Interference competition in the intertidal mud snail Hydrobia ulvae: egestion rates revisited

Egestion in the intertidal mud snail Hrdrobia ulvae was investigated under conditions in which coprophagy did not occur, both in laboratory populations (comprising 11 experimental densities from 1000 to 120,000 ind m(-2)) and in a field population with an average density of 50,000 ind m(-2) (24 test densities spanning the local range of < 5000 > 120,000 ind m(-2)). In contrast to an earlier study that suggested marked reduction in egestion rate in H. ventrosa as a consequence of interference competition, no such effect occurred in the natural population of H. ulvae, or in the experimental populations except at densities of at least 100,000 ind m(-2). Neither did any significant increase in inactivity or movement off the experimental substrata Occur with increasing density. These results are discussed in relation to resource limitation in natural densities of mud snails.