Mar Environ Res 2025 Feb 06;205:106999. doi: 10.1016/j.marenvres.2025.106999.

Benthic community phase shifts towards macroalgae beds compromise the recruitment of the long-spined sea urchin Diademaafricanum.

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The long-spined sea urchin Diadema africanum is a key herbivore in sublittoral ecosystems of the Eastern Atlantic, mediating shifts between productive macroalgae beds and barren grounds. Historical increases in D. africanum densities have been linked to predator overfishing and rising temperatures favoring settlement. However, recurrent mass mortality events have recently decimated populations, reversing benthic communities from urchin barrens to macroalgal beds across the Canary Islands. We investigated the influence of biological factors, such as adult density and macroalgal coverage, alongside physical variables, including substrate rugosity, crevice presence, and slope, on the distribution of newly settled D. africanum prior to mass mortality events. In 2004, when D. africanum densities were high, early recruits were positively associated with adult habitats and negatively with areas dominated by erect, non-calcareous algae. These findings highlight that a combination of adult conspecifics, high structural complexity, and crustose coralline algae favored D. africanum recruitment, while macroalgal habitats were unfavorable. Surveys in 2024, following recurrent mass mortalities, revealed a dramatic phase shift from encrusting corallines to erect algae and a recruitment failure of D. africanum. This recent shift has altered former favorable habitats, potentially compromising recruit survival and hindering population recovery.

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