Spatiotemporal patterns in Diadema antillarum settlement in the Florida Keys: implications for habitat restoration

The long-spined sea urchin (Diadema antillarum) is a key herbivore on Caribbean coral reefs, but mass mortality events have severely reduced its populations, diminishing grazing pressure and contributing to reef degradation. In the Florida Keys, where D. antillarum remains scarce, the extent and variability of larval settlement are poorly understood, yet this information is critical for predicting and managing population recovery. To address this, we quantified monthly settlement rates using standardized Astroturf plates deployed across a gradient of reef habitats at nine reef sites from 2017 to 2023. We found that settlement was significantly higher at offshore bank-barrier reefs than inshore patch reefs, with greater settlement observed in the middle/upper Florida Keys compared to the lower Keys. Seasonal variation indicated peak settlement in summer, suggesting that larval supply may be influenced by seasonal oceanographic processes. Despite evidence of settlement, recruitment success remains uncertain, as post-settlement factors such as predation or environmental conditions may limit survival. These findings provide key insights for D. antillarum restoration efforts, highlighting offshore reefs in the middle/upper Florida Keys as priority sites for enhancing settlement and improving post-settlement habitat to support population recovery. Future studies should investigate the biophysical mechanisms driving the spatial and temporal settlement patterns identified here, to develop a predictive framework for settlement events that can further inform population management.