Elysia catulus is a small species of sea slug, a marine gastropod mollusc in the family Plakobranchidae. This sea slug resembles a nudibranch, but it is not closely related to that order of gastropods, instead it is a sacoglossan. The specific name "catulus" comes from the Greek and means "little cat", referring to the superficial resemblance that the head of this slug bears to the head of a cat.