A small photophore-studded fish that rises from deep water to the surface layer each night.
A small fish, usually 2 to 15 cm, blunt-headed and large-eyed, with big silvery scales that come off at a touch and a small fleshy adipose fin behind the dorsal. The character is the photophores: round light organs set in defined groups along the belly and lower flanks, and their number and arrangement is what separates the roughly 250 species in the family. Juvenile sardines and silversides have no photophores; the marine hatchetfishes are deep bodied and strongly compressed; the barracudinas are slender and fanged. Identification from a photograph stops at the family. Species need the photophore groups counted, which in practice means the fish in hand, and a lanternfish that has been in a net or a beam for a while has usually lost enough scales to obscure the pattern anyway.
The family carries out the largest daily animal migration on the planet. They spend the day between roughly 300 and 1000 m, rise into the upper hundred metres after dark to feed on copepods and other small crustaceans, and drop again before first light. Collectively they are among the most abundant vertebrates in existence and they make up much of the deep scattering layer that shows on a ship’s echo sounder.
On a blackwater dive they are met in the upper part of that migration, near the lit downline, and they are one of the few subjects that behave like fish rather than like plankton: fast, aware of the diver, and gone in a direction of their own choosing.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
