MEPS 539:153-163 (2015)  -  DOI: https://doi.org/10.3354/meps11516

ABSTRACT: Boundary currents are spectacular oceanographic features whose role as the main drivers of dispersal is often taken for granted. However, numerous genetic studies of passively dispersing coastal species have challenged this idea, and have identified gene flow in the direction opposite to the currents, pointing to a role of nearshore circulation in facilitating dispersal. We explored the influence of the Agulhas Current in eastern South Africa on mitochondrial DNA gene flow in the rocky shore limpet Siphonaria serrata. Our study design was particularly suitable to address this issue because (1) levels of genetic structure are high, suggesting that migrants should be readily distinguishable from native individuals, and (2) the Agulhas Current flows very close to the coast in parts of the study area, and inshore dispersal in the opposite direction is unlikely. We identified a northern and a southern lineage, and although evidence for southward dispersal was particularly strong, gene flow analyses also inferred some northward dispersal. An investigation of this result revealed that the single haplotype responsible for this finding could not be clearly assigned to either lineage, and the northward dispersal scenario is thus questionable. We conclude that genetically inferred dispersal may not be biologically meaningful when lineage sorting among regional populations is incomplete and suggest that this was a problem in most previous studies investigating gene flow in southern African coastal organisms. Despite their considerable potential, genetic methods have so far contributed little towards clarifying the role of nearshore circulation in facilitating population connectivity.

KEY WORDS: Agulhas Current · Asymmetrical gene flow · Coalescent framework · Direct developer · Indian Ocean · Limpet · Phylogeography · Siphonaria serrata · Southern Africa