The lettuce sea slug (Elysia crispata) is a large (5cm) and colorful species of sea slug, a marine gastropod mollusk. The lettuce slug resembles a nudibranch, but it is not closely related to that clade of gastropods; it is classified as a sacoglossan, or sap-sucking sea slug.
Sacoglossans live by ingesting the cellular contents of algae, hence the adjective “sap-sucking.”
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