Coastal urbanization changes intertidal shorelines, by alteration and destruction of natural habitat and addition of new built habitat. In Sydney Harbour, New South Wales, Australia, up to 50% of the shoreline is composed of constructed habitat, particularly seawalls. Research has shown that many components of intertidal assemblages live on seawalls, but patterns of abundance and diversity are very variable. Seawalls differ physically from natural shores in a number of ways that are likely to influence distribution and abundances of intertidal molluscs, which are very important determinants of structure of intertidal assemblages.
This study examined diversity, abundances and frequencies of occurrence of intertidal molluscs on seawalls and on natural horizontal and vertical shores in numerous locations in Sydney Harbour and in natural boulder-fields and artificial boulder-fields created from rubble of seawalls. On seawalls, assemblages varied between tidal heights and among locations, but when data were combined across locations, there were some general patterns. Sessile bivalves (oysters and mussels) and many limpets were found in similar numbers on both habitats, or patterns varied inconsistently. Many coiled snails, in contrast, including whelks and grazing gastropods, plus opisthobranchs, which were either common or relatively sparse on horizontal shores, were not found on seawalls and found in intermediate frequencies on vertical shores. Similarly, common species of molluscs were found in natural and artificial boulder-fields in similar numbers, or patterns were not consistent, although rarer species were not found in these boulder-fields. Because molluscs are such important contributors to intertidal dynamics, these results have important implications for management of intertidal biodiversity in urbanized and altered environments.