Effects of contrasting tidal habitats on growth, survivorship and dispersal in an intertidal snail

We examined size-frequency distributions, growth rates, sizes of mating snails, dispersal and genetics in populations of the littorine Bembicium vittatum Philippi in two connected, but very different tidal ponds on Long Island, in the Houtman Abrolhos Islands, Western Australia. At one extreme, the "wet pond", snails are inundated by the tide each day. At the other extreme, the "dry pond", snails are at a higher tidal elevation, and are thus inundated only during spring tides, with intervening dry periods of up to 7 weeks. Snails in the wet pond are significantly larger, grow faster, and disperse more than those in the dry pond. Nevertheless, abundance and survivorship are higher in the dry pond, indicating a significant tradeoff of size and growth with survivorship. Reciprocal transplant experiments indicate that growth rate is a function of habitat: wet-pond snails grow faster than dry-pond conspecifics. Von Bertalanffy growth rate curves indicate that wet pond snails mature earlier and at a larger size than dry pond snails, indicating that differences in the size of mating snails are due to differences in growth rate. Measured rates of dispersal indicate that the wet pond, which is about 100 m long, is a single genetic neighbourhood. Genetic comparisons at four polymorphic allozyme loci support the interpretation of genetic mixing throughout the wet pond. Despite mixing over large distances in the wet pond, no movements were detected through the 20 m channel that connects the wet and dry ponds, and genetic comparisons confirmed isolation of the two populations. This study highlights the substantial ecological plasticity of B. vittatum, allowing persistence in a wide range of habitats.