In 1979, researchers in the Eastern Pacific Ocean scooped up a small, never-before-seen shark with distinctive pockets near its gills. Another “pocket shark,” as the animal was dubbed, was not seen again until 2010, when a National Oceanic and Atmospheric Administration ship found one in the Gulf of Mexico. But as Mindy Weisberger reports for Live Science, a new study has revealed that the two specimens do not belong to the same species—highlighting just how much scientists have yet to discover about the creatures that live in the mysterious deep.

Published in the journal Zootaxa, the paper describes an unusual shark that was discovered during a scientific survey of the Gulf of Mexico, while scientists were researching the feeding behaviors of sperm whales. The researchers used sonar to track the whales as they dove to 3,000 feet, then used nets to drag up prey from the area. Among those samples was a male pocket shark, which was frozen for later analysis.

Then, in 2013, NOAA biologist Mark Grace was processing specimens collected from these deep waters when he came across one animal that he could not identify.

“I knew it was a species of shark,” Grace said in 2015. “But I’d never seen anything like it before.”

The animal stretches just five-and-a-half inches long and, because of its bulbous head, looks a bit like a tiny whale. It has rows of small-but-sharp teeth and, like the 1979 specimen, two small pockets that produce a luminous fluid—“a feature that may aid in attracting prey or eluding predators,” according to the authors of the new study. The creature bears additional resemblances to the original shark, which is female; they share a general shape, and the shape and placement of their fins. It had previously been determined that the female belonged to the Dalatiidae, or kitefin shark family, and the specimen was formally classified as Mollisquama parini. But when Grace and his colleagues—among them Henry Bart and Michael Doosey of Tulane University—took a closer look at the second pocket shark, they realized that it represented an entirely new species.

The team didn’t want to dissect the animal, because it is so rare. So the researchers studied it using a series of advanced, but non-invasive technologies, like a dissecting microscope and high-resolution CT scans. They also had the specimen imaged at the European Synchrotron Radiation Facility in France, which possesses technologies that can produce x-rays 100 billion times brighter than those used in hospitals.

Through these methods, the researchers observed several key distinctions between Mollisquama parini and the second specimen—distinctions that could not be explained by sexual dimorphism, or differences in appearance between males and females of the same species. For one, the new specimen has 10 fewer vertebrae than Mollisquama parini. And in contrast to Mollisquama parini, it has a pit organ—which may help sharks detect stimuli, like water currents—on its jaw, and light-producing organs known as photophores distributed irregularly around its body. Photophores are known to exist on other sharks, and according to the study authors, these organs “may facilitate predatory behavior and render [sharks] practically invisible from below.”

Researchers dubbed the glow-in-the-dark creature Mollisquama mississippiensis, or the American pocket shark. And Grace says that the classification of this little animal shows the hidden richness of the world’s oceans, which cover 70 percent of our planet and yet remain largely unexplored. Scientists don’t know how many species lurk in the vast depths of the sea—a difficult territory to explore. And so the sea continues to turn up surprises.

“The fact that only one pocket shark has ever been reported from the Gulf of Mexico, and that it is a new species,” Grace says, “underscores how little we know about the Gulf —especially its deeper waters—and how many additional new species from these waters await discovery.”