TEMPORAL CHANGES IN AN INTERACTION BETWEEN AN INDIGENOUS LIMPET <i>SCUTELLASTRA ARGENVILLEI</i> AND AN ALIEN MUSSEL <i>MYTILUS GALLOPROVINCIALIS</i>: EFFECTS OF WAVE EXPOSURE

Abstract

A previous survey of 15 sites off the Namaqualand coast on the west coast of South Africa provided evidence of a competitive interaction between an alien mussel Mytilus galloprovincialis and an indigenous limpet Scutellastra argenvillei, and indicated that wave action mediates the strength of this interaction. In this study, the temporal persistence of these patterns was tested by selecting six sites, ranging from sheltered to very exposed, and monitoring them over a two-year period. The patterns remained consistent over time. Both S. argenvillei and M. galloprovincialis were perpetually absent from the most sheltered site. The limpet consistently dominated rock space at the semi-exposed site where mussel cover was always low. At exposed sites, M. galloprovincialis was dominant but its percentage cover varied temporally as a result of wave action, creating free space and allowing temporary expansion of limpet patches. However, with time the mussel recolonized the cleared rock space and repeatedly displaced the limpet from it. This provides additional observational evidence of competition for space between the alien invader and the indigenous limpet. It is concluded that S. argenvillei has a spatial refuge from M. galloprovincialis in areas with moderate wave action. However, the limpet is likely to become locally extinct on exposed shores because mussels outcompete it there and rapidly recolonize patches from which they have been temporarily eliminated by wave action.

Afr. J. mar. Sci. 25: 213–229