The Most Numerous Vertebrate

Marine biologists have decided that the most numerous vertebrate is the bristlemouth (genus Cyclodone), a small fish that lives in the middle ocean depths worldwide. There are trillions of them, possibly quadrillions, as many as 12 per square meter of ocean. Since they live far down in the dark, we know very little about them; indeed it has taken until quite recently for biologists to figure out how common they are worldwide. Mainly they live in the mesopelagic zone, between 100 and 1000 meters down, but they are found as deep as 5,000 meters (3 miles).

Still so much to learn about the world.