The bristlemouth, a small ugly genus of fish found in the ocean twilight zone, is probably the most common vertebrate on the planet – estimates go as high as the quadrillions.
Three hundred metres below the surface of the ocean, a little predator fish called the bristlemouth swims around in the murky twilight. It glows a little, as many fish do down there, and it spends its time eating plankton and miniature crustaceans, avoiding being eaten by larger predators, and making more bristlemouths. It’s not a big fish, between 2 and 20cm depending on the species, of which there are 32 in 8 related genera. But it is a common one.
It’s difficult to estimate how many fish are in the sea. Typical research methods include net trawling and sonar measurements. And every time someone trawls the mesopelagic twilight zone, whether they do so in the Pacific, or the Atlantic, or the Indian Ocean, what they find are bristlemouths. Lots of bristlemouths… more than any other fish by a significant margin.
So, if we take our best estimate of the range of the bristlemouth, and our best estimate of the number of bristlemouth in that range, we come up with an estimate for the number of bristlemouth in the world. And that estimate ranges from the trillions up into the quadrillions. Yup, that’s 1,000,000,000,000,000 bristlemouths swimming around. This little fish is easily the most common vertebrate (animal with a backbone) in the world.