Every beach an island—deep population divergence and possible loss of genetic diversity in Tylos granulatus, a sandy shore isopod

Biogeographic and phylogeographic patterns of sandy beach species are poorly understood, although these ecosystems are heavily impacted by anthropogenic pressures and are of elevated conservation concern. To contribute towards filling the knowledge gap on sandy beaches, we made use of phylogeographic approaches to determine levels of genetic structuring and diversity for Tylos granulatus, a large isopod with direct development distributed in South Africa and Namibia. Individuals (n = 214) were sampled from 9 locations encompassing the entire distribution range, and sequence data were generated for mtDNA cytochrome c oxidase subunit I (COI) and 16S. Results revealed high levels of population structuring between populations (ΦST = 0.11−0.96, p < 0.05), 2 deeply divergent lineages of T. granulatus and a new phylogeographic break in southern Africa. Northern populations are genetically more diverse, suggesting more stable evolutionary history compared to those in the south. Importantly, the patterns of divergence suggest unique evolutionary signals over short spatial scales (