If you’ve ever doubted just how inhospitable and unforgiving the deep sea can be, dive down and meet another fascinating animal surviving and thriving the dark depths of the ocean: the deep sea hatchetfish. These scrappy fish have two particular attributes worthy of a closer look—their appearance and their ability to counterilluminate.

Appearance

First off, the hatchetfish’s name is due to its shape, which bears resemblance to the blade of a hatchet, a tool used for chopping wood. This oddball fish has several more unique qualities, but the most recognizable one is its “anguished” facial expression. The gaping, downturned mouth and large, protruding eyes give it the appearance of being permanently shocked. Perhaps hatchetfish are surprised to still be alive in the tough conditions challenging the deep sea? 

In addition to withstanding the crushing pressure of the water they live in, hatchetfish are recognizable due to their small stature. Ranging from 1.1 inches (2.8 cm) to 4.7 inches (12 cm,) they are the definition of “bite size”.

Counterillumination

The hatchetfish has an interesting defense in the presence of predators: counterillumination. At this point, you  most likely have heard of bioluminescence, the ability for sea creatures to emit their own light. Hatchetfish possess the organs needed for bioluminescence which lie in rows along their bellies and shine blue. Counterillumination takes this a step further, allowing hatchetfish the ability to adjust the intensity and color of its light. 

In the top zone of the sea, where there is still some sunlight, many predators hunt by looking above for the silhouettes of these fish. When its lights are projected, the hatchetfish becomes invisible from below. 

This light also helps to camouflage the hatchetfish at deeper depths with no sunlight.. When light is thrust upon them in the darkness, their photophores funnel it outward, scattering it in a pattern that conceals them.

It’s great that despite their vulnerability, size and short life span (less than a year) the deep-sea hatchetfish is not currently at risk. Hatchetfish populations are found across temperate and tropical seas worldwide in the Atlantic, Pacific and Indian oceans. 

Help Ocean Conservancy keep our ocean healthy and trash free, so the hatchetfish and all beleaguered fish have one less problem confronting them!