Abstract

Strong biogeographic structure across the Southern Ocean has led to the recognition of multiple geographically restricted species in many coastal taxa. We examined evolutionary relationships within the Antarctic notothenioid fish genus Harpagifer using mitochondrial (COI, D-loop) and nuclear (ITS2, RHO) markers from seven nominal species distributed across southern South America, the Western Antarctic Peninsula, South Georgia, and sub-Antarctic islands of the South Indian Ocean. Genetic divergence was consistently low, with a maximum mitochondrial p-distance of similar to 1.2% between South American and Antarctic lineages and < 0.5% among Antarctic and sub-Antarctic Island taxa. Phylogenetic, species delimitation, and phylogeographic analyses recovered two shallowly divergent lineages broadly corresponding to Magellan (H. bispinis) and Antarctic-sub-Antarctic (H. antarcticus) distributions, with an estimated divergence during the Pleistocene (similar to 1.8-2.0 Ma). Most nominal island species were not supported as independent evolutionary lineages, and shared haplotypes were detected between Antarctica and geographically distant sub-Antarctic islands. These results indicate that diversification within Harpagifer is recent and substantially shallower than expected from its geographic distribution, supporting the existence of a single widespread Antarctic-sub-Antarctic species and highlighting that patterns of Southern Ocean regionalization are not consistently reflected in lineage diversification.