Nudibranchs /ˈnjuːdᵻbræŋk/ are a group of soft-bodied, marine gastropod molluscs which shed their shells after their larval stage. They are noted for their often extraordinary colours and striking forms. Currently, about 2,300 valid species of nudibranchs are known.
The word “nudibranch” comes from the Latin nudus, naked, and the Greek βραγχια, brankhia, gills.
Nudibranchs are often casually called sea slugs, but many sea slugs belong to several taxonomic groups which are not closely related to nudibranchs. A number of these other sea slugs, such as the photosynthetic Sacoglossa and the colourful Aglajidae, are often confused with nudibranchs.