DISPERSAL OF THE INTERTIDAL SNAIL, NODILITTORINA-PYRAMIDALIS, IN RESPONSE TO THE TOPOGRAPHIC COMPLEXITY OF THE SUBSTRATUM

The littorinid gastropod, Nodilittorina pyramidalis (Quoy & Gaimard), is commonly found at the top of rocky shores in New South Wales, Australia. On sandstone shores around Sydney, the snails are usually restricted to patches of topographically complex rock during low tide and are mainly found within the pits and crevices in these patches. Differences in densities and size-structures of populations on adjacent patches suggest little intermixing of these populations. Experiments showed that adult Nodilittorina had limited dispersal which tended to retain them within these topographically complex patches. This was a response to the topographic complexity of the habitat, but other factors were necessary to retain populations within these patches for more than a few days. Small snails were more responsive to the complexity of the habitat than large snails. In addition, Nodilittorina actively avoided topographically simple areas on these shores and rapidly returned to complex patches after experimental transplantation. These patterns of behaviour differed from those shown by the co-existing littorinid, Littorina unifasciata Gray, although both species are found in the same areas of the shore and feed in the same way on the same source of food.