Marine mass mortality events can drastically alter the structure and function of marine habitats, shifting the balance between organisms and leading to ecosystem degradation. The long-spined sea urchin Diadema antillarum plays a critical role in Caribbean coral reefs by maintaining the balance between corals and algae. Mass mortality of this urchin species has been linked to a pathogenic ciliate from the Diadema scuticociliatosis Philaster clade (DaScPc). Despite identifying the agent responsible, we still lack a fundamental understanding of the disease and its progression. By combining field monitoring, laboratory experiments, and molecular approaches, this project investigates how DaScPc, environmental conditions, and the microbiomes of Diadema interact to shape disease outcomes. These findings will be essential for predicting and potentially mitigating future urchin die-offs, thereby protecting the sensitive coral reef habitats they inhabit. This project includes the training and mentoring of graduate, undergraduate and high school students across four institutions, and the development of a global network of collaborators to facilitate monitoring and early detection of the disease. This project addresses two major questions surrounding mass mortality of the long-spined sea urchin Diadema antillarum. Aim 1 identifies environmental reservoirs of DaScPc and explores the environmental factors influencing its emergence and spread. This team conducts systematic time-series s