A small bubble shell of shallow sand, marked in fine red spiral lines over white.
A small shelled sea slug to about 2.5 cm, with most animals 15 to 20 mm. The shell is thin, white and rounded, marked with fine red or orange spiral lines crossed by finer lines running the other way, so the pattern reads as a red grid. The body is opaque white and extends well beyond the shell, with a broad squarish headshield in front and a thin iridescent blue line running round the edge of the foot and mantle. Very little else on sand at that size resembles it. The nearest relative likely to be seen alongside it is the rose-petal bubble shell Hydatina physis, which is considerably larger and banded in brown and white rather than gridded in red.
It occurs through the tropical and subtropical Indo-West Pacific, on sand and among algae in shallow water, ploughing along just below the surface and coming out onto open sand to crawl. It feeds on small sand-dwelling polychaete worms; gut contents have identified cirratulids specifically. Bubble shells of this kind leave a shallow furrow across the sand surface, and a furrow ending at a small mound is worth following. In the tropics the species is found intertidally and in the shallows; at the cool ends of its range, off Japan and New Zealand, it moves into deeper water.
Unlike a nudibranch it keeps a full external shell, into which the animal can withdraw, and the family it belongs to sits between the heavily shelled acteonids and the shell-less sea slugs. Sites and guides that file it under nudibranchs are using the term in its loose diving sense.
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.
