Insights on the last sea urchin Diadema africanum mass mortality suggest a worldwide Diadematid pandemic in 2022-2023

Iván Cano^1*Jacob Lorenzo-Morales^2,3,4Omri Bronstein⁵Carlos Sangil⁶José Carlos Hernández¹

• ¹Marine Community Ecology and Conservation. Departamento de Biología Animal, Edafología y Geología, Facultad de Ciencias, Universidad de La Laguna, La Laguna, Spain
• ²Instituto de Enfermedades Tropicales y Salud Pública de Canarias, La Laguna, Spain
• ³3 Departamento de Obstetricia y Ginecología, Pediatría, Medicina Preventiva y Salud Pública, Toxicología, Medicina Legal y Forense y Parasitología, La Laguna, Spain
• ⁴Centro de Investigacion Biomedica en Red Enfermedades Infecciosas, Madrid, Spain
• ⁵School of Zoology, Faculty of Life Sciences & Steinhardt Museum of Natural History, Tel Aviv University, Tel Aviv, Israel
• ⁶Universidad de La Laguna Departamento de Botanica Ecologia y Fisiologia Vegetal, San Cristóbal de La Laguna, Spain

The sea urchin Diadema africanum, a key herbivore in the shallow benthic ecosystems of the Canary Islands, has undergone recurrent mass mortality events (MMEs) in recent years. For the first time, this study assesses the extent and impact of a new MME across the archipelago during 2022–2023. Evidence was obtained through a combination of citizen science reports, visual censuses of adult populations, and larval settlement surveys employing both artificial larval collectors and visual surveys. Preliminary analyses of oceanographic conditions were conducted to explore potential environmental drivers of the reported MMEs. Citizen surveys documented widespread mortality across nearly all islands, which started in the westernmost islands of La Gomera and La Palma and spread eastward. Visual censuses revealed severe population declines, with adult densities decreasing by 73.8% in La Palma and 99.66% in Tenerife compared to 2021 levels. Notably, these represent the lowest records of D. africanum since the initiation of monitoring efforts across the seven main islands of the archipelago. Settlement surveys in Tenerife indicated a severe recruitment failure in 2023, with no settlers detected in larval collectors and no recruits observed in visual surveys conducted four months after the historical settlement peak reported for the species. Oceanographic data highlight the potential links between long lasting eastward currents and anomalous high-energy southern swells to the initial outbreaks at La Gomera and La Palma. The 2022–2023 MME occurred concurrently with other Diadematid die-offs worldwide, suggesting that the canaries may represent a potential link in a broader marine pandemic, linking the western Atlantic (Caribbean) with the Mediterranean Diadematid populations. The unprecedented scale of the reported collapse of D. africanum populations and near complete recruitment failure suggests a severe disruption of the species' life cycle, raising serious concerns about its persistence in the Canary Islands and the cascading consequences for ecosystem structure and function.