Transglobal spread of an ecologically significant sea urchin parasite

Abstract

AbstractMass mortality of the dominant coral reef herbivoreDiadema antillarumin the Caribbean in the early 1980s led to a persistent phase shift from coral-to algal-dominated reefs. In 2022, a scuticociliate most closely related toPhilaster apodigitiformiscaused further mass mortality ofD. antillarumacross the Caribbean, leading to >95% mortality at affected sites. Mortality was also reported in the related speciesDiadema setosumin the Mediterranean in 2022, where urchins experienced gross signs compatible with scuticociliatosis. However, the causative agent of the Mediterranean outbreak has not yet been determined. In April 2023, mass mortality ofD. setosumoccurred along the Sultanate of Oman’s coastline. Urchins displayed signs compatible with scuticociliatosis including abnormal behavior, drooping and loss of spines, followed by tissue necrosis and death. Here we report the detection of an 18S rRNA gene sequence in abnormal urchins from Muscat, Oman that is identical to thePhilasterstrain responsible forD. antillarummass mortality in the Caribbean. We also show that scuticociliatosis signs can be elicited inD. setosumby experimental challenge with the cultivatedPhilasterstrain associated with Caribbean scuticociliatosis. These results demonstrate thePhilastersp. associated withD. antillarummass mortality has rapidly spread to geographically distant coral reefs, compelling global-scale awareness and monitoring for this devastating condition through field surveys, microscopy, and molecular microbiological approaches, and prompting investigation of long-range transmission mechanisms.