Combtooth blennyBlenniidae

The name is about the teeth, and the teeth are odd. A blenny carries a single row of many fine, closely set teeth across the front of each jaw, and unlike the teeth of most fish they are not anchored in the bone but attached to the flexible tissue of the lip, so the whole row gives as it is dragged. It works like a squeegee. What it is dragged over is not seaweed but the film: the low mat of algae, bacteria and trapped sediment that coats every hard surface on a reef and is the largest single food supply in the place for anything with the gear to harvest it.

Part of the family threw that away. The fangblennies grew a pair of long curved canines in the lower jaw, some of them grooved and backed by a venom gland, and gave up grazing rock entirely. They take pieces out of other fish instead, feeding on scales, skin and mucus, a diet almost nothing else in the sea attempts. The victim loses a divot and turns to find nothing there.

The mimicry inside this family runs both ways. Fangblennies copy the cleaner wrasse down to the stripes and the hovering dance, which is how they get close enough to bite the customer that came in to be cleaned. And because a fanged blenny is a fish that predators learn to leave alone, a harmless grazing blenny in this sea copies the fanged one, and does nothing with the costume except live quietly inside it.

Thirty-five species are listed here, in every shallow habitat there is, from rock that is only underwater half the time to reef at forty meters. They are also among the few fish that will not leave: a blenny in a hole withdraws a couple of centimeters, waits, and comes back out to look at you, which makes an animal the size of a finger one of the easiest photographs on the reef.