Seaslug 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 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. 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 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.
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 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 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 are also often called "sea slugs".
Gastropod groups
Within the various groups of gastropods that are called "sea slugs" numerous families are within the informal taxonomic groupOpisthobranchia:
Aplysia dactylomela, photographed out of water 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 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 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 and a weight of 14 kilograms. 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 and there is at least one evolutionary transition to land.
