A blue and silver sea slug that floats upside down at the surface and eats siphonophores.
A small pelagic sea slug reaching about 4 cm, with a narrow tapering body and three pairs of fans of finger-like cerata spread out to each side. The upward-facing surface is silvery white and the downward-facing surface is deep blue, which is the same countershading arrangement as the violet snails and for the same reason: the animal floats upside down at the surface, held there by a bubble of gas swallowed into the stomach.
It preys on the floating siphonophores, chiefly Physalia, and on the by-the-wind sailor and the violet snails when it meets them. Stinging cells taken in with that food are not digested; they are moved through the gut into sacs at the tips of the cerata and stored there, so the slug carries the weapons of its prey. The concentrated result can sting more sharply than the siphonophore itself.
Distribution follows the drift of the surface community it feeds on, and on Indo-Pacific blackwater sites the animals appear in the same conditions as the violet snails, in the top few metres and often right at the surface. A second, smaller species with a broader body is recognised in the genus, and the two are not reliably separated in a photograph.
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.
