A review of the utility and application of relatedness and kinship in elasmobranchs

Understanding relatedness is fundamental to the fields of reproductive and social behaviour, trait-based analysis, population connectivity, and threatened species management. While relatedness has been studied in a broad range of terrestrial and aquatic taxa, research is often limited for species that are difficult to sample or persist in low densities. Elasmobranchs (sharks and rays) are one the most imperilled vertebrate clades in the world, with one third of all species being threatened with extinction. In this review, we examine the application of relatedness in studies of elasmobranchs, explore common challenges and limitations faced, provide best practice recommendations, and highlight knowledge gaps to guide future work. A systematic search identified 107 studies published between 2001 and November 2024 that focus on 65 species from 10 orders and 25 families. These studies used relatedness to investigate the reproductive behaviour, population genetics, demography, and social behaviour of elasmobranchs. Results highlight significant taxonomic biases, temporal patterns in marker use and research categories, and the need for a unified analytical framework to guide future work. This review provides a foundation for understanding how relatedness has been applied to date in studies of elasmobranchs and outlines future directions to advance the field.