Effects of sediment type and intra- and interspecific competition on growth rate of the marine snails Hydrobia ulvae and Hydrobia ventrosa

The combined effects of sediment grain size and intra- and interspecific co mpetition on growth rate of Hydrobia ulvae and H. ventrosa were investigate d in a 2-month laboratory experiment. Individuals of H. ulvae grew faster t han H. ventrosa at all competition intensities. Growth rate did not differ between fine- and coarse-grained sediments in any of the species. This sugg ests that other factors than the sediment quality cause the often observed size differences between Hydrobia snails from these sediment types in natur e. Competition from 1.6 to 1.9 times larger competitors affected growth of the smaller target snails. Competition between the species was asymmetrical and, in both species, per capita effect of competition was stronger from H . ulvae than from H. ventrosa, regardless of sediment type. From growth, Lo tka-Volterra competition coefficients were estimated to 0.2 from H. ventros a on H. ulvae and 1.8 from H. ulvae on H. ventrosa. Since competition betwe en the species seems to be independent of sediment grain size, the shift in numerical dominance in nature. from H, ulvae on coarse-grained sediment to H. ventrosa on fine-grained, could not be explained with different competi tive ability in different environments. (C) 2000 Elsevier Science B.V. All rights reserved.