THE INVASIVE BIOLOGY OF THE MUSSEL MYTILUS GALLOPROVINCIALIS ON THE SOUTHERN AFRICAN COAST

SUMMARY

The Palearctic mussel Mytilus galloprovincialis has recently invaded the southern African coast. Its present range extends from central Namibia to Hermanus, with an outlying population between Port Elizabeth and the Great Fish River. It is most abundant on the southern Benguela coast, where it occurs intertidally. It grows vigorously on suspended mussel ropes, but is absent from subtidal benthic substrata where the silt-tolerant Choromytilus meridionalis predominates. Intertidally, it outcompetes the indigenous mussel Aulacomya ater, by reason of its superior reproductive output, faster growth rate and greater tolerance to desiccation. It is a superior competitor to the intertidal limpet Patella granularis for primary rock space, which is a favoured settlement and recruitment medium for this species. The rate of Mytilus invasion in the Saldanha region between 1979 and 1988 was rapid, as evidenced by major changes in the diets of African black oystercatchers Haematopus moquini. Although at present largely confined to the cold temperate coasts of southern Africa, Mytilus grows rapidly in warm water and its eventual colonization of the warm temperate and subtropical coasts seems inevitable, as does consequent competition with the indigenous brown mussel Perna perna. No control measures are in operation. This species must be considered as out of control and uncontrollable, and research into its potential for commercial exploitation is needed.