A sacoglossan that keeps chloroplasts from the algae it eats. Two to five millimetres, on shallow algae.
Two things about this animal are wrong in almost every caption. It is not a nudibranch — it is a sacoglossan, a sap-sucking sea slug from a different order entirely. And it is probably not Costasiella kuroshimae. True kuroshimae was described from the Ryukyus, is minute at 2 to 5 mm, and has green cerata with white tips, white black-tipped rhinophores and a dark streak behind the eyes. The animal that everyone photographs in the Philippines and Indonesia — white-bodied, green cerata with pink to purple tips, a pink flush around the eyes, to about 12 mm — is an undescribed species informally labelled Costasiella sp. 3, and molecular work has found several more cryptic species that look the same. The characters in dispute are precisely the ones visible in a photograph, so a photograph cannot be identified to species. The face and the “ears” are the same in all of them: the ears are rhinophores and the black dots are eyes.
The solar-powered part is real but smaller than the reputation. Sacoglossans practise kleptoplasty — they suck the contents of algal cells and keep the chloroplasts alive inside their own tissue, which is what makes the cerata green. In this genus the retention is short-term; the long survival figures that circulate, over a month without food, come from a Caribbean relative, and the plastids look more like a stored reserve than a daily energy supply. The food alga is Avrainvillea, and that is where to look: patches of it on sheltered sand and rubble slopes in shallow water, down to 10 or 15 m.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
