Cross-shelf variation in the structure of molluscan assemblages on shallow, rocky reefs in subtropical, eastern Australia

Despite the fact that molluscs are one of the most widespread and conspicuous of all marine invertebrates, comparatively little is known about their ecology, especially with respect to biological interactions that drive community processes. We consequently assessed patterns of subtidal mollusc assemblages on subtropical rocky reefs in the Solitary Islands Marine Park (SIMP), eastern Australia. Shelled molluscs (specifically shelled gastropods and bivalves) were targeted to test if: (1) mollusc assemblages change with increasing distance from shore; (2) reef orientation (vertical and horizontal reef faces) influences assemblage structure; (3) sessile benthic communities influence the composition of mollusc assemblages. Multivariate analyses of community patterns indicated strong cross-shelf patterns. However, no significant differences were found for summary community variables (species richness, total abundance). Inshore sites were dominated by large herbivores (especially turbinids) and prey-specific ranellids. Mid-shelf assemblages comprised a mix of taxa that occurred in both inshore and offshore assemblages and were thus transitional between these shelf positions. Offshore assemblages were distinctly different to reefs closer to shore and were characterised primarily by the presence of sedentary (Vermetidae and Chamidae), and tropically-affiliated taxa. Relationships with reef orientations were found to be inconsistent and correlations with sessile benthos were relatively weak. The overall patterns are similar to those found for corals and fish in the region and most likely reflect the increasing influence of the East Australian Current offshore.