Seasonal, morphodynamic, and vegetation drivers of ghost crab (Ocypode quadrata) populations on subtropical sandy beaches.

Sandy beach ecosystems are increasingly affected by climatic variability and human pressures. The ghost crab Ocypode quadrata is widely used as a bioindicator of environmental conditions on sandy beaches. However, important gaps remain in understanding how natural sources of variability, such as seasonality and beach morphodynamics, influence its population dynamics. We evaluated seasonal and spatial variation in burrow density, biomass, carapace dimensions, and body condition, used here as morphological and population descriptors of O. quadrata, along morphodynamic gradient of sandy beaches in southern Brazil. Biomass and carapace dimensions, obtained through direct census, varied significantly among beaches and seasons, with higher values generally recorded on the reflective beach during summer, whereas intermediate and dissipative beaches showed higher values during winter. In contrast, body condition consistently peaked in autumn across all beaches. Burrow density, estimated through indirect census, did not vary significantly across seasons or beaches, but was positively associated with vegetation area, as were the other descriptors examined. These findings suggest that seasonal dynamics and habitat characteristics, including vegetation area, may be associated with variation in O. quadrata population descriptors across beaches differing in morphodynamic condition. Accounting for these natural sources of variability may support more reliable use of O. quadrata as a bioindicator in sandy beach ecosystems.