Very Odd Sacoglossan
In the previous post, I mentioned that there are at least five species of slugs in Box of Slugs 2.0. Four Elysia species were described in that post, but the one in the picture below is odd enough to warrant its own spotlight. This tiny beauty was clearly different from the Elysia, having branched rhinophores, and being adorned with cerata, outgrowths from the dorsal surface. It is more reclusive than the others, rarely making its presence known except for the occasional appearance in the early morning before lights-on. It is also very quick to become scarce, diving behind or into a clump of algae, if the lights come on when it is out and about. These behaviors, combined with my limitations as a photographer, explains the quality of the images so far.
When I first saw this creature, with its frilly cerata, I believed it to be a nudibranch. In general, marine aquarists are not pleased about seeing nudibranchs in their tanks. They tend to be specialist predators, focusing on one or a few species of prey items. Prey can be sponges, bryozoans, corals, or even other molluscs. As a result, nudibranchs can either be pests, destroying prized species, or, more likely, will starve to death in the absence of preferred foods
I had removed the slug to a small container, and was debating its fate, when Joanna made it clear that killing the slug was the less preferred option. Realizing that I did not yet know what I was dealing with, I took a few macro shots and started to go through field guides. Looking more closely, I realized that it was not a nudibranch, but a sacoglossan like Elysia. The lack of external gills on the posterior end (see photo below) mean that this animal is not a nudibranch, and the combination of cerata and branched rhinophores suggest that it is a species of Cyerce. Cyerce antillensis comes from the right part of the world to have ridden in on some macroalgae, and it lives on Penicillus, so it is a promising candidate. It is a variable species, and is the closest match I have found so far, but I have not yet seen a photo of Cyerce antillensis that is convincingly similar to our slug. In any case, the slug was returned to the Box of Slugs as a harmless curiosity.
The photos above and below show a body filled with green stuff, presumably chloroplasts removed from its food plant(s). According to the literature, however, Cyerce is not kleptoplastic.
With time and luck, opportunities for better photographs will present themselves. Who knows, there may even be enough of them to start seeing young ones.