The black, long-spined sea urchin, Diadema antillarum, is considered a keystone species for its importance in the maintenance of healthy reefs in its role as an herbivore. 90% of the population disappeared in a die-off in 1983 to 1984, but it is now making a slow comeback.
WCS along with University of Belize Environmental Research Institute currently monitors the status of the urchin population in the atoll. Specific threats include disease and dredging.
Preferred habitat for the urchin includes: sparse sea grass on shallow lagoon floor, medium sea grass on shallow lagoon floor, and forereef with dense massive and encrusting corals. Diffuse patch reef and dense sea grass on shallow lagoon floor are still considered good habitats, whereas dense patch reef and deep reef/wall/escarpment are marginal habitats.