What's believed to be the most numerous vertebrate on Earth is also one you've probably never heard of or seen—yet scientists say the bristlefish, a fanged creature that glows in the dark and lives deep down in the ocean, likely numbers in the thousands of trillions, reports the New York Times. To put the sheer volume of bristlemouth in context, the domestic chicken—the animal previously cited by an expert to be the most numerous vertebrate—comes in at around 24 billion. The tiny fish—which the Times notes have extremely wide mouths to show off their ample teeth—are smaller than a finger, but they seem able to survive by "counter-illiuminating" to blend in with their surroundings, as well as having the ability to switch from being a male when born to female later in life.
As Oceana.org notes, one of the reasons it's been so hard to pinpoint bristlemouth as the world's most abundant is that they live "in a difficult environment" and are "particularly fragile" when caught in trawler nets. And while researchers have suspected since the late 1800s that bristlemouth ruled the vertebrate roost, the US Navy didn't spot many of them during the Cold War when it studied ocean waters for submarine use, per the Times. Most people forgot about the bristlemouth for a while, until research in the 1990s started pulling bristlemouth by the netful back up to the surface. "We keep seeing lots of different critters we haven't seen before," says a California marine biologist. "The deeper you go, the stranger things get," he tells the Times, noting that while we've identified about 2 million species on Earth, there could be at least a million more undiscovered ones.