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Diving on the Costa Brava, if we look very closely at the small single celled alga Acetabularia mediterranea, known as Mermaid’s cup, we may find on the cup an amazing tiny animal with a rare and special ability. Many divers mistake it for a nudibranch but it is actually a sacoglossan, a "sap-sucking sea slug", named Elysia viridis.

Sacoglossans live by ingesting the internal contents of algae, hence the adjective "sap-sucking". Some sacoglossans simply digest the sap which they suck from the algae, but in some species like Elysia viridis the sea slugs sequester and utilize within their own tissues living chloroplasts from the algae they eat, a very unusual phenomenon known as kleptoplasty.



The sacoglossans are the only animals to employ this mysterious solar power strategy to gain energy directly from the sun. This earns them the title of the "solar-powered sea slugs", and makes them unique among animals. Sacoglossans have been known to survive for months living solely on the photosynthetic products of their acquired plastids.

Observant divers can find Elysia viridis sunbathing on Mermaid’s cup in shallow water, at midday, with its wing-like parapodia unfurled to gain maximum sun exposure; producing its own energy from the sun.
 

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The world grows more interesting every day. What a beautiful and fascinating animal. Thanks for sharing this insight.
 
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