The ecology and impacts of the introduced basket clam, Corbula gibba in Port Phillip Bay, Australia

This thesis describes the ecology and impact of the introduced European basket clam, Corbula gibba (Olivi, 1792), in Port Phillip Bay, Victoria. A bay-wide survey conducted in 1998 found C. gibba throughout the bay but it was significantly more abundant in deeper, muddier sites. Life history studies carried out at three sites found that two sites were dominated by a single cohort of large adults while the third site had one cohort of adults and one of juveniles. This juvenile cohort grew at a rate of about 4 mm2 per month but experienced high mortality, with only 0.5% surviving a six month period. Longer term abundance patterns available at St. Leonards, and supplemented by this study, found that, between 1969 and 1999, adult density varied between 0 and 106 individuals per m2 while juveniles were found in varying densities in 5 of 6 years sampled. The effect of anthropogenic disturbance on resident populations and subsequent invasion by C. gibba into disturbed sites was assessed experimentally using scallop dredging. Scallop dredging did not affect resident populations but the impact on subsequent invasion by C. gibba could not be determined as recruitment failed to occur over the study period and adult migration was not observed. The effect of ambient densities of C. gibba on endemic species such as the commercial scallop, Pecten fumatus, was investigated experimentally in the field. These experiments found that C. gibba significantly reduced the size and growth of juvenile P. fumatus to a greater extent than an equivalent number of conspecific scallops. Competition for shared food resources did not appear to be the mechanism underlying this impact, however, as dietary studies found that C. gibba was not a superior competitor. Additional studies found that C. gibba did not affect the recruitment of the native bivalves, Mytilus edulis and Electroma Georgiana, but significantly reduced conspecific recruitment. This effect is likely to have implications for the structure of C.gibba populations in the bay.