A flattened slug to about 6 cm (2.4 in), brown to olive and dusted with pale rings and blue flecks, its two side flaps folded over the back like a closed book. Unlike most of its relatives it does not sit on the weed it feeds from - it lies out on open sand and rubble, which is where a diver finds it, looking more like a piece of debris than an animal.
That exposure is the point. This is one of the solar-powered slugs: it pierces algal cells with a single-toothed radula, sucks out the contents and keeps the chloroplasts alive inside its own gut lining, where they go on photosynthesising for weeks. Spreading the flaps to the sun is not basking, it is feeding. An animal that has stocked up can survive months without eating at all, and the trick is rare enough in the animal kingdom that only this group of slugs manages it. Genetic work has shown the name covers several near-identical species, so what is called Plakobranchus ocellatus across the Indo-Pacific will eventually be split.
- flattened body with two side flaps folded over the back
- brown to olive with dense pale rings and blue flecks
- lies out on open sand rather than on algae
- length up to 6 cm
