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ABSTRACT Mass mortality of Diadematidae urchins, caused by theDiadema antillarumscuticociliatosis Philaster clade (DScPc),affected the Caribbean in spring 2022 and subsequently spread to the eastern Mediterranean, Red Sea, and western Indian Ocean. A key question around Diadematidae scuticociliatosis (DSc), the disease caused by the scuticociliate, is whether the urchin microbiome varies between scuticociliatosis-affected and grossly normal urchins. Tissue samples from both grossly normal and abnormalDiadema antillarumwere collected in the field during the initial assessment of the DSc causative agent and from an experimental challenge of DScPc culture on aquaculturedD. antillarum. Specimens were analyzed using 16S rRNA gene amplicon sequencing. Additional abnormal urchin samples were collected from the most recent outbreak site in the western Indian Ocean (Réunion Island). At reference (i.e., unaffected by DSc) sites,Kistimonasspp., Propionigeniumspp., andEndozoicomonasspp. were highly represented in amplicon libraries. DSc-affected urchin amplicon libraries had lower taxonomic richness and a greater representation of taxa related toFangia hongkongensisandPsychrobiumspp. Amplicon libraries of urchins experimentally challenged with the DSc pathogen had some shifts in microbial composition, butF. hongkongensiswas not a part of the core bacteria in DSc-challenged specimens. DSc-affectedEchinothrix diademafrom Réunion Island showed a similar high representation ofF. hongkongensisas that seen on CaribbeanD. antillarum. Our results suggest that DSc altersDiadematidaemicrobiomes and thatF. hongkongensismay be a candidate bacterial biomarker for DSc in environmental samples. The mechanism driving microbiome variation in host–pathogen interactions remains to be explored.IMPORTANCEThe mass mortality of Diadematidae urchins due toDiadema antillarumscuticociliatosis (DSc) has had significant ecological impacts, spreading from the Caribbean to the eastern Mediterranean, Red Sea, and western Indian Ocean. This study investigates whether the microbiome of urchins varies between those affected by DSc and those that are not. Using 16S rRNA gene amplicon sequencing, researchers found that DSc-affected urchins had lower taxonomic richness and a greater representation ofFangia hongkongensisandPsychrobiumspp. The findings indicate thatF. hongkongensiscould serve as a bacterial biomarker for DSc in environmental samples, providing a potential tool for early detection and management of the disease. Understanding these microbiome changes is crucial for developing strategies to mitigate the spread and impact of DSc on marine ecosystems. 

A scuticociliate ( Philaster sp.) causes mass mortality of the long-spined sea urchin Diadema antillarum .