AGGREGATION OF THE LITTORINID SNAIL LITTORINA UNIFASCIATA IN NEW-SOUTH-WALES, AUSTRALIA

Littorina unifasciata is a small littorinid snail found at mid- to high-shore levels on rocky shores in Australia. Much has been published on patterns of distribution and abundance of L. unifasciata on different shores, at different heights on the shore and in different microhabitats, but there is little information on patterns of aggregation of this species. Nevertheless, this species tends to form very distinct clusters when emersed during low tide. This study is a quantification of aggregation by L. unifasciata on different shores, levels on the shore and in different microhabitats. The use of patches of habitat of different size during low tide and high tide is also examined. Aggregation was variable from time to time and place to place and individual snails did not show a consistent tendency to aggregate on different occasions. Snails were less aggregated when they spread out to feed during high tide than when emersed during low tide, but this was only found at very small spatial scales because snails do not move far to feed. Therefore, the cues that operate to cause the snails to aggregate in same patches and not others appear to be operating at the scale of centimetres, rather than metres. This suggests that many of the traditional explanations for patterns of aggregation in intertidal animals, such as those having to do with desiccation or temperature, may not be important in this species. This phenomenon may be more widespread among intertidal animals than is currently recognised, but it will be necessary to examine patterns and test models about ecological processes at small spatial scales within shores, in addition to among shores, before this can be determined.