Sea slugs

Sea slug is a common name for some marine invertebrates that more or less resemble terrestrial slugs. Most creatures known as sea slugs are actually snails, i.e. they are sea snails (marine gastropod mollusks) that over evolutionary time 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.

The phrase "sea slug" is, however, also sometimes applied to taxa in other phyla, such as the sea cucumbers, which are not mollusks but echinoderms. Although the other animals sometimes called "sea slugs" are not gastropods, they are nonetheless soft-bodied, and their overall shape is slug-like.

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 serves as a warning to other animals of the sea slug's poisonous stinging cells. 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 (ceratia) 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.

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