Genetic testing confirms that sixgill sharks residing in the Atlantic Ocean are a different species than their counterparts in the Indian and Pacific oceans.
The sixgill sharks are a genus, Hexanchus, of deepwater sharks characterized by a broad, pointed head, six pairs of gill slits, comb-like, yellow lower teeth, and a long tail.
Though only two extant species—the Bluntnose sixgill shark (Hexanchus griseus and the bigeyed sixgill shark (Hexanchus nakamurai )—were originally known, a third has now been found to exist also.
By analysing mitochondrial genes, a team of scientists led by assistant professor of biological sciences at the Florida Institute of Technology, Toby Daly-Engel, determined that there are enough genetic differences between what had long been considered a single species of bigeyed sixgill shark to rename the Atlantic variety Hexanchus vitulus, or the Atlantic sixgill shark.
These differences found are of the same magnitude—about seven percent— as the genetic distance between both Atlantic and Indo-Pacific bigeye sixgill sharks and the bluntnose sixgill shark. Such variation far exceeds previous measures of species-level genetic divergence in elasmobranchs, even among slowly-evolving deep-water taxa.
Measuring up to six feet in length, Atlantic sixgill sharks are far smaller than their Indo-Pacific relatives, which can grow to 15ft or longer. Source: Florida Institute of Technology
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.