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]
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]
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".
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]
Within the various groups of gastropods that are called "sea slugs", numerous families are within the informal taxonomic group Opisthobranchia :
There is also one group of "sea slugs" within the informal group Pulmonata :
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]
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.
Sea angels are a large group of small free-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.
The clade Anaspidea, 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.
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.
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.
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.
Sacoglossa, commonly known as the sacoglossans or the "solar-powered sea slugs", are a superorder 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 they are sometimes called "sap-sucking sea slugs".
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.
Onchidiidae are a family of small, air-breathing sea slugs. They are shell-less marine pulmonate gastropod molluscs. Onchidiidae is the only family within the superfamily Onchidioidea.
Elysia crispata, common name the lettuce sea slug or lettuce 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, sea hares (Aplysiomorpha), and sap-sucking sea slugs (Sacoglossa).
Elysia ornata, commonly known as ornate elysia or ornate leaf slug, is a species of sea slug, a marine gastropod mollusk. This sea slug superficially resembles a nudibranch, yet it does not belong to that suborder of gastropods. Instead it is a member of the closely related clade Sacoglossa, the "sap-sucking" sea slugs.
Elysia pusilla is a species of small sea slug, a marine gastropod mollusk in the family Plakobranchidae. It is a sacoglossan.
Aplysia fasciata, common name the "mottled sea hare", or the "sooty sea hare", is an Atlantic species of sea hare or sea slug, a marine opisthobranch gastropod mollusk in the family Aplysiidae.
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.
Pteraeolidia ianthina is a sea slug, an aeolid nudibranch in the family Facelinidae. It is known as a blue dragon, a name it shares with Glaucus atlanticus and Glaucus marginatus.
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.
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.