If you're expecting it to be a tuna, flounder or goldfish, guess again. The most common type of fish on the planet is one humans rarely see. Why? Because it's small, ingeniously camouflaged and lives between 100 and 250 fathoms down in the ocean. It's the bristlemouth, a member of the genus Cyclothone.
The bristlemouth clan are humble, homely creatures with stubby bodies and tiny eyes, and they're mostly mouth and fins. Size-wise, they're unimposing. Most species grow to no more than three inches in length. But the bristlemouth has one outstanding piece of equipment: two rows of photophores, or light-emitting organs, on its underside, which it uses to prevent itself from a casting shadow its predators might spy. Its predators include shrimp, which gives you a sense of how puny this fish is.