Elysia is a genus of sea slugs, marine gastropod molluscs in the family Plakobranchidae. These animals are colorful sea slugs, and they can superficially resemble nudibranchs, but are not very closely related to them. Instead they are sacoglossans, commonly known as sap-sucking slugs.

Elysia sea slugs graze on algae and some species such as E. viridis and E. chlorotica hijack the chloroplasts for themselves (i.e. kleptoplasty). The chloroplasts end up lining the slug's digestive tract, enabling the slugs to survive solely by photosynthesis for several months at a time. This association is crucial for the development and maturing of the slug. Exactly how the slugs use the chloroplasts is unclear, as many of the proteins used by chloroplasts are encoded in the genome of the host cell. These proteins, numbering in the hundreds, are manufactured in the cell's nucleus, and then moved into the chloroplast, enabling it to survive. There has been a study regarding a member of the genus Elysia (E. chloritica), which states that there is horizontal gene transfer when the slugs ingest the algae. The study postulates that upon the ingestion of the algae, to keep the chloroplasts alive, E. chloritica steals genes via horizontal gene transfer to make proteins that maintain the chloroplasts. However, many other studies could not replicate the findings in the study and the claims remain disputed. Young specimens of E. atroviridis and E. marginata became known for their ability to regenerate the whole body from a severed head. This autotomy expels internal parasites.

This genus was previously sometimes considered to be in the family Stiligeridae, and was also previously placed in the family Elysiidae.

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