Distribution and feeding preference of a high intertidal littorinid

Two sites on the west coast of Vancouver Island, British Columbia, Canada, were selected to study the potential role of snail herbivory in influencing the algal community structure in the upper intertidal zone. Natural algal communities consisted of monospecific and mixed stands of three macroalgae, Mazzaella cornucopiae (=Iridaea cornucopiae) (Rhodophyta) and two fucoids, Fucus distichus and Pelvetiopsis limitata. Population densities of the snail, Littorina sp., varied over time in all stands, with high levels in the summer and low levels in the winter months. Highest snail densities were found in monospecific stands of M. cornucopiae. Laboratory feeding experiments conducted independently with each alga indicated significant preference for, and consumption of, M. cornucopiae. Results of repeated measures analysis, however, indicated that consumption patterns varied over the duration of the feeding experiment. When snails were provided with all three algae simultaneously, M. cornucopiae was the most preferred species, followed by P. limitata and F. distichus. The preference of the littorine for M. cornucopiae was detected by both edibility and attractiveness. Behaviors in response to tide and light were tested for the littorines, and showed that the snails moved more actively towards their food during 'low tide' than during 'high tide'. Light had no significant effect on their feeding behavior. This study suggests that Littorina sp., a major herbivore in this habitat, should have a significant effect on the abundance of M. cornucopiae. Furthermore, this preferential feeding by the littorinid may indirectly affect competitive interactions between M. cornucopiae and the two fucoids in the upper intertidal zones.