They are among the oldest creatures on earth, with ancestors dating back over 250 million years, much before dinosaurs, but their existence has been acknowledged after decades of uncertainty. It has now been confirmed that the sixgill sharks found in the Atlantic Ocean are not of the same species as the ones in Pacific and Indian oceans. A team of scientists led by Toby Daly-Engel of Florida Institute of Technology has named the new sea species as the Atlantic sixgill shark.
It has been a challenge to study the species as they reside at extreme ocean depths, occasionally reaching thousands of feet below the surface of the ocean. Scientists used 1,310 pairs of two mitochondrial genes to determine that there are enough genetic differences between what had long been considered a single species, Hexanchus nakamurai to rename the Atlantic variety, Hexanchus vitulus. The findings were published in the journal, Marine Biodiversity.
Atlantic sixgill sharks, measuring up to 6 ft in length, are far smaller than Indo-Pacific counterparts that touch the 15 ft-plus mark. They have unique, saw-like lower teeth and six gill slit, one more than other sharks.
Meanwhile, another sea species, beluga whale that spend summers feeding in the Arctic are diving deeper and longer to find food than in earlier years when sea ice covered more of the ocean for longer periods. Reduction in sea ice in the Arctic impacted animals like polar bears that rely on frozen surfaces for feeding, mating and migrating. But sea ice loss is changing the habitat of the Arctic and affecting other species in more indirect ways.
Two genetically distinct beluga populations spend winters in the Bering Sea, then swim north in the early summer as sea ice melts and open water allows them passage into the Beaufort and Chukchi seas. They feast all summer on fish there and invertebrates before travelling back south in the fall. Both populations are considered healthy.
The researchers analysed migration data collected intermittently from two different periods — referred to in the paper as “early” and “late” — for two beluga populations, covering the years 1993-2002 and 2004-2012. Satellite-linked tags attached to the whales tracked their movements around and away from the high Arctic feeding grounds. Dive-depth data were collected for only one population, the Chukchi belugas, because the other population’s tags did not have those capabilities. Researchers also tracked sea ice cover in the Arctic over these two periods and found that the ice declined substantially from the first to the second period.