The long-spined sea urchin, Diadema antillarum, is a common inhabitant of Caribbean coral reefs. These echinoderms, characterized by slender, dark spines, are found in shallow waters of the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico, inhabiting environments like coral reefs, seagrass meadows, and mangrove roots. Diadema antillarum is important for the balance of these underwater ecosystems.
Key Role in Reef Health
Diadema antillarum acts as a primary herbivore on Caribbean coral reefs, maintaining ecosystem balance. These urchins graze on macroalgae, preventing them from overgrowing and smothering young corals. By consuming algae, Diadema antillarum creates open space on the reef surface, allowing coral larvae to settle and grow. This grazing activity supports coral recruitment and the expansion of coral colonies.
Diadema antillarum is effective in controlling algal growth compared to other Caribbean grazers. Their efficient grazing keeps reef surfaces clear, benefiting coral settlement and growth. Without sufficient grazing, macroalgae can proliferate, outcompeting corals for light and space and hindering their ability to thrive. The urchins help maintain a healthy substrate for coral development, contributing to the overall resilience and biodiversity of the reef ecosystem.
Consequences of Population Decline
Declines in Diadema antillarum populations have led to ecological impacts on Caribbean reefs. A widespread mass mortality event occurred in 1983-1984, reducing urchin populations by 93% to 99% across the Caribbean and Western Atlantic. This event was caused by a pathogen, later identified as a scuticociliate. The loss of these grazers resulted in immediate changes to reef communities.
In the absence of Diadema antillarum, macroalgae proliferated, with some areas experiencing a 3,000% increase in algal biomass within six months. This led to a “phase shift” on many Caribbean reefs, where coral-dominated environments transformed into algal-dominated ones. The increased algal cover outcompeted and displaced corals, leading to degradation of reef structure and a loss of biodiversity. While some populations showed limited, patchy recovery in subsequent decades, they rarely reached pre-mortality densities.
A new mass mortality event affecting Diadema antillarum began in late January 2022, first observed in the U.S. Virgin Islands and rapidly spreading across at least 25 jurisdictions in the Caribbean. The signs of illness, including loss of tube feet control and spine loss, were similar to the 1983-1984 event, and lethality rates were as high as 99% in monitored populations. This recent die-off, also linked to a scuticociliate, further threatens the recovery of Caribbean coral reefs, which were already under stress from other factors like climate change and coral diseases.
Conservation and Restoration Initiatives
Efforts are underway to protect existing Diadema antillarum populations and restore depleted ones. Research focuses on understanding disease resistance and developing mitigation strategies for pathogens causing mass mortalities. Aquaculture programs are developing to rear Diadema antillarum in controlled environments for reintroduction to natural reefs. For example, the RAAK-PRO Diadema I project (2019-2024) developed methods for small-scale urchin restoration, now implemented across multiple Caribbean islands.
The subsequent RAAK-PRO Diadema II project (2024-2027) aims to upscale these methods by addressing cultivation bottlenecks, optimizing juvenile grow-out, and determining optimal restocking sizes and sites. Researchers are also investigating “Assisted Natural Recovery,” which involves providing suitable settlement substrates like bio-ball streamers to encourage larval settlement and survival in depleted areas. These initiatives, along with marine protected areas to safeguard remaining populations, are important for the long-term health and recovery of Caribbean coral reefs.