Chelidonura varians Chelidonura varians.jpg
The cephalaspidean Chelidonura varians
The sea hare Aplysia dactylomela, photographed out of water Aplysia dactylomela 2.jpg
The sea hare Aplysia dactylomela , photographed out of water

Within the various groups of gastropods that are called "sea slugs", numerous families are within the informal taxonomic group Opisthobranchia . The term "sea slug" is perhaps most often applied to nudibranchs, many of which are brightly patterned and conspicuously ornate. The name "sea slug" is also often applied to the sacoglossans (clade Sacoglossa ), the so-called sap-sucking or solar-powered sea slugs which are frequently a shade of green.

Another group of main gastropods that are often labeled as "sea slugs" are the various families of headshield slugs and bubble snails within the clade Cephalaspidea . The sea hares, clade Aplysiomorpha , have a small, flat, proteinaceous internal shell. The clades Thecosomata and Gymnosomata are small pelagic gastropods known as "sea butterflies" and "sea angels". Many species of sea butterflies retain their shells. These are commonly known as "pteropods" but are also sometimes called sea slugs; especially the Gymnosomata, which have no shell as adults.

The marine pulmonate Onchidella celtica Onchidella celtica 0377.jpg
The marine pulmonate Onchidella celtica

There is also one group of "sea slugs" within the informal group Pulmonata . One very unusual group of marine gastropods that are shell-less are the pulmonate (air-breathing) species in the family Onchidiidae, within the clade Systellommatophora . [8]

Diversity in sea slugs

Like many nudibranchs, Glaucus atlanticus can store and use stinging cells, or nematocysts, from its prey (Portuguese man o' war) in its finger-like cerata. [9] Other species, like the Pyjama slug Chromodoris quadricolor , may use their striking colors to advertise their foul chemical taste.

The lettuce sea slug ( Elysia crispata ) has lettuce-like ruffles that line its body. This slug, like other Sacoglossa, uses kleptoplasty, a process in which the slug absorbs chloroplasts from the algae it eats, and uses "stolen" cells to photosynthesize sugars. The ruffles of the lettuce sea slug increase the slug's surface area, allowing the cells to absorb more light.

Headshield slugs, like the Chelidonura varians , use their shovel-shaped heads to dig into the sand, where they spend most of their time. The shield also protects sand from entering the mantle during burrowing.

Peronia indica is a species of air-breathing sea slug, a shell-less marine pulmonate gastropod mollusk in the family Onchidiidae. [10]

The largest species of sea hare, the California black sea hare, Aplysia vaccaria can reach a length of 75 centimetres (30 in) and a weight of 14 kilograms (31 lb). [11] Most sea hares have several defenses; in addition to being naturally toxic, they can eject a foul ink or secrete a viscous slime to deter predators.

Some species of acochlidian sea slugs have made evolutionary transitions to living in freshwater streams [12] and there is at least one evolutionary transition to land. [13]

Related Research Articles

<span class="mw-page-title-main">Nudibranch</span> Order of gastropods

Nudibranchs belong to the order Nudibranchia, a group of soft-bodied marine gastropod molluscs that shed their shells after their larval stage. They are noted for their often extraordinary colours and striking forms, and they have been given colourful nicknames to match, such as "clown", "marigold", "splendid", "dancer", "dragon", and "sea rabbit". Currently, about 3,000 valid species of nudibranchs are known.

<span class="mw-page-title-main">Sea angel</span> Clade of gastropods

Sea angels are a large group of small free-swimming sea slugs, classified into six separate families. They are pelagic opisthobranchs in the clade Gymnosomata within the larger mollusc clade Heterobranchia. Sea angels were previously considered to be pteropods.

<span class="mw-page-title-main">Aplysiida</span> Clade of gastropods

The order Aplysiida, commonly known as sea hares, are medium-sized to very large opisthobranch gastropod molluscs with a soft internal shell made of protein. These are marine gastropod molluscs in the superfamilies Aplysioidea and Akeroidea.

<span class="mw-page-title-main">Opisthobranchia</span> Informal group of gastropods

Opisthobranchs is a now informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping.

<span class="mw-page-title-main">Aplysiidae</span> Family of gastropods

Aplysiidae is the only family in the superfamily Aplysioidea, within the clade Anaspidea. These animals are commonly called sea hares because, unlike most sea slugs, they are often quite large, and when they are underwater, their rounded body shape and the long rhinophores on their heads mean that their overall shape resembles that of a sitting rabbit or hare. Sea hares are however sea snails with shells reduced to a small plate hidden between the parapodia, and some species are extremely large. The Californian black sea hare, Aplysia vaccaria is arguably the largest living gastropod species, and is certainly the largest living heterobranch gastropod.

<span class="mw-page-title-main">Cephalaspidea</span> Order of gastropods

The order Cephalaspidea, also known as the headshield slugs and bubble snails, is a major taxon of sea slugs and bubble snails, marine gastropod mollusks within the larger clade Euopisthobranchia. Bubble shells is another common name for these families of marine gastropods, some of which have thin bubble-like shells. This clade contains more than 600 species.

<span class="mw-page-title-main">Sacoglossa</span> Clade of gastropods

Sacoglossa are a superorder of small sea slugs and sea snails, marine gastropod mollusks that belong to the clade Heterobranchia known as sacoglossans. There are 284 valid species recognized within this superorder. Sacoglossans live by ingesting the cellular contents of algae, hence they are sometimes called "sap-sucking sea slugs". 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 chloroplasts from the algae they eat, a very unusual phenomenon known as kleptoplasty, for the "stolen" plastids. This earns them the title of the "solar-powered sea slugs", and makes them unique among metazoan organisms, for otherwise kleptoplasty is known only among other euthyneurans and single-celled protists.

<span class="mw-page-title-main">Sea snail</span> Common name for snails that normally live in saltwater

Sea snails are slow-moving marine gastropod molluscs, usually with visible external shells, such as whelk or abalone. They share the taxonomic class Gastropoda with slugs, which are distinguished from snails primarily by the absence of a visible shell.

<i>Elysia crispata</i> Species of gastropod

Elysia crispata, common name the lettuce sea slug or lettuce slug, is a large and colorful species of sea slug, a marine gastropod mollusk.

<span class="mw-page-title-main">Cerata</span> Anatomical structures found in nudibranch sea slugs

Cerata, singular ceras, are anatomical structures found externally in nudibranch sea slugs, especially in aeolid nudibranchs, marine opisthobranch gastropod mollusks in the clade Aeolidida. The word ceras comes from the Greek word "κέρας", meaning "horn", a reference to the shape of these structures.

<span class="mw-page-title-main">Rhinophore</span> Anatomy of groups of marine gastropods

A rhinophore is one of a pair of chemosensory club-shaped, rod-shaped or ear-like structures which are the most prominent part of the external head anatomy in sea slugs, marine gastropod opisthobranch mollusks such as the nudibranchs, sea hares (Aplysiomorpha), and sap-sucking sea slugs (Sacoglossa).

<span class="mw-page-title-main">Diverticulum (mollusc anatomy)</span>

As applied to mollusks, the Neo-Latin term diverticulum is an anatomical feature. The term is most often encountered in the plural form as "diverticula", "hepatic diverticula", or "digestive diverticula", which are anatomical terms for organs which are visible from the outside of the body in a clade of sea slugs known as aeolid nudibranchs, marine opisthobranch gastropod molluscs.

<i>Elysia pusilla</i> Species of gastropod

Elysia pusilla is a species of small sea slug, a marine gastropod mollusk in the family Plakobranchidae. It is a sacoglossan.

<span class="mw-page-title-main">Acochlidiacea</span> Order of molluscs

Acochlidiacea, common name acochlidians, are a taxonomic clade of very unusual sea snails and sea and freshwater slugs, aquatic gastropod mollusks within the large clade Heterobranchia. Acochlidia is a variant spelling.

Strubellia paradoxa is a species of freshwater slug, a shell-less freshwater gastropod, an aquatic gastropod mollusk within the clade Acochlidiacea.

<i>Elysia diomedea</i> Species of gastropod

Elysia diomedea is a species of sea slug found throughout the Pacific coast of Central America.

<i>Elysia trisinuata</i> Species of gastropod

Elysia trisinuata is a species of sea slug, a marine gastropod mollusc in the family Plakobranchidae. This sea slug resembles a nudibranch but is not closely related to that order of gastropods, instead belonging to another clade, Sacoglossa, the "sap-sucking" sea slugs.

Elysia serca, the seagrass elysia or Caribbean seagrass elysia, is a species of sea slug, a marine gastropod mollusc in the family Plakobranchidae. Although this sea slug resembles a nudibranch, it is not a nudibranch; it belongs to the clade, Sacoglossa, the "sap-sucking" sea slugs. It was first described by Marcus in 1955 from specimens found in Brazil.

<i>Coryphella verrucosa</i> Species of gastropod

Coryphella verrucosa, is a species of sea slug, an aeolid nudibranch, a marine gastropod mollusc in the family Coryphellidae.

<i>Costasiella ocellifera</i> Species of gastropod

Costasiella ocellifera is a small (5–13 mm) species of sea slug, a shell-less marine gastropod mollusk in the family Costasiellidae. Costasiella ocellifera, and other members of the Costasiellidae family are often mistakenly classified as nudibranchs because they superficially resemble other species of that group, but they are actually a part of the Sacoglossa superorder of sea slugs, also known as the “sap-sucking sea slugs,” "crawling leaves" or the "solar-powered sea slugs." C. ocellifera was discovered by Simroth in 1895, and was initially classified as Doto ocellifera. The Brazilian species, Costasiella liliana, is a synonym of C. ocellifera.Costasiella ocellifera shows long-term retention of functional kleptoplasty.

References

  1. ↑ Yonow, Nathalie (2015), Rasul, Najeeb M.A.; Stewart, Ian C.F. (eds.), "Sea Slugs: Unexpected Biodiversity and Distribution", The Red Sea: The Formation, Morphology, Oceanography and Environment of a Young Ocean Basin, Springer Earth System Sciences, Berlin, Heidelberg: Springer, pp. 531–550, doi:10.1007/978-3-662-45201-1_30, ISBN   978-3-662-45201-1 , retrieved 2024-02-09
  2. ↑ Thompson, T. E. 1976. Biology of opisthobranch molluscs, vol. 1, 207 pp., 21 pls. Ray Society, no. 151.
  3. ↑ Byatt, Andrew; Fothergill, Alastair; Holmes, Martha (2002). The Blue Planet: A Natural History of the Oceans. BBC. DK. ISBN   978-0-7894-8265-5.
  4. ↑ "Sea Slug". encyclopedia.com. Oxford University Press. 18 May 2018.
  5. ↑ Choi, Charles Q. (29 December 2006). "Human, Sea Slug Brains Share Genes for Alzheimer's and Parkinson's". Scientific American.
  6. ↑ Schmitt, V.; Anthes, N.; Michiels, N.K. (July 2007). "Mating behaviour in the sea slug Elysia timida (Opisthobranchia, Sacoglossa): hypodermic injection, sperm transfer and balanced reciprocity". Frontiers in Zoology. 4 (17). BioMed Central Ltd: 17. doi: 10.1186/1742-9994-4-17 . PMC   1934903 . PMID   17610714.
  7. ↑ Thompson, T. E., & Brown, G.H., 1984. Biology of opisthobranch Molluscs. Vol. 2. Ray Society; London. 1-229, p.49
  8. ↑ "How sea slugs fall in love". freshscience.org. 2007. Retrieved 2 May 2014.
  9. ↑ Thompson, T. E.; Bennett, I. (1969-12-19). "Physalia Nematocysts: Utilized by Mollusks for Defense". Science. 166 (3912): 1532–1533. Bibcode:1969Sci...166.1532T. doi:10.1126/science.166.3912.1532. ISSN   0036-8075. PMID   17742854.
  10. ↑ MolluscaBase (2019). MolluscaBase. Peronia indica (Labbé, 1934). Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=1033079 on 2019-10-05
  11. ↑ "WoRMS - World Register of Marine Species - Aplysia californica J. G. Cooper, 1863". www.marinespecies.org. Retrieved 2024-02-09.
  12. ↑ Schrödl, Michael; Neusser, Timea P. (2010-01-01). "Towards a phylogeny and evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia)". Zoological Journal of the Linnean Society. 158 (1): 124–154. doi: 10.1111/j.1096-3642.2009.00544.x . ISSN   0024-4082.
  13. ↑ Kano, Yasunori; Neusser, Timea P.; Fukumori, Hiroaki; Jörger, Katharina M.; Schrödl, Michael (2015-10-01). "Sea-slug invasion of the land". Biological Journal of the Linnean Society. 116 (2): 253–259. doi: 10.1111/bij.12578 . ISSN   0024-4066.
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