The sixgill sharks of the genus Hexanchus (Hexanchiformes, Hexanchidae) are large, rarely encountered deep-sea sharks, thought to comprise just two species: the bluntnose sixgill Hexanchus griseus (Bonaterre, 1788) and the bigeye sixgill Hexanchus nakamurai (Teng, 1962). Their distribution is putatively worldwide in tropical and temperate waters, but many verified records for these species are lacking, and misidentification is common. Taxonomic uncertainty has long surrounded H. nakamurai in particular, with debate as to whether individuals from the Atlantic constitute a separate species. Using 1,310 base pairs of two mitochondrial genes, COI and ND2, we confirm that bigeye sixgill sharks from the Atlantic Ocean (Belize, Gulf of Mexico, and Bahamas) diverge from those in the Pacific and Indian Oceans (Japan, La Reunion, and Madagascar) with 7.037% sequence divergence. This difference is similar to the genetic distance between both Atlantic and Indo-Pacific bigeye sixgill sharks and the bluntnose sixgill shark (7.965% and 8.200%, respectively), and between the entire genus Hexanchus and its sister genus Heptranchias (8.308%). Such variation far exceeds previous measures of species-level genetic divergence in elasmobranchs, even among slowly-evolving deep-water taxa. Given the high degree of morphological similarity within Hexanchus, and the fact that cryptic diversity is common even among frequently observed shark species, we conclude that these results support the resurrection of the name Hexanchus vitulus Springer and Waller, 1969 for bigeye sixgill sharks in the northwest Atlantic Ocean. We propose the common name “Atlantic sixgill shark” for H. vitulus, and provide new locality records from Belize, as well as comments on its overall distribution.
This evening, we have an exciting paper about describing a new species of hexanchid from 2018!!
Using DNA testing, the authors of this paper sought out to determine just how different, genetically, species like Hexanchus griseus (bluntnose sixgill shark) and Hexanchus nakamurai (bigeye sixgill shark) were from each other, and more specifically, Hexanchus nakamurai from the Indo-Pacific and Hexanchus cf. nakamurai from the northwest Atlantic. A total of 45 hexanchids were sampled. Each was measured for fork length, total length, depth captured, and a 5mm fin clipping was taken for the DNA testing. Twenty-three H. cf. nakamurai were sampled from Belize, the Bahamas, and the northern Gulf of Mexico. They were compared to five H. griseus samples, four H. nakamurai samples (from the Indo-Pacific), and one Heptranchias perlo (sharpnose sevengill shark) sample.
The results were fascinating. The genetic variation between H. cf. nakamurai (Gulf of Mexico) and H. nakamurai (Indo-pacific) was 7.037%. These are sharks that were originally thought to be the same species. Lets compare that to the genetic difference between the nakamurai and other hexanchids. H. griseus varied 7.964% from H. cf. nakamurai and 8.1% from H. nakamurai.
H. cf. nakamurai varied 8.572% from H. perlo, and H. nakamurai varied 9.125% from H. perlo. Inter-species genetic variation, in contract, was only within the range of 0.011% - 0.340%. The genus Squalus, from the order Squaliformes, in comparison, shows about 4% variation between different species of dogfish.
The authors propose and conclude that Hexanchus cf. nakamurai be its own species: Hexanchus vitulus, the Atlantic Sixgill Shark! And so, a new species was identified! That brings us, now, to six living species of hexanchids! The frilled shark, the bluntnose sixgill shark, the bigeye sixgill shark, the atlantic sixgill shark, the broadnose sevengill shark, and the sharpnose sevengill shark! This is amazing news, and really displays the powerful utility of DNA sequencing.
