Genetic diversity in the American elephantfish (Chimaeriformes: Callorhinchus callorynchus) and among its congeners

Understanding genetic population structure and connectivity is essential for effective species-specific management and conservation strategies. The American elephantfish Callorhinchus callorynchus is targeted and retained as incidental catch in commercial and recreational fisheries in Chile and Argentina. Its wide-ranging distribution across southern South America may require transnational co-operation to ensure sustainable use, but its current population structure is not known. In this work, we analysed the levels of genetic diversity and differentiation within C. callorynchus in South America using two mitochondrial markers, the control region (CR) and the cytochrome oxidase subunit I gene (CO1). Moreover, we assessed levels of genetic diversity within, and divergence among, the three extant callorhinchids (genus Callorhinchus), a group that exhibits allopatric geographical distributions in the southern hemisphere. Overall, sequence analyses of the mitochondrial CR and the CO1 revealed extremely low levels of sequence variation both within and among Callorhinchus species. Genetic homogeneity was found throughout the range of C. callorynchus coupled to low-frequency haplotype sharing across spatially distant locations in Chile and Argentina, suggesting gene flow along the South American coast. Moreover, our analyses supported a scenario of recent population expansion of the species in South America. Given the absence of dispersive eggs or juvenile stages in chondrichthyans, gene flow is mainly mediated by actively swimming adults. Based on the available data, gene flow in callorhinchids appears to occur along continuous coastal regions, with deep oceanic waters serving as strong barriers. Findings here provide an important baseline for future research on dispersal and gene flow in holocephalans.