Ocean SunfishesMolidae

A sunfish has no tail. The caudal fin and the skeleton that would carry it are lost during development, and the back of the animal finishes in a clavus: a stiff scalloped rudder built out of the rear ends of the dorsal and anal fins and edged with a row of small bony nubs. The vertebral column simply stops. What is left reads as a fish cut in half behind the head, which is where the old nickname of a swimming head comes from, while the Latin name means millstone.

It is a pufferfish. Molidae sit in the same order as the puffers, triggerfishes, filefishes and boxfishes, a group that traded speed away for other kinds of defense, and the sunfish inherited the family mouth: teeth fused into a parrot-like beak set in an opening that never fully closes.

And it does not lie at the surface because it likes the sun. Fish carrying temperature loggers turn out to be working animals, making repeated dives into cold water below the thermocline after siphonophores and jellies, then returning to the warm surface layer to put the heat back, and warming up far faster than they cooled down. The drifting sunbather is a fish between shifts. The food is thin stuff, too. Siphonophores and jellies are largely water, so a great deal of the sea has to go through the animal, and the dives run one after another through the day.

Three are listed here and none of them is a sighting anybody can arrange. These are animals of open ocean basins, and in this sea a mola is a story somebody tells for years afterward rather than something a dive site can be built around.