A slender, near-transparent fish that shelters inside a sea cucumber and feeds outside it at night.
A pale, almost colourless fish, translucent enough over the front half that the backbone and the silvery gut show through, tapering from a blunt head to a fine point with no separate tail fin and no pelvic fins. Most animals seen are 10 to 20 cm. The anus sits far forward, under the throat, which is the anatomical oddity that goes with the way the fish lives. In a light it looks like a length of wet thread with eyes.
The shelter is a sea cucumber. The fish finds the holothurian’s rear opening, turns, and backs in tail-first, sometimes taking several attempts, and spends the day inside the host’s body cavity or respiratory trees. At night it comes out to hunt small crustaceans over the sand and returns before dawn. Species of Carapus take nothing from the host and use it only as a shelter; species of Encheliophis feed on the host’s tissue and gonads. Sea stars and large bivalves are used by some species in the same way.
Finding one means working the sea cucumbers rather than the sand. A fish part-way out, with the head clear and the body still inside, is the frame most people are after, and it happens in the first part of a night dive and again close to dawn. The larvae are unmistakable in their own right: a long pelagic stage carrying a trailing filament from the nape, taken regularly on blackwater dives.
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.