Competition for Food Between the Intertidal Gastropods Acmaea Scabra and Acmaea Digitalis

The limpets Acmaea scabra and A. digitalis graze on encrusting microalgae in the upper intertidal and splash zones in central California, differing in habitat distribution but with considerable overlap. Controlled field experiments tested for interspecific competition in a habitat where both species were common but where A. scabra was similar than A. digitalis. Removal of A. digitalis from fenced exclosure plots resulted in significant growth increases in A. scraba, correlated with pronounced increases in the algal food supply. A converse effect on A. digitalis occurred in one of three experiments. It is concluded that in the habitat studied, limpet grazing strongly limited algal crop most of the year, and interspecific competition (exploitation type) for food limited growth rates and maximum size in A. scraba.