SanddiverTrichonotus

It goes into the seabed at speed, head first, without slowing down. A sanddiver does not burrow the way a crab burrows; it swims into the bottom and keeps going, and a patch of sand with thirty fish hanging above it is bare a second later with no sign of where they went. They come back out the same way, nose first, and resume hovering as though nothing had happened.

That single trick is the whole living. A sanddiver feeds on small animals picked out of the water just above open sand, in a place with no coral, no ledge and nothing whatever to get behind. Every other small fish that feeds in the water column does it within a body length of something solid to bolt into. This one has no structure at all, so it carries its refuge underneath itself, and that is the reason it can occupy a habitat that is otherwise empty of fish its size.

The males are worth stopping for. The front rays of the dorsal fin are drawn out into long filaments, and a male raises them and works them at females while patrolling above the group, which hovers a hand's width off the bottom with everything facing into the current. The whole aggregation drops into the sand together when a shadow crosses it.

The bottom has to be right. These are fish of clean, coarse, current-swept sand, and they do not persist where the sediment has gone fine and silty or where the bottom is dredged and repeatedly anchored, so their presence says something specific about the ground they are on. The family is small, fewer than a dozen species across the Indo-Pacific, and one of them is listed here: a slender fish of about fifteen centimeters that most divers cross without ever registering, on the sand flats they are swimming over to get somewhere else.