UnicornfishesNasinae

Nobody has explained the horn. It grows forward off the forehead as the fish matures, in some species out past the tip of the mouth, and it is not a weapon: unicornfish fight with the blades on the tail and no one has recorded the horn doing anything at all. It is not a digging tool, it is not a lever, and it is not restricted to males, which rules out the tidiest answer available. The standing guesses are a signal within the species and some effect on the way water moves over the head, and neither has been settled. Several species never grow one and get by perfectly well.

In at least one of them the horn arrives with a change of profession. Young lined unicornfish crop weed off the bottom; adults, once the horn is out past the snout, feed on plankton in midwater instead. Whether the horn causes the switch or simply happens alongside it is unknown, though a forward-pointing spike is awkward equipment for working a flat surface.

They are surgeonfishes, and the tail says so with a difference. A surgeonfish carries a single folding blade that lies hidden in a groove until it is needed. A unicornfish carries a pair of fixed bony keels on each side of the tail base that never fold away at all, usually outlined in bright blue. There is no sheathing them, so they are advertised instead, and they are used the same way, in a sideways swipe delivered in passing.

Five species are listed here, up to about 70 cm (28 in). Some have left the reef surface entirely and hang off drop-offs and in channels where current runs, gray crowds facing into the flow and picking it over; others stayed with the weed and are among the few fish that will eat the tough leathery algae nothing else bothers with.