Antipodean white sharks on a Mediterranean walkabout? Historical dispersal leads to genetic discontinuity and an endangered anomalous population

Abstract

The provenance of white sharks (Carcharodon carcharias) in the Mediterranean is both a conundrum and an important conservation issue. Considering this species's propensity for natal philopatry, any evidence that the Mediterranean stock has little or no contemporary immigration from the Atlantic would suggest that it is extraordinarily vulnerable. To address this issue we sequenced the mitochondrial control region of four rare Mediterranean white sharks. Unexpectedly, the juvenile sequences were identical although collected at different locations and times, showing little genetic differentiation from Indo-Pacific lineages, but strong separation from geographically closer Atlantic/western Indian Ocean haplotypes. Historical long-distance dispersal (probably a consequence of navigational error during past climatic oscillations) and potential founder effects are invoked to explain the anomalous relationships of this isolated 'sink' population, highlighting the present vulnerability of its nursery grounds.

Occurrence of great white shark, <I>Carcharodon carcharias</I> (Elasmobranchii: Lamniformes: Lamnidae), off Corsica (northern Mediterranean): historical and contemporary records

“…Consequently, of the 191 specimens reported in the literature, 45 specimens having less than 4 m in total length were juveniles, and 146 other specimens had a larger size, exceeding 4 m, were adults, the latter significantly outnumbered the former (χ 2 = 38.37, P < 0.05). Gubili et al (2010) sequenced mitochondrial DNA in four specimens of Mediterranean great white sharks, collected from four different sites at different periods of the year, and noted that juvenile sequences were identical. Gubili et al (2010) showed little genetic differentiation from Indo-Pacific lineages, but strong separation from geographically closer Atlantic and western Indian Ocean haplotypes.…”

“…Gubili et al (2010) sequenced mitochondrial DNA in four specimens of Mediterranean great white sharks, collected from four different sites at different periods of the year, and noted that juvenile sequences were identical. Gubili et al (2010) showed little genetic differentiation from Indo-Pacific lineages, but strong separation from geographically closer Atlantic and western Indian Ocean haplotypes. These observations suggest that an isolated Mediterranean population of white sharks remains a justified hypothesis.…”

DNA from historical and trophy samples provides insights into white shark population origins and genetic diversity

“…Nonetheless, genetic analysis of Australian white shark populations suggests estimates of contemporary effective population sizes approach levels at which adaptive potential may be lost (Blower et al 2012). A similar concern was expressed following the observation that several contemporary white sharks sampled from across the Mediterranean all had the same Pacific clade mitochondrial haplotype (Gubili et al 2011). In Turkish waters white sharks are considered extinct in the Sea of Marmara, although contemporary records of neonates in the northern Aegean Sea suggest nearby breeding grounds (Kabasakal 2014).…”

“…Nevertheless, each overlap contained only a single polymorphic site, the remaining amplicons allowing adequate haplotypic assignment (Pardini et al 2001). Two additional primers were designed to target polymorphisms diagnostic of potential Atlantic haplotypes as distinct from those of Mediterranean or Pacific origin (Gubili et al 2011). Of these, only one (D-loop7) was successfully used in this analysis, yielding a 206 bp product.…”

“…We have also used this method to recover sequences from dried skin and jaw cartilage of Mediterranean and Pacific Ocean white sharks collected 34 to 129 yr ago. This resource was used to test whether the prevailing Pacific origin hypothesis (Gubili et al 2011) of Mediterranean white sharks can be refuted by sequencing the D-loop of more individuals.…”