Insights Into Insular Isolation of the Bull Shark, Carcharhinus leucas (Müller and Henle, 1839), in Fijian Waters

Kerstin B J Glaus,Mahmood Shivji,Brian Stockwell,Sharon A Appleyard,Ciro Rico,Eric Clua,Amandine D Marie,Juerg M Brunnschweiler

doi:10.3389/fmars.2020.586015

Abstract

The bull shark (Carcharhinus leucas) is a large, mobile, circumglobally distributed high trophic level predator that inhabits a variety of remote islands and continental coastal habitats, including freshwater environments. Here, we hypothesize that the barriers to dispersal created by large oceanic expanses and deep-water trenches result in a heterogeneous distribution of the neutral genetic diversity between island bull shark populations compared to populations sampled in continental locations connected through continuous coastlines of continental shelves. We analyzed 1,494 high-quality neutral single nucleotide polymorphism (SNP) markers in 215 individual bull sharks from widespread locations across the Indian and Pacific Oceans (South Africa, Indonesia, Western Australia, Papua New Guinea, eastern Australia, New Caledonia, and Fiji). Genomic analyses revealed partitioning between remote insular and continental populations, with the Fiji population being genetically different from all other locations sampled (FST = 0.034–0.044, P &lt; 0.001), and New Caledonia showing marginal isolation (FST = 0.016–0.024, P &lt; 0.001; albeit based on a small sample size) from most sampled sites. Discriminant analysis of principal components (DAPC) identified samples from Fiji as a distinct cluster with all other sites clustering together. Genetic structure analyses (Admixture, fastStructure and AssignPOP) further supported the genetic isolation of bull sharks from Fiji, with the analyses in agreement. The observed differentiation in bull sharks from Fiji makes this site of special interest, as it indicates a lack of migration through dispersal across deep-water trenches and large ocean expanses.

Highlights

Reduced genetic variation, population isolation and genetic drift may lead to genetic divergence and speciation among terrestrial organisms inhabiting islands (Grant, 1985)
Genotyping by sequencing using DArT-SeqTM resulted in 59,601 single nucleotide polymorphism (SNP) prior to quality control filtering (Carson et al, 2014), which in turn resulted in 1,494 loci using a 99% call rate, read depth >7, and 2% minor allele frequencies (MAF)
Among the 1,494 SNPs that passed all quality control filters for the 215 individuals, zero SNPs were identified as outlier loci putatively under positive selection (FDR 1%)

Summary

Introduction

Population isolation and genetic drift may lead to genetic divergence and speciation among terrestrial organisms inhabiting islands (Grant, 1985). While terrestrial organisms inhabiting oceanic islands generally show significant levels of genetic differentiation when compared to their continental conspecifics, marine taxa with large dispersal capabilities are expected to show little divergence as their habitats appear to be continuous (Roberts, 1997). Data is limited when it comes to the quantification of population structure in coastal shark species inhabiting both island and continental coastal waters. Such studies can be vital for the identification of management units and metapopulations. An analysis examining the population structure of the tiger shark on a global scale using a suite of mitochondrial and nuclear (microsatellites) genetic markers revealed population isolation for the Hawaiian Islands (Bernard et al, 2016)

Methods

Results

Conclusion