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.1,2,3,4,5,6,7,8,9,10,11,12 Recurring mass mortality events (MMEs) of Diadema species Gray, 1825 have been recorded globally,13,14,15,16,17,18,19,20,21,22,23 the most notorious and ecologically significant of which occurred in the Caribbean in 1983,14,17,19,20 contributing to the shift from coral to algal-dominated ecosystems.17,24,25 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)26,27,28 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.13,15,18 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.29,30,31,32 We issue a warning regarding the further expansion of mortalities and call for immediate monitoring and conservation efforts for these key ecological species.

Mass mortality of the invasive alien echinoid Diadema setosum (Echinoidea: Diadematidae) in the Mediterranean Sea.

The sea urchin Diadema setosum is an ecological key species across its range, particularly on coral reefs. In 2006 D. setosum was first observed in the Mediterranean Sea, and since, it has proliferated to occupy the entire Levantine Basin. Here we report the mass mortality of the invasive D. setosum in the Mediterranean Sea. This is the first report of D. setosum mass mortality. The mortality spans over 1000 km along the Levantine coast of Greece and Turkey. The current mortality shows similar pathologies to previously reported Diadema mass mortality events, suggesting pathogenic infection as the cause of mortalities. Maritime transport, local currents, and fish predation of infected individuals may distribute pathogens at varying geographical scales. Due to the proximity of the Levantine Basin to the Red Sea, the risk of pathogen transport to the native Red Sea D. setosum population is imminent-with potentially catastrophic consequences.