The long-spined sea urchin, Diadema antillarum, is a key species within the tropical reef ecosystems of the Caribbean Sea and the wider Western Atlantic. It controls algal growth, maintaining the delicate balance of the reef environment. Mass mortality events, such as the die-off in 1983-1984 and a subsequent event in 2022, have provided region-wide evidence of its profound ecological role. These die-offs, which reduced populations by over 97%, underscore the vulnerability of the entire ecosystem to the loss of this single invertebrate. Understanding the consequences of the urchin’s disappearance is fundamental to comprehending the current degraded state of many Caribbean reefs.
The Role of the Long-Spined Urchin as a Primary Grazer
The Diadema antillarum is an efficient herbivore, and its primary ecological function is to graze the reef substrate clean of algae. It consumes fast-growing, short algal filaments known as turf algae, along with other microscopic life. By scraping these surfaces almost bare, the urchin creates the necessary “clean” space for the settlement of coral larvae. This continuous grazing activity ensures that the hard, calcium carbonate structure remains available for new coral colonies. Without the urchin’s high-intensity grazing, this open space is quickly overgrown, directly inhibiting the next generation of corals from establishing a foothold.
Immediate Consequence: Proliferation of Macroalgae
The immediate result of losing the long-spined sea urchin is the rapid increase of algae, a process known as release from herbivory. Within months of the 1983 die-off, the biomass of algae on some reefs increased by as much as 3,000%. The turf algae that the urchins cropped short quickly grow taller and transition into fleshy macroalgae, or seaweed. This dense macroalgae is often unpalatable to other grazers, including many species of herbivorous fish. The unchecked growth of this seaweed creates a thick blanket over the reef, dominating the benthic landscape.
Structural Shift: The Loss of Coral Dominance
The proliferation of macroalgae initiates a cascade that fundamentally alters the reef structure. The dense algal canopy smothers existing adult corals by blocking sunlight and inhibiting the necessary water flow required for respiration and waste removal. Even more damaging is the effect on coral recruitment, as the overgrown substrate prevents the microscopic coral larvae from settling; macroalgae can also release chemicals that are toxic to settling coral larvae, further reducing their survival rates. This combination of reduced adult survival and blocked recruitment accelerates a phenomenon known as a phase shift—the transition of the ecosystem from a coral-dominant to an algal-dominant state. The complex, three-dimensional structure of the former coral reef is replaced by a relatively flat, less diverse environment dominated by tangled seaweed, and this loss of structural complexity reduces the habitat and shelter available for countless fish and invertebrate species, leading to an overall decline in biodiversity and ecosystem health.
Pathways for Ecosystem Resilience and Recovery
Despite the damage caused by the urchin die-offs, some reefs show potential for resilience and recovery, primarily through the action of other herbivores. Parrotfish are capable of grazing down some types of macroalgae, and their protection through marine managed areas can help control algal growth; however, parrotfish alone are not as effective as a dense population of Diadema at completely clearing the hard substrate for coral settlement. Localized recovery of the urchin, which has occurred in some areas like Jamaica since the 1980s, has demonstrated that the phase shift can be reversed. In these recovery zones, increased urchin density has led to a reduction in macroalgae and a corresponding increase in the density of juvenile corals. Efforts to restore populations through aquaculture and outplanting are underway, aimed at enhancing this natural recovery process and rebuilding the reef’s capacity for self-maintenance.