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 actually gastropods, i.e. they are sea snails (marine gastropod mollusks) that over evolutionary time have either completely lost their shells, or have seemingly lost their shells due to having a greatly reduced or internal shell. The name "sea slug" is most often applied to nudibranchs, as well as to a paraphyletic set of other marine gastropods without obvious shells. [1]

In biology, a common name of a taxon or organism; also known as a vernacular name, English name, colloquial name, trivial name, trivial epithet, country name, popular name, or farmer's name; is a name that is based on the normal language of everyday life; this kind of name is often contrasted with the scientific name for the same organism, which is Latinized. A common name is sometimes frequently used, but that is by no means always the case.

Marine biology The scientific study of organisms that live in the ocean

Marine biology is the scientific study of marine life, organisms in the sea. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather than on taxonomy.

Slug common name for an apparently shell-less terrestrial gastropod mollusc

Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word slug is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a small internal shell, particularly sea slugs and semislugs.


True sea slugs have enormous variation in body shape, color, and size. Most are partially translucent. The often bright colors of reef-dwelling species implies that these are under constant threat of predators, but the color can serve as a warning to other animals of the sea slug's toxic stinging cells or offensive taste. Like all gastropods, they have small razor-sharp teeth, called radulas. Most sea slugs have two pairs of tentacles on their head used primarily for sense of smell, with a small eye at the base of each tentacle. 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 specialize for food, including certain jellyfish, bryozoans, sea anemones, and plankton as well as other species of sea slugs. [2] [3]

The radula is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the oesophagus. The radula is unique to the molluscs, and is found in every class of mollusc except the bivalves, who use instead cilia, waving filaments that bring minute organisms to the mouth.

Cerata anatomical structures found in nudibranch sea slugs

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

Plankton Organisms that live in the water column and are incapable of swimming against a current

Plankton are the diverse collection of organisms that live in large bodies of water and are unable to swim against a current. The individual organisms constituting plankton are called plankters. They provide a crucial source of food to many small and large aquatic organisms, such as bivalves, fish and whales.

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. [4] 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 end up labeled with the common name "slug".

Sea snail common name for snails that normally live in saltwater

Sea snail is a common name for 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.

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

Nudibranchs (clade Nudibranchia) are a large group of marine gastropods which have no shell at all. These may be the most familiar sort of sea slug, at least to scuba divers; nudibranchs, although most are not large, are often very eye-catching because so many of 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". [5]

Nudibranch order of molluscs

Nudibranchs are a group of soft-bodied, marine gastropod molluscs which 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," or "sea rabbit." Currently, about 3,000 valid species of nudibranchs are known.

Gastropod groups

Within the various groups of gastropods that are called "sea slugs" numerous families are within the informal taxonomic group Opisthobranchia :

Opisthobranchia infraclass of molluscs

Opisthobranchs is now an 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.

The phrase "sea slug" is perhaps most often applied to nudibranchs, many of which are brightly patterned and conspicuously ornate.
For two examples see the images of Nembrotha aurea and Glossodoris atromarginata within this article.
The name "sea slug" is also often applied to the sacoglossans (clade Sacoglossa ), the so-called sap-sucking or solar-powered sea slugs; these are frequently some shade of green.
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
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 hare Aplysia dactylomela, photographed out of water Aplysia dactylomela 2.jpg
The sea hare Aplysia dactylomela , photographed out of water
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.

There is also one group of "sea slugs" within the informal group Pulmonata :

Pulmonata order of molluscs

Pulmonata, or "pulmonates", is an informal group of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills. The group includes many land and freshwater families, and several marine families.

The marine pulmonate Onchidella celtica Onchidella celtica 0377.jpg
The marine pulmonate Onchidella celtica
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. [6]

Diversity in sea slugs

Like many nudibranchs, Glaucus atlanticus can store and use stinging cells from its prey (Portuguese man o' war) in its finger-like cerata. 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.

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). 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.

Related Research Articles

Sea angel order of molluscs

Sea angels are a large group of extremely small, swimming sea slugs, not to be confused with Cnidarians, classified into six different families. They are pelagic opisthobranchs in the clade Gymnosomata within the larger mollusc clade Heterobranchia. Sea angels were previously referred to as a type of pteropod.

Cephalaspidea suborder of molluscs

The clade 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.

Sacoglossa order of molluscs

Sacoglossa, commonly known as the sacoglossans or the "sap-sucking sea slugs", are a clade of small sea slugs and sea snails, marine gastropod mollusks that belong to the clade Heterobranchia. Sacoglossans live by ingesting the cellular contents of algae, hence the adjective "sap-sucking".

<i>Elysia chlorotica</i> species of mollusc

Elysia chlorotica is a small-to-medium-sized species of green sea slug, a marine opisthobranch gastropod mollusc. This sea slug superficially resembles a nudibranch, yet it does not belong to that clade of gastropods. Instead it is a member of the clade Sacoglossa, the sap-sucking sea slugs. Some members of this group use chloroplasts from the algae they eat for photosynthesis, a phenomenon known as kleptoplasty. Elysia chlorotica is one of these "solar-powered sea slugs". It lives in a subcellular endosymbiotic relationship with chloroplasts of the marine heterokont alga Vaucheria litorea.

<i>Elysia viridis</i> species of mollusc

Elysia viridis, the sap-sucking slug, is a small-to-medium-sized species of green sea slug, a marine opisthobranch gastropod mollusc in the family Plakobranchidae.

Metarminoidea superfamily of molluscs

Metarminoidea is a provisional taxonomic superfamily of colourful sea slugs, aeolid nudibranchs, marine opisthobranch gastropod molluscs in the clade Nudibranchia.

<i>Elysia crispata</i> species of mollusc

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


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 (Nudibranchia), Sea Hares, (Aplysiomorpha) and sap-sucking sea slugs (Sacoglossa).

Diverticulum (mollusc)

As applied to mollusks, the New 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 catulus</i> species of mollusc

Elysia catulus is a small species of sea slug, a marine gastropod mollusc in the family Plakobranchidae. This sea slug resembles a nudibranch, but it is not closely related to that order of gastropods, instead it is a sacoglossan. The specific name "catulus" comes from the Greek and means "little cat", referring to the superficial resemblance that the head of this slug bears to the head of a cat.

<i>Elysia diomedea</i> species of mollusc

Elysia diomedea is a species of sea slug, a marine gastropod mollusc in the family Plakobranchidae.

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 rufescens is a species of sea slug, a marine gastropod mollusc in the family Plakobranchidae. This sea slug resembles a nudibranch but is not classified in that order of gastropods, instead belonging to a closely related clade, Sacoglossa, the "sap-sucking" sea slugs. This species was first described by Pease from Tahiti in 1871.

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.

Elysia bangtawaensis 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 but is part of a rather different clade, the sacoglossans, also known as the "sap-sucking" sea slugs.


A cnidosac is an anatomical feature that is found in the group of sea slugs known as aeolid nudibranchs, a clade of marine opisthobranch gastropod molluscs. A cnidosac contains cnidocytes, stinging cells that are also known as cnidoblasts or nematocysts. These stinging cells are not made by the nudibranch, but by the species that it feeds upon. However, once the nudibranch is armed with these stinging cells, they are used in its own defense.


  1. Thompson, T. E. 1976. Biology of opisthobranch molluscs, vol. 1, 207 pp., 21 pls. Ray Society, no. 151.
  2. Byatt, Andrew, Alastair Fothergill, and Martha Holmes. The Blue Planet: A Natural History of the Oceans. New York, NY: DK, 2001. Print. sea slugs
  3. "sea slug". The Columbia Encyclopedia, 6th ed.. 2015. 10 Nov. 2015<>.
  4. "Mating behaviour in the sea slug Elysia timida". BioMed Central Ltd. Retrieved 2014-05-02.
  5. Thompson, T. E., & Brown, G.H., 1984. Biology of opisthobranch Molluscs. Vol. 2. Ray Society; London. 1-229, p.49
  6. "How sea slugs fall in love" . Retrieved 2014-05-02.