Despite having evolved some 250 million years ago, sixgill sharks are still some of the most mysterious creatures living in the oceans. So elusive are these deep-sea predators that researchers have only just figured out that there is a new species living in the Atlantic.
Sixgill sharks are unusual among sharks for being the only extant species to have an extra pair of gill slits (while a few others have yet another pair still). The sixgills have long been split into two species – the bluntnose sixgill shark (Hexanchus griseus) that can live 2,500 meters (8,200 feet) beneath the surface and the bigeye sixgill shark (Hexanchus nakamurai) that is smaller and tends to live closer to the surface, although still out of the reach of most biologists.
The bigeye sixgill has been found in most major oceans, and until now was considered a single species. But a new paper reveals that the sharks that live in the Atlantic Ocean are a different species than those found in the Indian and Pacific Oceans. This newly described Atlantic species has now been called Hexanchus vitulus.
Publishing their work in the journal Marine Biodiversity, the team used genetics to settle the debate of whether or not there were multiple species of the deep-sea predators. By analyzing 1,310 base pairs of two mitochondrial genes, they were able to confirm that – despite looking almost identical – the Atlantic population was indeed different enough to be elevated to species level.
“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,” explains Toby Daly-Engel, who co-authored the paper describing the new species, in a statement.
This is important because even though little is known about the sharks or even the deep abyss in which they live, fishing vessels are increasingly probing the depths in the search of new stock.
“Because we now know there are two unique species, we have a sense of the overall variation in populations of sixgills,” says Daly-Engel. “We understand that if we overfish one of them, they will not replenish from elsewhere in the world.”