Population genetic divergence as consequence of past range expansion of the smooth hammerhead shark Sphyrna zygaena

Abstract

The Smooth Hammerhead Shark Sphyrna zygaena is a species with anti-tropical distribution in the eastern Pacific from California USA, to southern Chile with a remarkably gap in abundance in equatorial areas between southern Mexico and Central America. For temperate species, warm waters can represent oceanographic barriers limiting the gene flow between adjacent populations. The mtDNA control region was used to infer the phylogeography and historical demography of the Smooth Hammerhead in the eastern Pacific. An AMOVA comparing locations grouped into northern and southern eastern Pacific equatorial areas indicated significant genetic differentiation. Each population had a common haplotype separated by two mutational differences suggesting that divergence occurred very recently. The pattern of genetic differences matches with the anti-tropical distribution of S. zygaena in the eastern Pacific. Gene diversity was lower in the northern population when compared to the southern area, a pattern indicative of a recent founder event. Broadly, this pattern suggests a recent population expansion occurring 25,000–10,000 years. bp, during the last glacial cycle, when the drop of surface sea temperatures created favorable conditions at equatorial latitudes to cross the thermal barrier imposed by warm waters at equatorial latitudes. These genetically distinct population groups might lead to delineate a management plan that considers two separated stocks in the eastern Pacific.