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.
True sea slugs have enormous variation in body shape, color, and size.
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. 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".
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".
Within the various groups of gastropods that are called "sea slugs" numerous families are within the informal taxonomic groupOpisthobranchia
There is also one group of "sea slugs" within the informal groupPulmonata
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
The largest species of sea hare, the California black sea hare, Aplysia vaccaria