Nudibranch aka Sea Slugs
A nudibranch (pronounced /ˈnjuːdɨbræŋk/) is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, shell-less marine opisthobranch gastropod mollusks, which are noted for their often extraordinary colors and striking forms. The clade Nudibranchia is the largest clade within the heterobranchs, with more than 3,000 described species.
The word "nudibranch" comes from the Latin nudus, naked, and the Greek brankhia, gills.
Nudibranchs are often casually called "sea slugs", a non-scientific term. This has led some people to assume that every sea slug must be a nudibranch. Nudibranchs are very numerous in terms of species, and are often very attractive and noticeable, but there are a wide variety of other kinds of sea slugs, and these belong to several taxonomic groups that are not very closely related to nudibranchs. A fair number of these other sea slugs are colorful, and can be confused with nudibranchs.