Marine Hatchetfishes (Sternoptychinae) are a subfamily  of small pelagic deep sea fish with a peculiar body shape. Their bodies are pressed in from each side, making them very flat and somewhat reminiscent of a hatchet!

3D Model of Argyropelecus aculeatus, a species known as the “Lovely Hatchetfish”. It only feeds at night, when migrating to shallower waters.

There are around 40 species of hatchetfish, but they all share the same body shape. The thin, blade-like body allows the Hatchetfish to better hide in the open mesopelagic zone. They only appear as a thin sliver to hungry predators stalking below.

Picture of Argyropelecus hemigymnus, known as the "Half-naked hatchetfish" due to lacking scales on its tail. Another characteristic of this species is the black dots on its tail. (Credit: Joan Angel-Soto)

Picture of Argyropelecus hemigymnus, a species known as the “Half-naked hatchetfish”. This name may come from it lacking scales on its tail, appearing half naked! The back dots on its tail are also a main characteristic of this species. (Photo: Joan Angel-Soto)

In addition to this, they have photophores – bioluminescense organs allowing them to produce their own light. The photophores cover their belly and are used for counter-illumination, matching the light coming down from the surface water to aid hiding their silhouettes from predators hunting below.

The purple dots are photophores – bioluminescense organs that produce light. (Photo: Joan Soto-Angel)

Hatchetfish also participate in diel vertical migration, rising up towards the surface during night to feed on small crustaceans and larvae, and back to the deep during daytime. With their peculiar body, rows of photophores and large upward turning eyes, the hatchetfish is well equipped to survive in the deep, dark depths of the ocean twilight zone.

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