A small transparent sea slug that lives its whole life in open water and eats siphonophores.
A small clear slug-shaped animal with no shell, no obvious foot, and a compact orange or brown digestive gland showing through the body wall. It swims by flexing the whole body from side to side and is most often seen in the beam as a pale comma travelling steadily in one direction. Larval fish and some pteropods are confused with it at a glance; the swimming action settles it.
Cephalopyge trematoides is a nudibranch that has abandoned the bottom entirely. There is no crawling stage in the adult life; the animal is born, feeds, mates and dies in the water column. It feeds on siphonophores, particularly Nanomia, and is often photographed attached to or beside one. The species is cosmopolitan in warm seas and has been recorded from the surface down to several hundred metres, though it is a surface-layer animal at night.
Records from this region are thin rather than absent. It is a regular but uncommon blackwater subject at the sites where blackwater diving is run in Indonesia and the Philippines, and it is one of the few open-ocean molluscs that can reasonably be named to species in the water, because the combination of a shell-less body, the swimming action and the siphonophore association is not shared by much else.
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.
