Elysia chlorotica, also called emerald green sea slug or eastern emerald elysia, species of sea slug belonging to the family Elysiidae (order Sacoglossa) and known for its ability to photosynthesize food. It was among the first members of the animal kingdom thought to be capable of producing chlorophyll, a pigment found in nearly all photosynthetic plants that use solar energy to transform carbon dioxide into carbohydrates. Members of this species appear as wide, rippling, green leaves with snail-like heads. They inhabit the shallow salt marshes and inlets of North America’s Atlantic coast from Florida to Nova Scotia. Over their life span of 9–10 months, they can grow to a length of 1–6 cm (0.4–2.4 inches).

Eastern emerald elysia (Elysia chlorotica). Mary Tyler—Mary Rumpho/University of Maine

The photosynthetic ability of Elysia chlorotica appears to come from the temporary incorporation of chloroplasts (photosynthesizing structures within plants) from Vaucheria litorea, a yellow-green alga it consumes, into cells that surround E. chlorotica’s digestive tract. Chloroplasts and other plastids (small bodies involved in the synthesis and storage of foodstuffs) can continue to photosynthesize nutrients for the animals for up to several months. It is unknown, however, how much E. chlorotica relies on the photosynthetic capacity of the chloroplasts it brings into its body for energy, because individuals can survive long periods of darkness (possibly through digesting their stored plastids). E. chlorotica also assimilates the genes of V. litorea into its genetic structure; however, those genes do not appear to be active in the animal.