IMIS

Using production of faecal pellets as a proxy for feeding rate, possible reciprocal effects of the widespread and abundant mudsnails Hydrobia ulvae and Hydrobia ventrosa on the rare Hydrobia acuta neglecta were investigated under the sea-water salinity conditions in which all three species occur naturally in East Anglia, UK.

Over a density range equivalent to 1000–34 000 m^-2, H. acuta and H. ventrosa (though not H. ulvae) at some point displayed an intraspecific reduction in egestion with increase in density. For H. acuta, every 5000 extra snails above a threshold of 16 000 m^-2 reduced egestion by up to 8%; and for H. ventrosa, every 5000 extra snails above a threshold of 8000 m^-2 reduced egestion by an average 7%.

Keeping the density of one test species constant at 4000 m^-2 whilst varying that of a sympatric one from 0 up to 30 000 m^-2 indicates that H. ulvae has a marked effect on H. acuta at all densities, decreasing its egestion by 10% for every 5000 m^-2H. ulvae also present, but that H. acuta has no such reciprocal effect on H. ulvae. H. ventrosa had no effect on egestion in H. acuta, although H. acuta did have some minor effect on H. ventrosa, on average decreasing its egestion by 3% for every 5000 m^-2H. acuta present.

The intensity of intraspecific competition within H. acuta, therefore, exceeds that exerted on H. acuta by H. ventrosa, and likewise intraspecific effects within H. ventrosa generally outweigh any interspecific effect from H. acuta. The effect of H. ulvae on H. acuta, however, exceeds that intraspecifically within H. acuta.