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A ciliate belonging to theDiadema antillarumscuticociliatosis (DaSc)-associatedPhilasterclade (DaScPc) caused catastrophic long-spined urchin mass mortality in spring and summer of 2022. The ciliate can be grown in culture in both the presence and absence ofD. antillarumtissues, suggesting that it may persist outside its host by consuming microorganisms or dissolved organic nutrients. We hypothesized that DaScPc was present outside its host during and after mass mortality and absent prior to 2022. We examined DaScPc in DNA extracted from 500 swabs of sympatric metazoa and abiotic surfaces, and plankton samples, collected at 35 sites in the Caribbean in 2022 and 2023. DaScPc was detected on corals, turf algae, and a sponge, predominantly at sites with active or prior DaSc. We examined whether it was present prior to 2022 by surveying extracted DNA from Caribbean corals and water collected near corals by PCR and by mining publicly available transcriptomes and metagenomes for DaScPc rRNAs. These efforts yielded no DaScPc genes. We further hypothesized that DaScPc may recruit to the specific corals detected in field surveys, and that these may then infect naïve hosts. A mesocosm experiment to test DaScPc recruitment suggested that, while it recruited to corals, it did so inconsistently between coral species. Incubation of corals that recruited DaScPc with naïve urchins yielded inconclusive results since urchins died without characteristic DaSc signs. Overall, our results suggest that DaScPc may occur outside its urchin host, and that it may have been absent in the region prior to 2022. 

Sea star wasting disease (SSWD) refers to a suite of gross signs affecting Asteroidea species. These include epidermal lesions, everted viscera, arm autotomy, and ultimately, full body disintegration leading to mortality. The common sea starAsterias rubensis a keystone species in the coastal Northeast Atlantic and may be susceptible to the disease. While the precise aetiology of SSWD remains poorly understood, environmental instability, including rising sea temperatures, has been linked to SSWD outbreaks. To investigate this connection, an experiment was conducted to quantify disease sign expression inA. rubensunder elevated temperature. We exposed sea stars to either elevated temperature (18°C) or a control treatment (12°C) for a 14 d period. We quantified the presence of disease signs associated with SSWD, the progression of signs, and survival of individuals. Elevated temperature induced a greater number of signs consistent with SSWD and also resulted in mortality for some of those animals. Furthermore, larger individuals were more likely to show increased signs of disease. Our results provide evidence that signs associated with SSWD increase with elevated temperature.