A mysterious pathogen is causing widespread deaths among sea urchin populations worldwide, with severe impacts in the Canary Islands. The reef-grazing species Diadema africanum has reached historic lows, threatening marine ecosystems. Scientists report halted reproduction and near-extinction risks in affected areas.
Sea urchins like Diadema africanum serve as key ecosystem engineers in subtropical and tropical waters, grazing on seaweed and seagrass to protect corals and calcifying algae while providing food for various marine species. However, unchecked population growth, fueled by overfished predators and global warming, has previously led to 'urchin barrens'—devastated seafloors stripped of plant life.
In the Canary Islands, D. africanum numbers had risen since the mid-1960s, prompting unsuccessful biological control efforts from 2005 to 2019. The situation reversed dramatically in February 2022, when mass die-offs began off La Palma and Gomera in the archipelago's west. The outbreak spread eastward over the year, with infected urchins showing reduced activity, erratic movements, unresponsiveness to stimuli, and eventual loss of flesh and spines.
This event echoes prior incidents: in 2008 and 2018, diseases killed about 93% of the species off Tenerife and La Palma, and 90% near Madeira, with some recovery after 2008. Unlike before, the 2022 crisis saw no rebound, followed by a second mortality wave in 2023. Surveys across 76 sites on the seven main islands from summer 2022 to 2025 revealed all-time low abundances, including a 74% decline in La Palma and 99.7% in Tenerife since 2021.
Reproduction has collapsed, particularly along Tenerife's eastern coast. Traps in September 2023 captured few larvae during peak spawning, and no juveniles appeared in January 2024 surveys. 'Our analyses showed that the current abundance of D. africanum across the Canary Islands is at an all-time low, with several populations nearing local extinction,' said Iván Cano, a doctoral student at the University of La Laguna.
The die-offs coincide with similar events in the Caribbean, Mediterranean, Red Sea, Sea of Oman, and western Indian Ocean. Suspected culprits include scuticociliate ciliates like Philaster or amoebae such as Neoparamoeba branchiphila, possibly linked to unusual wave activity or transport via currents and shipping. 'Reports from elsewhere suggest that the 2022-2023 die-off in the Canary Islands was another step in a broader marine pandemic, with serious consequences for these key reef grazers,' Cano concluded. The pathogen remains unidentified, and while Southeast Asia and Australia have been spared so far, further spread cannot be ruled out.