Lanterneye fishesAnomalopidae

The light under the eye cannot be switched off, so the fish grew a lid. The organ burns without a pause, for reasons the species page gives, and a permanent lamp on a small fish in a dark cave is a liability rather than an asset. The family's entire design is the answer to it. In the genus that lives in this sea, a black membrane rises from below and draws across the organ like an eyelid. Other members of the family solve it the other way and rotate the whole organ downward into a pocket in the head. Neither is dimming a lamp. Both are putting a shutter across one.

Which turns a light into speech. Work on this species in the 🌊 Red Sea found the blink rate climbing while a fish drove off another of its own kind, and found that it would attack an artificial light organ set at the mouth of its cave as readily as a real intruder. These fish hold territory in pairs at a cave entrance, in the dark, and they hold it with light, since display and color are worth nothing where they live.

The same shutter is the escape route. A flash, then black, then a hard turn: whatever was tracking the light keeps going along the line the light was traveling, and the fish is no longer on it.

One species is listed here, and there is a way to see it. It comes shallow on moonless nights and stays deep when the moon is up, so a night dive on a new moon, at a wall with cave mouths in it, is the realistic chance. What you are looking for is not a fish. It is a pair of small blue-green lamps working along a rock face, going out and coming back on.