Recruitment and biotic interactions of the blue mussel, Mytilus edulis and the ribbed mussel, Geukensia demissa

Competition between populations on rocky intertidal shores can lead to the exclusion of one or more species from the assemblage. Research suggests that competition in soft-sediment assemblages leads to a reduction in growth but rarely increases mortality rates and therefore, does not result in exclusion. Additionally, the effect of competition on assemblage structure is modified by the amount of recruitment to a site. I proposed that if competition for food exists among suspension feeders, the intensity of this interaction would change with variation in recruitment. To explore this possibility, I chose two bivalves, Mytilus edulis and Geukensia demissa, that co-occur in a New Jersey salt marsh. At this site, juvenile M. edulis often recruit onto the shells of adult G. desmissa. I examined three aspects of these populations. First, I quantified the amount of M. edulis recruitment to the site. Recruitment was monitored weekly in three microhabitats during the spring of 1994–1996. Recruitment was variable at both temporal scales and two spatial scales. The second aspect of these bivalve populations examined was their microhabitat specific mortality rates. A failure-time model was used to determine the median number of days each species survived during the summer of 1995. Median survival time of G. demissa was much greater than the median survival time of M. edulis. Geukensia demissa survived best in pools but M. edulis survived longer in Spartina patches. The third aspect of this research was to determine if competition exists between these populations. I transplanted juvenile M. edulis onto the shells of adult G. demissa. Geukensia demissa were collected after approximately four months and growth was determined by the amount of new shell growth and by dry body and gonad weights. Gonad weight was significantly reduced for G. demissa grown in the presence of M. edulis. These results suggest that although M. edulis is short lived, sufficient recruitment results in reduction of the amount of gonad tissue in G. demissa but is unlikely to influence assemblage structure