Resurrection of the sixgill shark Hexanchus vitulus Springer & Waller, 1969 (Hexanchiformes, Hexanchidae), with comments on its distribution in the northwest Atlantic Ocean
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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.
KeywordsSystematics Mitochondrial DNA Phylogenetics Speciation Elasmobranchs
The authors thank the Save Our Seas Foundation, the Summit Foundation, the International Union for Conservation of Nature, the Rufford Foundation, the University of West Florida, Florida Institute of Technology, and the Deep-C Consortium through the Gulf of Mexico Research Initiative who provided financial support for this project. I.E.B. and R.T.G. thank the Belize Fisheries Department and the fishers and captains in Belize, especially D. Castellanos, R. Lima, E. Muschamp, M. Alamina, E. Cuevas, and D. Garbutt. R.D.G. thanks E. and A Brooks, S. Williams, D, Chapman, L, Howey-Jordan, D, Abercrombie and L. Jordan for assistance in collecting samples in the Bahamas, and J. Imhoff and C. Peterson for assistance in collecting samples from the Gulf of Mexico. T.D.E. thanks J. Eble, M. Pfleger, C. Hitchcock, and A. Koch for help with genetic analyses, M. Boudreau and C. Meyer for logistical support, and J. Kiszka for H. nakamurai tissue from La Reunion. Thanks also to J. Carlson, who oversaw the bottom longline observer program and implemented program-wide biological sampling. S.J.B.G. and M.P.E. thank the observers, C. Aguero, P. Bear, J. Combs, M. Lee and J. Patterson, and port sampler D. Fable for collecting the samples. Sixgill sharks were captured in Belize under Fisheries Permit 9-16 and in the USA under NOAA Fisheries Highly Migratory Species Division Exempted Fishing Permits and the US Federal Register (HMS-EFP-07-01, HMS-EFP-08-01, HMS-EFP-09-01, FR Doc. E9-20489).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed by the authors.
Sampling and field studies
All necessary permits for sampling and observational field studies have been obtained by the authors from the competent authorities and are mentioned in the acknowledgements, if applicable.
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