Marine Mussels. Elizabeth Gosling

Чтение книги онлайн.

Figure 3.13 Performance of three indigenous South African mussels, Aulacomya atra, Perna perna and Choromytilus meridionalis, relative to Mytilus galloprovincialis. (a) Growth rate (mm in the first four years). (b) Total annual reproductive output as a percentage of body mass. (c) Survival rate after 24 months at a shore height experiencing 50% exposure to air per tidal cycle.

      Source: From Branch & Steffani (2004). Reproduced with permission from Elsevier.

Another example of interspecific competition is the contribution of M. galloprovincialis to the displacement of M. trossulus along much of its historic range in southern California (Shinen & Morgan 2009). Other examples are those between M. californianus and the sea palm, Postelsia palmaeformis, on the Pacific coast (Dayton 1973; Blanchette 1996). The sea palm grows quickly and may overgrow M. californianus, eventually causing mussels to be torn free in the waves. This creates the space necessary for settlement of spores and recolonisation by Postelsia. Then there is the competition between M. galloprovincialis and the large indigenous limpet, Scutellastra argenvillei, on the west coast of South Africa, where the mussel is capable of forming dense, almost monospecific stands low on the shore. A survey indicated that at wave‐exposed locations, the abundance of M. galloprovincialis changes with exposure (Steffani & Branch 2003). At such 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, becoming confined to patches within a matrix of mussel beds. 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 are 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.

The American slipper limpet, Crepidula fornicate, was unintentionally introduced to Europe in the 1870s with oysters imported for farming purposes. Since the limpet is a filter feeder, trophic competition and associated negative effects when epizootic on bivalves have been assumed (Figure 3.14). Thielges et al. (2005) were the first to experimentally test the effects of C. fornicata on survival and growth of its major basibiont, M. edulis. In two field experiments (each lasting two weeks), epigrowth by C. fornicata resulted in a four‐ to eightfold reduction in survival of mussels, equivalent to a mortality of 28 and 30%, respectively. Shell growth in surviving mussels with attached C. fornicata was three to five times lower compared to unfouled mussels, but similar to that with artificial limpets. As a causative agent, interference competition in the form of changes in small‐scale hydrodynamics due to C. fornicata stacks was suggested. This could result in a high energy expenditure for byssus production of mussels. In general, interference and not exploitation competition is suggested to be the major impact of epizootic C. fornicata on its basibionts (organisms that are host to epibionts) in Europe.