Elysia chlorotica is a “solar-powered” marine sea slug that sequesters and retains photosynthetically active chloroplasts from the algae it eats and, remarkably, has incorporated algal genes into its own genetic code. It is emerald green in color often with small red or white markings, has a slender shape typical of members of its genus, and parapodia (lateral "wings") that fold over its body in life. This sea slug is unique among animals to possess photosynthesis-specific genes and is an extraordinary example of symbiosis between an alga and mollusc as well as a genetic chimera of these two organisms.
To obtain algal chloroplasts Elysia chlorotica slugs use their radula (tooth) to pierce a filament of the alga Vaucheria litorea and suck out its contents. The ingested algal cytoplasm and nuclei move through the gut but algal chloroplasts are trapped and concentrated in vacuoles along branches of the digestive tract. While inside an algal cell, functional chloroplasts use proteins encoded by their own genes as well as others encoded by genes within the algal nucleus. Within a sea slug, however, isolated chloroplasts can not receive proteins from the algal genome. Remarkably, these chloroplasts remain functional anyway because the slug genome includes the algal genes necessary for plastid function. Elysia chlorotica probably gained these algal genes through lateral (or horizontal) gene transfer. One possible vector is a virus that infects the sea slug and carried pieces of algal DNA (Pierce et al., 2003).