Knowledge of population structure, connectivity and effective population size remains limited for many marine apex predators, including the bull shark Carcharhinus leucas. This large-bodied coastal shark is distributed worldwide in warm temperate and tropical waters, and uses estuaries and rivers as nurseries. As an apex predator, the bull shark likely plays a vital ecological role within marine food webs, but is at risk due to inshore habitat degradation and various fishing pressures. We investigated the bull shark's global population structure and demographic history by analysing the genetic diversity of 370 individuals from 11 different locations using 25 microsatellite loci and three mitochondrial genes (CR, nd4, cytb). Both types of markers revealed clustering between sharks from the Western Atlantic and those from the Western Pacific and the Western Indian Ocean, with no contemporary gene flow. Microsatellite data suggested low differentiation between the Western Indian Ocean and the Western Pacific, but substantial differentiation was found using mitochondrial DN A. Integrating information from both types of markers and using Bayesian computation with a random forest procedure (ABC-RF), this discordance was found to be due to a complete lack of contemporary gene flow. High genetic connectivity was found both within the Western Indian Ocean and within the Western Pacific. In conclusion, these results suggest important structuring of bull shark populations globally with important gene flow occurring along coastlines , highlighting the need for management and conservation plans on regional scales rather than oceanic basin scale.
Microsatellite genotypes of the bull shark samples
Sampling information are indicated for each individual (date of sampling, sampling locality, sex, total length) as well as the identified mitochondrial haplotypes for each gene (control region, nicotinamide adenine dinucleotide dehydrogenase subunit 4, cytochrome b) and the concatenated mitochondrial haplotype (CR-nd4-cytb). Microsatellite genotypes for the 25 loci are also indicated (2 alleles per locus, 3-digits per allele). Missing data are indicated by "NA" or by a question mark.