Experimental studies of interactions between two unionids, Elliptio waccamawensis and Leptodea ochracea: Variation in negative and positive effects

We conducted a field experiment in Lake Waccamaw, North Carolina, to elucid ate the responses of two unionids (Elliptio waccamawensis and Leptodea ochr acea) to varying population densities. Field enclosures at two depths maint ained each species in low, mixed culture, and high densities; intra- and in terspecific interactions were inferred from changes in growth, tissue mass, and shell mass. The experiment lasted one year (May, 1995-1996), with inte rim data collected in September, 1995. E. waccamawensis did not respond to density manipulations over the annual experimental period; however, its gro wth was significantly reduced in the presence of L. ochracea during summer at the deeper site. This negative effect on E. waccamawensis is consistent with competition for resources, and a laboratory experiment estimating muss el grazing rates provided supporting evidence. For L. ochracea, density had no effect on annual growth at the deeper site; at the shallow site, howeve r, high density significantly increased growth. Winter data revealed both n egative and positive density-dependence. L. ochracea in high density treatm ents grew significantly less at the deep site but demonstrated increased gr owth at the shallow location. Thus, both negative and positive interactions occur in this mussel assemblage and vary with season and depth.