SPATIAL PATTERNS OF SHELL SHAPE OF 3 SPECIES OF COEXISTING LITTORINID SNAILS IN NEW-SOUTH-WALES, AUSTRALIA

The shape and relative weight of the shell have been shown to vary intraspecifically and interspecifically in a number of species of gastropods, including many different littorinids. These differences give rise to different shell forms in different habitats. In those species which have non-planktotrophic development, differences in shell form among shores have been usually explained in terms of natural selection because exposure to waves supposedly favours light shells with large apertures, while predation by crabs on sheltered shores favours elongated, thick shells with smaller apertures. Differences in shell shape among species found at different heights on the shore have been explained in terms of resistance to desiccation and temperature. Such variables would tend to act on a relatively broad-scale, i.e. causing differences among heights on a shore or among shores. Rates of growth, which might vary at much smaller scales within a shore, have also been shown to affect the shapes of many shells. In this study, the shape and relative weight of shells of three species of co-existing littorinids (Littorina unifasciata, Bembicium nanum and Nodilittorina pyramidalis) were measured. These species all have planktotrophic development and they are found on many shores where there is no evidence that they are preyed upon by crabs. Before explanations of shell shape are proposed, it is necessary that patterns of variation, within different parts of a shore and among different shores are clearly documented. These patterns were measured at a number of different spatial scales within and among replicate shores with different amounts of wave exposure. Large and small specimens were included to allow intraspecific comparisons among snails of different sizes found at different heights on the shore. The results showed significant differences among shores in shape and relative weight of shells, but these differences could not be explained by exposure to waves. In addition, snails of different sizes and different species did not show the same patterns although they were collected from the same sites. Importantly, the shell shape of Littorina unifasciata varied significantly among sites at approximately the same height within a shore. These differences could not be clearly correlated with density, mean size nor exposure to waves. The only consistent pattern was a decrease in relative aperture size in specimens living higher on the shore. Models that have commonly been proposed to explain shape and relative weight of shells in other species of gastropods are not adequate to explain the small- and large-scale variation of the measurements described here. It is proposed that any selective advantage of shell morphology and the effects of any variables on the development of shell morphology in these species can only be identified after appropriately designed and replicated field experiments.