Hunts other sea slugs by following their slime trails, and swallows them whole.
A slender polycerid to about 10 cm, black to deep blue-black with three or four fine longitudinal lines of yellow or pale green, orange-red rhinophores and a small gill plume set well back. It looks like a stretched, narrow Nembrotha, which is appropriate, because Nembrotha is what it eats.
It is a specialist predator of other polycerid sea slugs. It locates prey by following the mucus trail, catches up, and engulfs the animal whole through a distensible mouth — prey and predator are often close to the same length, and a recently fed Roboastra is visibly distended. Trail-following has been demonstrated experimentally in the genus, including the ability to tell the direction of travel from the trail itself. A Roboastra moving fast and straight is hunting, and following it is the more interesting option. Recorded from about 5 to 30 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.
