JawfishesOpistognathidae

The mouth that names this family is a digging tool. A jawfish's jaws run back well past the eye and open wider than the head is deep, and most of what they do is move sand: the fish bites a load out of the floor of its shaft, carries it up, and spits it clear of the entrance.

What it is building is a vertical burrow with a chamber at the bottom, propped at the rim with material chosen and carried in one piece at a time, which the species pages go into. It is not finished, ever. Sand runs, walls fall in, and a jawfish spends part of every day rebuilding the same hole. That work is the reason the animal behaves the way it does. The burrow, not the fish, is the territory, and everything a jawfish does happens within a body length or two of the top of it: feeding on what drifts past the entrance, displaying at the neighbors, and dropping out of sight when a shadow arrives.

So a sand flat that looks like empty ground between two pieces of reef is a settlement, laid out on a grid, spaced by how far apart the digging has to be. Divers swim over it at speed on the way to somewhere with coral on it.

The catch is that the investment cannot be moved. A shaft filled by a storm, an anchor or a careless fin has to be dug out again or written off, and the fish that lost it has to cross open sand it is not built to cross. Three species are listed for this sea, one of them known here and nowhere else and another from very few records, and all of them are found by hanging still over open sand and waiting for a head to come back up.