Sixgill sharks are among the oldest vertebrates on the planet—they evolved over 250 million years ago—but they still remain elusive and mysterious. These aquatic critters dwell at extreme ocean depths, making them very difficult to study, and scientists are still learning new things about them. As Vittoria Traverso of Atlas Obscura reports, a team of marine biologists recently cleared up decades of uncertainty surrounding the Atlantic-dwelling sixgill shark, confirming that it is, in fact, a different species from its counterparts in the Indian and Pacific oceans.

As the researchers explain in a study published in Marine Biodiversity, sixgill sharks belong to the genus Hexanchus, which was long thought to be comprised of just two species: the bluntnose sixgill and the bigeye sixgill. These creatures dwell in tropical and temperate oceans around the world, and are distinguished by their saw-like lower teeth and unusual number of gills; most sharks only have five. For some time, however, uncertainty has surrounded the classification the bigeye sixgill (or Hexanchus nakamurai), with some researchers suggesting that Atlantic sixgills might constitute a separate species from those that dwell in the Indian and Pacific oceans.

Hoping to put an end to the debate, a team led by Toby Daly-Engel of the Florida Institute of Technology analyzed 1,310 base pairs of two mitochondrial genes from sharks in the Atlantic, Indian and Pacific oceans, according to Sci-News. They found enough genetic differences to classify Atlantic sixgills as a new species, which they have named Hexanchus vitulus.

“We showed that the sixgills in the Atlantic are actually very different from the ones in the Indian and Pacific Oceans on a molecular level, to the point where it is obvious that they're a different species even though they look very similar to the naked eye," Daly-Engel says in a statement.

In the past, sixgills have only infrequently come into contact with humans, but that could change as commercial fishing moves increasingly into deep oceans. Being able to properly classify sixgill sharks (and other deep-sea creatures) may prove vital to ensuring their survival in the future.

"Because we now know there are two unique species, we have a sense of the overall variation in populations of sixgills,” Daly-Engel explains in the statement. “We understand that if we overfish one of them, they will not replenish from elsewhere in the world.”