GobiesGobiidae

No family of marine fish has more species in it. More than two thousand gobies have been described, and the number moves every year, which is what happens to a group of animals that are small, sedentary and easily cut off from each other: a population on one patch of rubble does not mix much with the population on the next one, and given enough time it stops being the same fish.

The equipment that made that possible is under the chin. In a goby the two pelvic fins are fused into a shallow cup, and the cup works as a suction disc. Most of the family has also given up the swim bladder, so a goby does not hover; it holds on. That one change turns surfaces into addresses. A coral branch, the stem of a sea whip, the inside of a sponge, the roof of a cave, wave-hammered rock in the first meter of water: a goby can be stuck to any of them, in current that would roll an ordinary fish off the bottom.

Being small is the other half of it. A goby fits into spaces that are not habitat for anything else - a burrow dug and maintained by a snapping shrimp, the gap between the branches of one coral head it will never leave - and that is a whole living nobody else can take. The price is a short life at the bottom of the food chain, and the family answers by maturing early and breeding often.

Fifty-four species are listed here, and a diver notices perhaps three of them. The one everybody meets is a shrimpgoby posted at its burrow mouth on open sand, propped on its fins, dropping out of sight as you close the last meter. The rest are on the underside of things.