Hatchet fish

Sladen's hatchetfish is a 7 cm scrap of living tinfoil from the open ocean. Its body is so deep and so thin that it looks less like a fish than like a blade someone dropped in the water. It spends the daylight hours down in the dark and climbs towards the surface every night, a commute it makes for its whole life.

Also known as: lowcrest hatchetfish, sladen's hatchet fish, sladen's hatchetfish, sladen's hatchtefish, lowcrest hachetfish, silvery hatchetfish, silvery hatchetfish, hawaiian hatchet fish, lowerest hatchetfish, sladen's hatchet fish, lowcrest hatchetfish

Its close relatives share the same silver blade of a body, so shape will not separate them. Look at the two short spines set at an angle behind the belly. On this species they point opposite ways: the front one is blunt and aims forwards, the rear one points back.

The body is deep and squeezed flat from side to side, with a keel, a raised ridge, running along the belly. The back is dark and the flanks are silvery. The eyes are tubular and aim upwards. Small luminous spots, called photophores, dot the gill cover and the belly. The front dorsal fin is low and long, about three times longer than it is tall, and a second, soft-edged fin sits behind it on a long base.

This is an open-ocean fish of warm and mild seas right around the world. It has no link to reefs or coastlines and hangs instead in open midwater, down to nearly 2,900 m. Its day is split in two: deep water while the sun is up, then a climb towards the surface at dusk and a return at dawn. It feeds on drifting animals, among them krill, small shrimp-like amphipods and copepods. The eggs and young drift in the plankton.