PipefishesSyngnathinae

A pipefish wears its skeleton on the outside. The body is a chain of bony rings, ridged and interlocking, sitting where scales would be on any other fish, and the joints between them allow very little sideways bend. That matters more than it sounds, because a fish swims by throwing a wave down its own body. A pipefish cannot make that wave.

So it does something else. A small fin on the back beats fast enough to blur, the pectoral fins behind the head work as oars, and the animal hovers, reverses and holds against a current with nothing about it visibly moving. It is slow and it costs almost nothing, which is fortunate, because a pipefish has no stomach at all. Food goes from the throat into the intestine and out again, nothing is stored, and the fish picks small crustaceans off rubble and weed one at a time from first light until dark.

The male carries the eggs, and how he carries them is the most interesting thing in the group, because the pipefishes show the whole series at once. In some, the female glues her eggs to a bare strip of skin on his underside and they ride there in the open water. In others, folds of skin rise on either side and half cover them. In others again the folds meet and seal into a closed pocket that he opens only to release the young. Line those up and the seahorse stops being a special case: it is the far end of the same series, a pipefish that stood upright and shut the pouch completely.

Telling them apart is arithmetic more than art. The rings can be counted, the number along the trunk and the number along the tail are fixed for a species, and a photograph that shows the body cleanly will settle an identification that pattern and color cannot. Nineteen species are listed here, most of them between a matchstick and a pencil in size, lying against rubble or hooked into weed. Divers see few of them, and not because they are rare.