10 Weird Creatures From the Mariana Trench

Deep-sea Hatchetfish
Hatchetfish can actually shift the intensity of their bioluminescence based on the light available from above to optimize camouflage. © 2015 HowStuffWorks, a division of Infospace LLC

There are a lot of odd-looking fish in the sea, but not many of them resemble humans' hand tools. The deep-sea hatchetfish resembles a silvery swimming hatchet.

There are more than 40 species of hatchetfish. All of them have ridiculously skinny bodies, and many of them have shiny scales, too, which adds to the metallic, hatchetlike appearance. They're small fish, and even the biggest types grow only to about 6 inches (15 centimeters) long. Their delicate looks belie serious ruggedness, because these fish are found in depths pushing nearly 5,000 feet (1,524 meters).

Hatchetfish have bioluminescent bodies, and they can alter the brightness of their glow depending on how much light is filtering from above. In doing so, they're counterilluminating their bodies in a clever camouflage technique. Their dim, self-produced light reduces their silhouettes, making it much more difficult for predators to spot them from below.