Competitive interactions between two species of intertidal herbivorous gastropods from Victoria, Australia

The competitive interactions between two species of intertidal herbivorous gastropods, Austro-cochlea constricta (Lamarck) and Bembicium nanum (Lamarck), were examined experimentally at two times of the year on a rocky shore in Victoria, Australia. There was little mortality of either species during the experiment, except for A. constricta at the greatest experimental densities in summer/autumn. Competition for food does not appear to regulate the densities of either species, which contrasts markedly with the strong intraspecific and interspecific competitive interactions between herbivorous gastropods determined from similar but more extensive experiments on rocky shores in New South Wales. For each species, the effects of competition for food on body weights were greater in summer/autumn than in winter/spring, corresponding to seasonal changes in the abundance of macroalgae on the shore. For A. constricta, intraspecific competition had a stronger negative effect on body weight than interspecific competition with B. nanum, whereas there was no such difference between intraspecific and interspecific competition for B. nanum. The exclusion of the pulmonate limpet Siphonaria diemenensis Quoy et Gaimard in winter/spring had a negative effect on body weights of A. constricta but no effect on B. nanum. These competitive interactions may be a result of the different feeding behaviours of the two species of snail.