Species-specific responses of resident crabs to vertical habitat complexity on intertidal oyster reefs

•Crab responses to vertical habitat complexity on intertidal oyster reefs differed.•Densities of a filter feeder were positively correlated with rugosity.•Flatback mud crab densities were positively correlated with rugosity and mussels.•Common mud crabs were more evenly distributed among rugosity treatments.•Consistent relationships between rugosity and crab densities occurred among sites.

Habitat vertical complexity is an important physical feature of marine systems that can influence factors such as predator-prey interactions and recruitment. We examined how vertical structure on intertidal Eastern oyster Crassostrea virginica reefs affected resident crab distributions by deploying trays of varying rugosity at two sites in Charleston, South Carolina. Trays were collected after fifteen weeks and oyster spat and resident macrofauna were counted and measured. Densities of oyster spat and the filter-feeding, invasive, green porcelain crab Petrolisthes armatus were strongly positively associated with rugosity. Flatback mud crab Eurypanopeus depressus densities were positively associated with rugosity and with scorched mussel Brachiodontes exustus densities. The common mud crab Panopeus herbstii exhibited a more even distribution across oyster rugosity treatments and was the only species that commonly occurred in the low rugosity treatment. Differences in crab sizes between sites were attributable to abundances of smaller individuals and may reflect spatial variation in recruitment potential. Differences in crab size distributions among treatments were caused by more relatively large crabs in the high rugosity treatments, where greater vertical structural complexity provided more refuge for larger individuals. The relationships between rugosity and faunal densities were consistent at both sites, indicating that the rugosity metric can be used to estimate changes in spat and associated faunal densities when vertical habitat complexity on oyster reefs is reduced, for instance, through harvesting or sedimentation.