The enemy within and without: Sympatric reservoirs and potential reemergence of the Diadema antillarum scuticociliatosis Philaster clade that causes long-spined urchin mass mortality

The Diadema antillarum scuticociliatosis Philaster clade (DaScPc) was responsible for catastrophic long-spined urchin mass mortality in Spring and Summer 2022 throughout the eastern Caribbean. Despite a growing understanding of its virulence, key knowledge gaps remain around its provenance, environmental distribution and potential to colonize sympatric organisms. We surveyed 634 surface swabs and plankton samples collected at 35 sites in the Caribbean for DaScPc and recovered it primarily from Siderastrea siderea (Hexacorallida), turf algae, and the barrel sponge Ircinia compana, and predominately at a site with active DaSc and near the site of DaSc emergence in the region. Amplicon (16S rRNA) surveys of DaScPc positive swabs revealed that its load was significantly related to richness, diversity and evenness of bacterial communities, but was not related to any single bacterial taxon. Survey of coral DNA extracts and water collected adjacent to corals prior to 2020 by PCR, and consultation of Transcriptome Shotgun Assembly (TSA; NCBI) and metagenomes at IMG-MER, revealed no evidence for DaScPc, suggesting the pathogen is a recent introduction to the region. An experiment to test recruitment suggested that DaScPc may grow on coral surfaces but did not identify consistent patterns between coral species. Finally, an experiment aimed at examining reemergence from corals yielded inconclusive results, since urchins died with signs characteristic of DaSc. Overall, our results suggest that DaScPc inhabits organic matter rich environments, possibly supporting bacteria that are their prey, and that the scuticociliate may persist for months after DaSc introduction.