CardinalsApogonidae

The male holds the eggs in his mouth, and for as long as he holds them he does not eat.

A cardinalfish clutch is not a loose scatter of eggs. They are bound to one another by fine threads into a single ball, and that is what makes the thing portable: the male takes it at spawning and carries it in his throat, rolling and reshuffling the mass so that clean water keeps moving through the middle of it. A brooding male is obvious once you know the shape. The jaw is distended, the head looks wrong, and he sits a little apart from the group with his mouth shut on a week or more of work.

The rest of the family's biology follows from that decision. A reef fish this size normally spawns into open water, releases eggs by the thousand and accepts that almost none of them come back; a cardinalfish makes a small clutch and guards nearly all of it through to hatching. The accounting is not sentimental. Males have been recorded swallowing part of a brood, and sometimes the whole of it, when the cost of the fast outweighs what the eggs are worth.

By day they are the easiest fish on the reef to find and the least rewarding to watch: a packed cloud of silver bodies under a ledge or inside a cave mouth, holding station in the shade, with an eye that takes up much of the head. The feeding is done in the dark, out over open reef, where divers are not.

Forty-seven species are listed here, and separating them is a real problem - a stripe through the eye, a spot at the base of the tail, a bar that comes and goes with mood. Several of them are known from this sea and nowhere else.