Genetic structure and evolutionary history of two near near-shore shark species

Sara Krause

Abstract: Global phylogeography and identification of management units of the Galapagos shark (Carcharhinus galapagensis)

Although shark conservation is recognized as a high priority, with approximately one-third of all sharks and rays threatened with extinction, further research is needed for effective conservation management. Through the application of molecular approaches, conservation genetics provides valuable insights into population size, stock structure, demographic history, and genetic variability, crucial to informing successful conservation strategies. This study aimed to genetically assess the circumglobally distributed Galapagos shark (Carcharhinus galapagensis) to provide conservation and management implications. We analyzed genome-wide single nucleotide polymorphisms to evaluate the global population structure, estimate contemporary and historical effective population size (Ne), and test hypotheses for the directionality of population expansion and the influence of biogeographic processes on demographic history. Structure analysis identified a genetically distinct population in the Atlantic at Ascension Island, along with two previously confirmed Pacific stocks. Comparably low genetic diversity combined with low Ne of the geographically and genetically isolated Atlantic population raises conservation concerns and suggests a potential evolutionary significant unit. Contrasting patterns of genetic diversity and population divergence reject a hypothesized East Pacific origin with westward expansion, leaving the species' phylogeography inconclusive. We highlight the importance of assessing a species' demographic history to improve understanding of present-day patterns of genetic diversity, structure, and connectivity and suggest regional management to preserve isolated Galapagos shark populations.

Abstract: Grey nurse shark (Carcharias taurus) sampled off the Northern Territory of Australia genetically linked to the West Australian population

The critically endangered grey nurse shark (Carcharias taurus) is a circumglobally distributed coastal species, particularly susceptible to anthropogenic activities. Overexploitation has resulted in severe global population declines, including in Australian waters, where two genetically distinct populations are known to exist on the east and west coasts of the continent. However, occasional sightings of grey nurse sharks in the Timor Sea have been reported, and an individual was caught in Northern Territory waters. In this small-scale study, we aimed to assign this single C. taurus individual encountered outside its known distribution range to a population, based on previously identified mitochondrial haplotypes for this species. The alignment of mtDNA control regions of said individual with confirmed eastern and western Australian samples revealed a haplotype unique to Western Australia (haplotype E), assigning the individual caught in Northern Territory waters to the Australian west coast population. This result encourages consideration of conservation and management measures specifically for the Northern Territory and highlights the need for future research on the understudied grey nurse shark in the Timor Sea.