An international team of marine biologists from the United States and Belize has confirmed that sixgill sharks residing in the Atlantic Ocean are a different species than their counterparts in the Indian and Pacific oceans. The team’s findings were published online this month in the journal Marine Biodiversity.
An adult Atlantic sixgill shark (Hexanchus vitulus) swims in the waters off Belize. Image credit: Ivy Baremore, MarAlliance.
The sixgill sharks of the genus Hexanchus are large, rarely encountered deep-sea sharks.
With ancestors dating back over 250 million years, well before dinosaurs, they are among the oldest creatures on Earth.
Yet the fact that they reside at extreme ocean depths, sometimes thousands of feet below the surface, has made them especially challenging to study.
Using 1,310 base pairs of two mitochondrial genes, Dr. Toby Daly-Engel from Florida Institute of Technology and co-authors determined there are enough genetic differences between what had long been considered a single species, the bigeye sixgill shark (Hexanchus nakamurai), to rename the Atlantic variety 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,” Dr. Daly-Engel explained.
Measuring up to 6 feet (1.8 m) in length, Atlantic sixgill sharks are far smaller than their Indo-Pacific relatives, which can grow to about 15 feet (4.5 m) long.
They have unique, saw-like lower teeth and six gill slits, as their name suggests. Most sharks have five gill slits.
“With their new classification, Atlantic sixgill sharks will now have a better chance at long-term survival,” Dr. Daly-Engel said.
“Because we now know there are two unique species, we have a sense of the overall variation in populations of sixgills.”
“We understand that if we overfish one of them, they will not replenish from elsewhere in the world.”
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Toby S. Daly-Engel et al. Resurrection of the sixgill shark Hexanchus vitulus Springer & Waller, 1969 (Hexanchiformes, Hexanchidae), with comments on its distribution in the northwest Atlantic Ocean. Marine Biodiversity, published online February 13, 2018; doi: 10.1007/s12526-018-0849-x
