No divergent evolution, despite restricted connectivity, between Atlantic and Indian Ocean goby populations

Southern Africa is a marine biodiversity hotspot that not only comprises faunal elements from the Atlantic and Indian Oceans, but also large numbers of endemic species. Using mitochondrial and nuclear DNA sequence data, we explored whether genetic structure in the endemic coastal goby Psammogobius knysnaensis, a species whose range straddles both biomes, is linked to the boundary between the two oceans. Subtle genetic structure was identified between Atlantic and Indian Ocean populations, with genetic diversity being lower in the Atlantic, and particularly on the west coast. Our results point to partial isolation between the populations associated with each biome that is most likely driven by the region’s oceanography, but unlike in other species, there is no evidence for distinct regional evolutionary lineages that are likely adapted to the environmental conditions prevalent in each region. The exclusive presence of P. knysnaensis in sheltered habitats (estuaries and lagoons) may protect this species from the severe impacts of cold water upwelling on the west coast.