You are here:
EXPERIMENTAL STUDIES OF INTERACTIONS BETWEEN TWO UNIONIDS, ELLIPTIO WACCAMAWENSIS AND LEPTODEA OCHRACEA: VARIATION IN NEGATIVE AND POSITIVE EFFECTS
Citation:
DiDonato, G. T. AND A. E. Stiven. EXPERIMENTAL STUDIES OF INTERACTIONS BETWEEN TWO UNIONIDS, ELLIPTIO WACCAMAWENSIS AND LEPTODEA OCHRACEA: VARIATION IN NEGATIVE AND POSITIVE EFFECTS. JOURNAL OF FRESHWATER ECOLOGY 16(2):303-316, (2001).
Description:
Historical records indicate dense and diverse assemblages of unionid mussels (Bivalvia: Unionidae) in a variety of aquatic communities, but the potential structuring role of competitive interactions among these organisms has not been adequately examined. We report here the results of a field experiment in Lake Waccamaw, North Carolina, designed to elucidate the responses of two abundant unionid mussels, Elliptio waccamawensis and Leptodea ochracea, to varying population densities. Field enclosures (0.25m2) established at two depths maintained each species in low, mixed culture, and high density conditions; intra- and interspecific interactions were inferred from changes in growth, tissue mass, or shell mass. The experiment was conducted from May, 1995, to May, 1996, and interim data were collected in September, 1995, to investigate seasonal (i.e., summer, winter) variation. Statistical analyses consisted of multivariate and univariate ANCOVA followed by preplanned contrasts to identify specific density responses. E. waccamawensis did not respond to density treatments over the full experimental period; however, growth of individual E. waccamawensis was significantly reduced in the presence of L. ochracea during summer at the deeper site. The negative effect of L. ochracea on E. waccamawenisis is consistent with competition for reduced resources, and a laboratory experiment estimating mussel grazing rates provided supporting evidence for that interaction mechanism. For L. ochracea over the annual period, density had no effect on growth at the deeper site; however, high densities at the shallow site significantly increased growth. Winter data revealed both negative and positive density-dependence. Individual L. ochracea in high density treatments grew significantly less at the deep site but demonstrated increased growth at the shallow location. Thus, interactions consistent with both competition and facilitation occur in this mussel assemblage and vary witih season and depth. While evidence of facilitation has emerged from studies of marine benthic invertebrates, this is a novel discovery for freshwater mussels.