LongfinsPlesiopidae

You can read the size of the space a fish lives in off the shape of its fins.

A longfin is named for the ones that give it away: a broad soft dorsal and anal fin running most of the body, a rounded tail, long pelvic fins trailing under the throat. None of that is equipment for speed. A fish that crosses open water is stiff-bodied with a forked tail, arranged for going in a straight line and going fast. This is the opposite arrangement, built to turn inside its own length, back out of a dead end, and hold still against surge in a space narrow enough to touch on both sides. The body is a description of a hole.

The rest follows. Longfins hunt shrimp and small fish at dusk and through the night, from an entrance rather than in the open, which is what the large eye is for. Each one holds a crack or an overhang and stays with it. In a torch beam a longfin reads as a dark fish scattered with pale points, and it does not bolt so much as slide backward out of the light.

The family also bluffs. The best-known member of it turns itself into a moray eel when frightened, and the males sit with the eggs when there are eggs to sit with, both of which their own pages describe properly.

Plesiopidae is a small Indo-Pacific family, richest around 🇦🇺 Australia, and this sea holds four listed species, one of which would not reach the edge of a coin. They are not rare. They are unlooked for, and any diver who spends a few minutes with a light on the roof of a cave instead of on the reef face will start meeting them.