SPATIAL COMPARISONS OF POPULATIONS OF AN INDIGENOUS LIMPET SCUTELLASTRA ARGENVILLEI AND AN ALIEN MUSSEL MYTILUS GALLOPROVINCIALIS ALONG A GRADIENT OF WAVE ENERGY

Abstract

In the 1970s, the Mediterranean mussel Mytilus galloprovincialis invaded the South African coast and spread rapidly to dominate much of the West Coast, indicating either the opportunity to occupy a vacant niche or its superior competitive capability over indigenous species. In Namaqualand on the West Coast it appears to compete with a large indigenous limpet, Scutellastra argenvillei, which is capable of forming dense, almost monospecific stands low on the shore. A survey on the Namaqualand coast indicated that the abundance of M. galloprovincialis changes with wave exposure. At wave-exposed locations, the mussel covered up to 90% of the primary substratum, whereas in semi-exposed situations it was never abundant. As the cover of M. galloprovincialis increased, the abundance and size of S. argenvillei on rock declined and it became confined to patches within a matrix of mussel bed. Both species were absent from sheltered shores and diminished where wave action was extreme. Comparisons with previous surveys indicated that exposed sites now largely covered by the alien mussel were once dominated by dense populations of the limpet. Therefore, the results of this survey provide circumstantial correlative evidence of a competitive interaction between M. galloprovincialis and S. argenvillei, and suggest that wave action mediates the strength of this interaction. The presence of mussel beds provides a novel settlement and living substratum for recruits and juveniles of S. argenvillei, albeit at much lower densities than in limpet patches. Adult limpets were virtually excluded from the mussel beds owing to their large size, which indicates the unsuitability of this habitat as a replacement substratum after competitive exclusion from primary rock space.

Afr. J. mar. Sci. 25: 195–212