Spatial and temporal patterns in the dynamics and use of pen shells (Atrina rigida) as shelters in St. Joseph Bay, Florida

Mollusc shells are important as settlement substrates for sessile organisms and as shelters or nest sites for many mobile benthic animals in soft-bottom marine habitats. Processes that influence the availability or quality of shells impact certain shell-using species. In St. Joseph Bay, Florida, USA, the availability of shells of dead Atrina rigida (Sol.) (pen shells) for shelters or nests influence the distribution of an assemblage of mobile benthic animals (crabs, fish, and octopus) and regulates reproduction in one blenny. Because the shells are produced by living organisms, their abundance and quality (e.g., size) will be influenced by the dynamics of the A. rigida population. Processes that alter, destroy, or remove shells affect their availability or quality (e.g., degree of fouling). Species-specific shelter preferences and the strength of intra- and interspecific competition for shelters will influence resource availability for each species. I examined patterns of natural variation in A. rigida population and shell resource dynamics that impact the abundance and quality of shell shelters: I also examined patterns of resource use within the occupant assemblage. Strong spatial variation in A. rigida recruitment, growth, and mortality rates created a patchy, temporally fluctuating distribution of shelters for shell-using species in St. Joseph Bay, Blennies and clingfish only guarded eggs in new shells with little fouling cover, but toadfish nested in older, heavily fouled shells. Stone crabs and wharf crabs occupied large shells in greater proportions than small shells. Overall occupancy of pen shells and the relative abundance of shell-using species fluctulated among years. Resource availability for shell-using species in St. Joseph Bay is the product of complex interactions between A. rigida population dynamics, rates of shell fouling and loss, and annual variation in the abundance of shell occupants.