A night animal whose enormous rose-pink mantle spills out well beyond the thin striped shell it cannot withdraw into.
A sea slug that still has a shell, and the shell is the smaller half of the animal. It is thin, brittle, almost translucent, and wrapped in fine brown spiral lines on a cream ground. Over and around it flows a mantle of a colour nothing else on the sand shares — rose to magenta, edged in a paler line, frilled at the margins and far too large to be pulled back inside. The animal is not defended by the shell in any useful sense; it simply carries it.
It is a hunter. Hydatina feeds on burrowing polychaete worms, ploughing just under the surface of the sand and taking them whole, and the head shield at the front is the tool for it: a flattened, muscular plate that pushes sediment aside and keeps it out of the mantle cavity. The trail is often easier to find than the animal — a shallow raised furrow in the sand, ending where the slug has gone under.
Almost entirely nocturnal. On a night dive over black sand it is one of the few animals that reads as bright pink under torchlight at a distance, which makes it one of the easier good subjects to spot and one of the harder ones to photograph well: the mantle is close to the limit of what a strobe can hold without burning out, and the shell’s fine spiral lines are lost at anything above a modest power.
The name is a nuisance. Rose petal bubble shell, paper bubble and green-lined paper bubble are all applied to this animal and to related species in the family, so a common name alone is not an identification.
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.
