Genetic connectivity and phylogeography of the night shark (Carcharhinus signatus) in the western Atlantic Ocean: Implications for conservation management

The night shark, Carcharhinus signatus, is a mesopelagic, semi-oceanic shark species found only in the Atlantic Ocean. It is one of the most frequently caught sharks in pelagic longline fisheries and is classified as Vulnerable by the International Union for the Conservation of Nature (IUCN). Despite their prevalence in commercial fisheries, the population genetic structure of the night shark has not been assessed. The present study investigated the genetic diversity, genetic connectivity, and phylogeography of the species throughout the western Atlantic Ocean, based on complete mitochondrial control region (mtCR) sequence data (n = 152) and genotypic data from nine nuclear microsatellites (n = 119). The mtCR sequence revealed 19 haplotypes, with overall haplotype and nucleotide diversities of 0.74 (+/- 0.027) and 0.0034 (+/- 0.0019), respectively, whereas the nuclear microsatellite observed and expected heterozygosities were 0.408 and 0.421, respectively. There was significant population structure (CYRILLIC CAPITAL LETTER EFST = 0.429; P < 0.01) and isolation by distance (r = 0.65, P = 0.03) based on mtCR sequence data, but no genetic differentiation based on nuclear microsatellite analyses. The phylogenetic analyses support the existence of two matrilineal lineages, which diverged during the Pleistocene. Mitochondrial demographic analyses indicated a historical bottleneck effect followed by population expansion during the Pleistocene, whereas nuclear microsatellites did not detect a recent or a strong bottleneck. For conservation purposes, we advocate that the species should be considered to comprise at least two management units (MUs) in the western Atlantic Ocean. MU-specific catch quotas should be implemented throughout the range of the species given its low genetic diversity and vulnerability to overexploitation.