Influences of tidal conditions, temperature and desiccation on patterns of aggregation of the high-shore periwinkle, Littorina unifasciata, in New South Wales, Australia

Abstract

The small high-shore littorinid, Littorina unifasciata, is the most abundant gastropod on many shores in New South Wales, Australia. It is usually found in dense clusters while emersed during low tide. Previous work has shown that the degree of aggregation varies spatially among shores, between different heights on the shore, between habitats with different aspect and among replicate sites in the same general area. Factors that might influence the degree of aggregation were not identified. In this study, aggregation at different stages of the tide and under different weather conditions was examined to test the models that aggregation and dispersal in L. unifasciata occur in response to the drying and wetting of the substratum by the tides or weather. Aggregation was found to be greater when the substratum was dry, irrespective of the state of the tide. It was proposed that environmental conditions would be harsher during neap than spring tides and when the time of low tide was nearer mid-day and, hence, snails would be more aggregated at these times. Aggregation was therefore examined with respect to the timing and duration of low tide. Although aggregation could be correlated with the time of low tide in some mid-shore sites, there was no such correlation higher on the shore. In addition, the time of low tide accounted for very little of the variability in aggregation from time to time. Finally, the influence of high temperatures and desiccation on the tendency to aggregate and the effects of aggregation on the temperatures and water reserves of the snails were examined experimentally in the field. Aggregated snails had larger water reserves and greater temperatures than did solitary animals, but neither the temperature of the snails, nor their water reserves, influenced their tendency to aggregate.