Elysia chlorotica, also referred to as the eastern emerald elysia, is a medium-to-small-sized green sea slug. It is a member of the Sacoglossa clade, also known as sap-sucking sea slugs, and is famous for kleptoplasty—the process of harnessing chloroplasts from ingested algae for photosynthesis. It has an endosymbiotic relationship with the chloroplasts of the saltwater heterokont alga Vaucheria litorea, enabling it to utilize the sun's energy to survive. Distributed along North America's east coast, from Florida to Nova Scotia, it lives in salt marshes, tidal pools, and shallow creeks at a depth of 0.5 meters or less.
Elysia chlorotica begins life as a red-pigmented brown juvenile. When it eats Vaucheria litorea, it absorbs the chloroplasts from the alga and transforms into a bright green color, blending with its surroundings. The chloroplasts continue to function for months, allowing the slug to produce energy through photosynthesis without the need for food. Scientists originally thought that horizontal gene transfer was involved in maintaining chloroplasts, but subsequent studies challenged this theory. In spite of the doubt, Elysia chlorotica's capacity to store chloroplasts for long periods is an amazing survival mechanism.
The slug metamorphoses from a shelled veliger larva to a juvenile, ultimately becoming a leaf-like adult after consuming Vaucheria litorea. The adult lateral parapodia assist with camouflage and photosynthesis. They are generally 20–30 mm long but occasionally up to 60 mm long. With a lifespan of around 9–10 months, Elysia chlorotica can photosynthesize for extensive periods but can apparently live in the dark, indicating alternative sources of energy. Its peculiar biology continues to intrigue researchers, providing insight into potential use in bioengineering and symbiotic energy production.