Some nudibranchs (a kind of sea slugs) feed on cnidarians (sea anemones, jellyfish, corals, and tiny sea anemone-like animals called hydrozoans). Many of these cnidarians form symbiotic relationships with single-celled algae and profit from the energy produced when these algae photosynthesize. When a nudibranch eats a cnidarian, it gets a mouthful of algae. It could digest the algae and the cnidarian, or it could do something far more interesting. Some species of nudibranchs are able to steal the algae and form their own symbioses with the algae that they steal from their cnidarian prey!
Stealing and then storing the algae requires certain adaptations which had to evolve in the species that can do this. For example, cells are needed for algae storage and those cells need to recognize that these algae should not be digested like the rest of the material that entered the digestive system when the slug consumed the cnidarian. We don't yet know how these slugs recognize the algae, but some slugs do and others don't, meaning some slugs can steal algae and others just digest them, losing out on the valuable energy they could get from ongoing photosynthesis.
In this paper, we examined the nudibranch, Phidiana lynceus to see if it can steal algae from the hydrozoans it eats and if it gets energy produced via photosynthesis. We found that it evolved all of the adaptations needed to steal and store symbiotic algae and profit from their photosynthetic goodness, but prefers to hang out in the dark, so it doesn't get any of the energetic benefits that photosynthesis would provide. For now, this slug remains a paradox... a slug that could photosynthesize but chooses not to by crawling under a rock.
Photos: The hydrozoans on which Phidiana lynceus feeds and from which it steals algae (a), and Phidiana crawling under a rock (b). The brownish color in the cerata (finger-like appendages on the slug's back) are the algae stored in special cells.
