Mass mortality of diadematoid sea urchins in the Red Sea and Western Indian Ocean

Sea urchins are primary herbivores on coral reefs, regulating algal biomass and facilitating coral settlement and growth.^{123456789101112} Recurring mass mortality events (MMEs) of Diadema species Gray, 1825 have been recorded globally,^{1314151617181920212223} the most notorious and ecologically significant of which occurred in the Caribbean in 1983,^14171920 contributing to the shift from coral to algal-dominated ecosystems.¹⁷²⁴²⁵ Recently, first evidence of Diadema setosum mass mortality was reported from the eastern Mediterranean Sea.²³ Here, we report extensive mass mortalities of several diadematoid species inhabiting the Red Sea and Western Indian Ocean (WIO)²⁶²⁷²⁸ including first evidence of mortalities in the genus Echinothrix Peters, 1853. Mortalities initiated in the Gulf of Aqaba on December 2022 and span the Red Sea, the Gulf of Oman, and the Western Indian Ocean (Réunion Island), with population declines reaching 100% at some sites. Infected individuals are characterized by spine loss and tissue necrosis, resulting in exposed skeletons (i.e., tests) and mortality. Molecular diagnostics of the 18S rRNA gene confirm the presence of a waterborne scuticociliate protozoan most closely related to Philaster apodigitiformis in infected specimens—identical to the pathogen found in the 2022 Caribbean mass mortality of Diadema antillarum.¹³¹⁵¹⁸ Collapse of these key benthic grazers in the Red Sea and Western Indian Ocean may lead to algal dominance over corals, threatening the stability of coral reefs on a regional scale.^29303132 We issue a warning regarding the further expansion of mortalities and call for immediate monitoring and conservation efforts for these key ecological species.