|Karen N. Pelletreau et al|
The main thing about this particular marine mollusc is that it can photosynthesise for long periods of time. Various animals, notably corals, form symbiotic relationships with algae in which the the animal provides a home and protection for algae and the algae provide food for the animal. But this curious sea slug goes a step further—it eats a particular type of algae, Vaucheria litorea, and actually assimilates chloroplasts, tiny light absorbing subcellular units that make energy from sunlight.
Other relatives of E chlorotica do the same, but usually, the choloroplasts, lacking the supportive environment of the algae die in a few days. Somehow, E chlorotica keeps the algae alive for months, sometimes until the sea slug itself reaches old age and dies. The thing I find most pleasing about this whole arrangement is how much like a leaf this sea slug looks.
E chlorotica lives in a vanishingly small range on the eastern seaboard of the USA, and it's poorly studied. How it keeps the chloroplasts functioning for so long might be because the slug itself has integrated some of the algal genes that support the chloroplasts, although the science on this is out. And the rareness of the animal and the difficulties with studying it in the lab mean answers might not be forthcoming any time soon.
Picture source: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0097477