Coastline topography affects the distribution of indigenous and invasive mussels

Coastline topography has important effects on nearshore oceanography, larval trans- port, settlement and the adult distribution of benthic organisms. The resultant physical regime also influences interactions between invasive and indigenous species. Such interactions can alter inter- tidal communities dramatically, including the local extinction and replacement of native species. We examined the effect of bays and their associated headlands on the distribution of indigenous (Perna perna) and invasive (Mytilus galloprovincialis) mussels along 500 km of the south coast of South Africa. Within this single biogeographic region, mussel cover was estimated at 22 sites across 4 bays and the intervening open coast. Given that mussel biomass is greater at intermediate levels of wave exposure and that wave exposure is strongly dependent upon coastline topography, we hypothesised that mussel cover would be greater in bays, and that bays would specifically favour M. galloprovin- cialis which is more easily disturbed by strong waves. The 2 species show partial vertical separation into 3 zones within the lower eulittoral zone. Both species had significantly greater cover within bays. There was, however, an interaction between bay and zone for P. perna, and the effect of bays was strongest within the preferred zones of each species. Although the overall effect of bay was stronger for M. galloprovincialis than for P. perna, this resulted from the strong spatial structure identified for the M. galloprovincialis distribution using semivariogram analysis. Overall findings illustrate how coastline topography and local processes operate in synchrony to affect the dynamics of invasive and indigenous intertidal species.