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 tem- perate 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 elas- mobranchs, 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 BAtlantic sixgill shark^ for
H. vitulus, and provide new locality records from Belize, as well as comments on its overall distribution.

Toby S. Daly-Engel1, Ivy E. Baremore, R. Dean Grubbs, Simon J. B. Gulak, Rachel T. Graham, Michael P. Enzenauer