MEPS 246:197-209 (2003)  -  doi:10.3354/meps246197

ABSTRACT: Growth rates, condition indices and shell shapes of Mytilus galloprovincialis, an invasive alien mussel that has colonised the west coast of South Africa, were measured at a series of sites with different wave exposure regimes in 2 regions 500 km apart. Wave action at the sites was determined by maximum wave-force dynamometers. M. galloprovincialis grew faster and had higher condition values on exposed than on sheltered shores, probably due to higher food availability at sites with greater water flow. Growth rates and condition values, however, declined at sites experiencing extreme wave action. This resulted in a polynomial relationship between the growth coefficient K, and wave force. Mussels grew slower in the southern region, where food concentrations are lower. The shells of M. galloprovincialis tended to be lower and narrower at exposed sites, perhaps reducing the effect of hydrodynamic forces. Mussel shells were thickest on sheltered and extremely exposed shores. The results are discussed in the context of the impact that M. galloprovincialis is likely to have on indigenous species. Because M. galloprovincialis is scarce and slow-growing at sheltered sites, its competitive effects there are likely to be minimal, whereas on exposed sites its density, growth and condition are highest and its effects will peak, diminishing again at extremely exposed sites.

KEY WORDS: Growth rate · Condition · Mussels · Wave force · Rocky intertidal · South Africa