Species → Long-spined sea urchin
Long-spined sea urchin
Diadema antillarum
What if it disappeared?
Impact MajorAfter 95 to 99% of Caribbean long-spined sea urchins died in 1983-1984 — the last major algae grazer remaining after decades of herbivorous fish overfishing — the Discovery Bay reef in Jamaica went from over 50% coral cover to less than 5%, while fleshy algae invaded more than 90% of the substrate.
DocumentedDetail
Hughes, T.P. (1994). Catastrophes, phase shifts, and large-scale degradation of a Caribbean coral reef. Science, 265(5178), 1547–1551.
Long-term monitoring at Discovery Bay shows that the already long-standing overfishing of herbivorous fish had made the reef dependent on the urchin as the last line of grazing defense. Its sudden collapse, caused by a pathogen that spread from the Atlantic entrance of the Panama Canal, thus removed the last major check on algal proliferation — not the sole cause of the reef's decline, but the trigger that tipped an already weakened system.
A new mass mortality struck the species starting in January 2022, this time caused by an identified pathogenic ciliate (Philaster apodigitiformis): mortality reaching 99% in affected areas, spreading over more than 1,300 km from north to south in four months — on a species whose recovery since 1983 had already reached only about 12% of its original density.
DocumentedDetail
Hylkema, A. et al. (2023). The 2022 Diadema antillarum die-off event: Comparisons with the 1983-1984 mass mortality. Frontiers in Marine Science.
Unlike the 1983 epidemic, whose exact pathogen was never formally identified, the 2022 event was attributed to a specific scuticociliate, since found in other Diadema-genus urchins in the Mediterranean, the Red Sea, and the western Indian Ocean — a sign that this type of epidemic may not stay confined to the Caribbean.
Combined with warming-linked coral bleaching events, the urchin's failure to recover its pre-1983 densities may have locked a large share of Caribbean reefs into an algae-dominated state rather than a coral-dominated one — but disentangling the urchin's exact share from that of warming and pollution remains difficult reef by reef.
ExtrapolatedDetail
Extrapolation à partir des données de rétablissement (Lessios 2016, revue des 30 ans) et de la littérature générale sur le déclin des récifs caribéens : le mécanisme local de Discovery Bay n'a pas été répliqué comme bilan systématique à l'échelle de toute la région.
The urchin's recovery varies widely from island to island — better in the eastern Caribbean, nearly nonexistent elsewhere — which makes a single conclusion for the region as a whole risky without a reef-by-reef comparison.
What's missing to confirm it: A systematic comparison between reefs where the urchin has recovered better and reefs where it remains absent, controlling for warming and local pollution, is still missing at the scale of the whole Caribbean.
Documentation
-
Hughes (1994) — Catastrophes, phase shifts, and large-scale degradation of a Caribbean coral reef, Science
Accessed August 7, 2026
-
Hylkema et al. (2023) — The 2022 Diadema antillarum die-off event, Frontiers in Marine Science
Accessed August 7, 2026
-
Lessios (2016) — The Great Diadema antillarum Die-Off: 30 Years Later, Annual Review of Marine Science
Accessed August 7, 2026
Last verified: August 14, 2026