Earth’s deep sea

Hatchetfish: deep-sea mirrors and river fliers

By the Fish of Ganymede team · 2 min read · Updated 3 November 2026

Hatchet fish: two silver, big-eyed alien fish with belly lights from Fish of Ganymede

Two quite different groups of fish are called hatchetfish, because both have a deep, narrow belly shaped like the blade of a hatchet. One lives in the deep sea and hides with light; the other lives in South American rivers and can leap out of the water. They are not close relatives.

Deep-sea hatchetfish

Marine hatchetfish are small, mostly under 10 cm, very flat and shiny silver. They live in the twilight zone, roughly 200 to 1,500 metres down, and some rise toward the surface at night to feed. Their big eyes point upward, and in some species they are tube-shaped, to catch the faint light from above and the shapes of animals against it.

A silver, big-eyed alien fish with a row of belly lights, drawn by the game — A silver, big-eyed deep fish from the game, with a line of belly lights like a hatchetfish.

Hiding with light

From below, any fish shows up as a dark shadow against the dim light coming down from the surface. Hatchetfish fix this with rows of light organs along the belly that point downward. They can match the brightness of the light above, so their outline disappears. This trick is called counter-illumination.

Their mirror-like silver sides add to the disguise, reflecting the water around them from the side.

Freshwater hatchetfish

Freshwater hatchetfish, such as the marbled hatchetfish and the silver hatchetfish, come from rivers and streams in South and Central America and are popular in aquariums. They are usually 3 to 6 cm long and live just under the surface.

Their deep chest holds strong muscles for the side fins, so they can burst out of the water and skim or glide a short way to escape a predator. In an aquarium they need a tight lid, a peaceful group of their own kind and calm water near the top.

Deep-sea fish in the game

Fish of Ganymede’s silvery, big-eyed deep fish with glowing belly lines are a nod to hatchetfish. Read about the other real deep-sea fish behind the game, or dive in, free.

Questions

Can hatchetfish fly?

Freshwater hatchetfish can leap out of the water and skim or glide a short distance. Deep-sea hatchetfish cannot.

How big do hatchetfish get?

Deep-sea hatchetfish are mostly under 10 cm; freshwater hatchetfish are usually 3 to 6 cm.

Are deep-sea and freshwater hatchetfish related?

Not closely. They share the name because both have a deep, hatchet-shaped belly.