Nudibranch
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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.
Image: Berghia coerulescens (Laurillard, 1830)
Image: Chromodoris lochi (AA3)
Glaucus atlanticus is an example of a nudibranch that has its cerata positioned like wings instead of on its back.
Nudibranchs (Nembrotha kubaryana) eating Clavelina tunicate colonies
Sea slug
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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 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. The name "sea slug" is often applied to nudibranchs and a paraphyletic set of other marine gastropods without apparent shells.
The nudibranch Nembrotha aurea is a gastropod.
A sea cucumber also looks slug-like and is sometimes loosely called a "sea slug", but it is not a gastropod.
The nudibranch Glossodoris atromarginata
The sacoglossan Elysia crispata