Habitat-associations of turban snails on intertidal and subtidal rocky reefs.

Patchiness of habitat has important influences on distributions and abundances of organisms. Given the increasing threat of loss and alteration of habitats due to pressures associated with humans, there is a need for ecologists to understand species' requirements for habitat and to predict changes to taxa under various future environmental conditions. This study tested hypotheses about the generality of patterns described for one species of marine intertidal turban snail for a different, yet closely-related species in subtidal habitats along the coast of New South Wales, Australia. These two closely-related species live in similar habitats, yet under quite different conditions, which provided an opportunity to investigate how similar types of habitats influence patterns of distribution, abundance and size-structure in intertidal versus subtidal environments. For each species, there were similar associations between biogenically structured habitat and densities. The intertidal species, Turbo undulates, were more abundant, with greater proportions of small individuals in habitats formed by the canopy-forming alga, Hormosira banksii, the solitary ascidian, Pyura stolonifera or the turfing red alga, Corallina officinalis compared to simple habitat (bare rock). Similarly, more Turbo torquatus were found in biogenically structured subtidal habitat, i.e. canopy-forming algae, Ecklonia radiata, mixed algal communities ('fringe'), or turfing red algae (Corallina officinalis and Amphiroa aniceps) than where habitat is simple (barrens). Small T. torquatus were more abundant in areas of turf and 'fringe', while large snails were more abundant in areas of kelp and barrens. These patterns were found at each location sampled (i.e. eight intertidal and two subtidal rocky reefs) and at all times of sampling, across each environment. This study highlighted the consistent influence of biogenically structured habitats on the distribution, abundance and size-structure of intertidal and subtidal turban snails and forms a basis for increasing the understanding of the potential underlying processes causing such patterns.