Mechanisms and consequences of competition between an alien mussel, Mytilus galloprovincialis, and an indigenous limpet, Scutellastra argenvillei

The influence of an alien mussel Mytilus galloprovincialis (Lamarck) on an indigenous limpet Scutellastra argenvillei (Krauss) was investigated on the west coast of South Africa by comparing four situations in which limpets occupied rock patches surrounded by mussels: (1) 'established patches' of high-density adult limpets; (2) 'experimentally cleared patches' from which mussels were manually removed; (3) 'naturally disturbed patches' where mussels had been eliminated by wave action; and (4) 'half-density' patches formed by thinning established patches. Limpets in established patches were most effective in retarding settlement and lateral invasion by mussels, but all patches shrank due to mussel encroachment. As patches shrank, limpet density fell in established and naturally disturbed patches due to emigration. Limpets living on mussel beds were small, sparse and never achieved sexual maturity; 60% shifted to rock, whereas only 7% moved from rock to mussels. The limpets showed highest fidelity to established patches (79% after 12 months) and lowest fidelity in naturally disturbed patches (20%). Thinning of established patches reduced fidelity to 26%. Mussel beds did not provide a suitable alternative substratum for S. argenvillei after they displaced the limpets from rocks. Moreover, elimination of mussels by waves failed to allow S. argenvillei to re-establish dense, adult populations resembling those prior to arrival of the mussel. Adults of S. argenvillei feed collectively by trapping kelp blades. On the topographically complex mussel beds limpets cannot feed this way. In sum, on exposed shores where M. galloprovincialis achieves maximal recruitment and growth, S. argenvillei is incapable of preventing mussel encroachment and is likely to become completely displaced. Semi-exposed shores do, however, offer a refuge preventing global extinction of the limpet.