A shell-less pelagic sea slug that flies on two wings and hunts sea butterflies.
A small pelagic snail with no shell at all, clear or faintly pink, with the gut and gonad showing through as an orange, red or brown core, and a pair of wing-like parapodia set just behind the head that beat continuously. Most tropical ones are well under 2 cm. The confusion is with the sea butterflies, the thecosome pteropods, which have their own entry here and which keep a shell or a glassy pseudoconch and have broader, more sail-like wings; also with small heteropods and with the pelagic nudibranchs. This entry is pitched at the order, because the characters that separate the families and genera are the arrangement of buccal cones, hook sacs and suckered arms inside the head, and a photograph will not settle which one it is. Pneumodermopsis, Pneumoderma, Hydromyles, Cliopsis and Thliptodon are among the genera in the region.
They are gastropods that have abandoned the shell entirely, and they are predators. Most are specialists on the thecosome pteropods, and the mouthparts are the whole apparatus: eversible buccal cones that grip and hold the prey’s shell, hook sacs carrying rows of chitinous hooks that draw the soft body out of it, and in some genera a pair of suckered arms as well. All of that is retracted in a swimming animal and everted only during an attack, so a photograph of a sea angel feeding shows a different animal from a photograph of one in transit.
They swim steadily rather than drifting, which is usually what picks them out: a salp hangs in the beam and a sea angel does not, and the beat of the wings is generally visible before the body is. They occur through the water column and are commonly met in the upper part of a blackwater dive.
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.
