Cusk-eelsOphidiidae

This fish keeps its hips under its chin. In a cusk-eel the pelvic fins have moved right forward to the throat and stopped working as fins: each is a pair of fine filaments trailed along the ground in front of the head and used to feel for what is buried in it. The species listed here goes further and adds barbels around the snout and lower jaw, which is why it collects names like bearded cusk and goatsbeard brotula. It reads the bottom by touch, after dark.

The other thing this family does is talk. Ophidiids carry one of the more elaborate sound rigs in any fish: muscles running onto the swim bladder, a stretchable window in the bladder wall, and in some species a small bone at its front end that rocks when the muscles pull. In several of them the whole apparatus belongs to adult males alone, so the sexes differ more inside than out and their calls differ to match. The calling happens at night and in the breeding season, from fish nobody can see. Drop a hydrophone over what looks like empty sand and there is a chorus on it.

The group as a whole is aimed at darkness rather than at depth in itself, although depth is where it holds its record and the species page has that. The same body works in a reef crevice at night, in a burrow in a sand flat, and on a continental slope in permanent black: long and tapering, with dorsal, tail and anal fins run together into one unbroken fringe, so the animal reverses into a hole as easily as it went in.

One species is listed here. On a night dive it is a head in a crack, or a slow brown shape crossing open sand and then not there.