FilefishesMonacanthidae

One filefish here hides by smell. The longnose filefish eats nothing but the polyps of Acropora coral, and it takes on the chemical signature of what it eats closely enough that in trials a reef predator hunting by odor responded to the fish no more than it responded to the coral. It was the first vertebrate shown to disguise its smell rather than its outline, and the disguise is made entirely of dinner.

The bill for that specialization comes due when the coral does. A fish that eats one genus of coral has nowhere to go when a bleaching event takes it, and populations of these filefish drop out of reefs where the Acropora has died even though the reef structure is still standing. This sea has its own version of the animal, found here and nowhere else, alongside the wider Indo-Pacific one.

The family as a whole is triggerfishes with the armor taken off: same order, same body plan, thinner and more flexible. The skin has no ordinary scales, only minute bristles set into it, and the result is rough enough that dried filefish hides were used to smooth wood, which is where the English name comes from. The first dorsal spine locks upright, and the catch that holds it is described on one of the species pages. Below, the pelvis swings down a flap of skin that makes the fish abruptly twice as deep, so one that has reversed into a crevice is wedged there deliberately and cannot be pulled out by the tail.

The rest of the family eats what nothing else will: sponges, hydroids, seagrass, algae, gorgonian polyps, tunicates. They travel by rippling the dorsal and anal fins, tail folded away, which gives them a slow hover, the ability to reverse, and a habit of hanging head-down beside a coral head while working over it.

Ten species are listed here, from fish the length of a finger to one over a meter, and several are small, plain, brown and effectively unrecorded in this sea. Their eggs are laid on the bottom rather than released into the current, and in some species the male stays with them, which is unusual company for the group they belong to.