Diving on the Costa Brava, if we look very closely at the small single celled alga Acetabularia mediterranea, commonly known as Mermaid's cup, we may find on the cup an amazing little creature, often mistaken by divers for a nudibranch. It is part of the order Opisthobranchia, and is a sacoglossan named Elysia timidis.
Sacoglossa, commonly known as the "sap-sucking sea slugs", are a clade of small sea slugs and sea snails, marine gastropod molluscs, that belong to the clade Heterobranchia. Sacoglossans live by ingesting the internal contents of algae, hence the adjective "sap-sucking".
Elysia timidis maintains the chloroplasts of the alga Acetabularia mediterranea, without digesting them, and metabolises the photosynthetic products. Sunbathing in shallow water at midday, with its parapodia unfurled to gain maximum sun exposure; this tiny animal is able to produce its own energy from the sun.
The sacoglossans are the only animals to employ this mysterious solar power strategy to gain energy directly from the sun. Some sacoglossans simply digest the sap which they suck from the algae, but in some species the slugs sequester and utilize within their own tissues living chloroplasts from the algae they eat, a very unusual phenomenon known as kleptoplasty. This earns them the title of the "solar-powered sea slugs", and makes them unique among animals.
Many of the organisms (e.g. Elysia spp.) resemble winged slugs with a pair of cephalic tentacles. In photosynthetic members of the group, the wings, or parapodia, can be unfurled to maximise the area of the organism that is struck by sunlight. Others (e.g. Placida spp.) have cylindrical cerata extending from the dorsal surface. The majority of the organisms are between one and three centimetres in length; they are typically uniform in colour by virtue of adapting ingested chloroplasts into their own cells and not easy to spot.