Diadema antillarum10 years after mass mortality: still rare, despite help from a competitor

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Abstract
The black sea urchin Diadema antillarum was until 1983 an important component of Caribbean coral reef communities, affecting the distribution and abundance of all major guilds of sedentary organisms. Between 1983 and 1984 this species suffered the most extensive and severe mass mortality ever recorded for a marine animal. Continuous monitoring in Panama shows that in the subsequent 10 years D. antillarum densities remained at < 3.5% of their pre-mortality levels. Despite pre-1983 evidence that D. antillarum competed with other echinoids, there has been no competitive release by other sea urchin species. Reef-wide inclusions and exclusions of echinoids indicate that: (i) the low rate of Diadema recruitment does not result from absence of settlement cues for the larvae or from lack of protection by conspecific adults but from paucity of larvae in the water column; and (ii) Echinometra viridis-an echinoid previously shown to compete with adult Diadema-actually facilitates the latter's recruitment. The lack of recovery of D. antillarum despite its high fecundity, planktonic larvae and the assistance of E. viridis, demonstrates that unique disturbance events in the history of a species can have long-lasting effects on its abundance, independently of community-level processes.