Ophiuroids in a Bahamian Saltwater Lake: The Ecology of a Paleozoic‐like Community

This study examined predation pressure as a determinant of ophiuroid abundance in Carribean shallow marine communities. The density of Ophiothrix oerstedii in Sweeting Pond, a saltwater lake on Eleuthera Island, Bahamas, was orders of magnitude higher (mean density up to 434.2 individuals/m 2 ) than at nearby coastal sites (maximum density 3.21 individuals/m 2 ). Ophiothrix occurred cryptically off the coast but fully exposed in the lake. Experiments in which ophiuroids were placed in open arenas revealed that Ophiothrix off the coast experienced dramatically higher predation pressure from fishes than they did in the lake, which contained none of the common Caribbean reef predators. Analysis of the diets of potential Ophiothrix predators in Sweetings Pond confirmed that the predation rate on these brittlestars was negligible. Density was directly related to the degree of microtopographical heterogeneity in Sweetings Pond. When the heterogeneity of experimental plots was artificially increased in a flat area of the lake by adding artificial bivalve clumps, introduced Ophiothrix remained in those plots, whereas they emigrated from unmanipulated control plots. Raised surfaces appear to confer an advantage to the suspension—feeding Ophiothrix. This population is strongly reminiscent of certain Paleozoic crinoid— and ophiuroid—dominated communities, and the absence of predatory fishes may have been important to the persistence of those fossil communities as well.