This is a sea slug (Elysya chrolotica), it feeds on algae (Vaucheria litorea) and partially digests them.
Keeping the chloroplasts viable, it incorporates them into its digestive system and is able to photosynthesize.
This makes it an autotrophic being! In other words, it synthesizes its own food with the incorporated plastids.
Thanks to this strategy, it can live for months without taking a bite, simply feeding on sunlight.
This clever strategy has made Elysia famous as the first animal capable of photosynthesis.
Biologists were intrigued by how the slug could be so selective as not to digest a part of the algae, precisely the part it needed for its purpose.
But its merit goes far beyond that.
It turns out that this mollusk of barely six centimeters has in its chromosome genes from the algae that are indispensable to maintain in good condition the chloroplasts that it steals from it.
So it has also become the first case of functional gene transfer from one multicellular species to another, to the envy of many researchers.
DOI: 10.1093/molbev/msy061
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