ECOLOGICAL RELEASE IN A BAHAMIAN SALT WATER LAKE: OCTOPUS BRIAREUS (CEPHALOPODA) AND OPHIOTHRIX OERSTEDII (OPHIUROIDEA) (MELLUSCA, ECHINODERMATA, PREDATION)

Sweetings Pond, an isolated salt water lake on Eleuthera Island, Bahamas, supported persistent, high-density populations of the cephalopod Octopus briareus and the epifaunal, suspension-feeding ophiuroid, Ophiothrix oerstedii. Both of these species were orders of magnitude more abundant than populations of conspecifics off the Eleuthera coast. In contrast, the abundance and diversity of predatory reef fishes were very low in the lake.;Ophiothrix oerstedii occurred rarely and cryptically off the coast but abundantly and fully exposed in Sweetings Pond. Experiments in which ophiuroids were placed in open arenas in the lake and off the coast revealed that predation pressure from fishes was dramatically higher at the coastal site. Diet analysis of fishes and invertebrates in Sweetings Pond confirmed that the predation rate was negligible on O. oerstedii there. Density was directly related to the degree of small-scale topographical heterogeneity of the substratum.;When O. oerstedii were transplanted from the coast to the lake, they switched to a fully exposed life mode. Conversely, Sweetings Pond ophiuroids transplanted to the coast became cryptic. This result indicates that refuging behavior is primarily a phenotypic response to biotic conditions. The O. oerstedii apparently assess fish predator presence by the frequency of non-lethal attack that they experience.;Sweetings Pond is reminiscent of certain Paleozoic communities in being dominated by an epifaunal, suspension-feeding echinoderm and a cephalopod carnivore. This anachronistic community persists due to the absence of predation by fishes, providing modern-day, ecological-time support for the "Mesozoic marine revolution" hypothesis.;Octopus briareus occupied dens within natural formations in Sweetings Pond. Using artificial cavities, field experiments on den selection revealed the minimum length and diameter requirements of adult O. briareus. A nearest neighbor experiment demonstrated that two individuals would not simultaneously occupy the same or contiguous artificial dens, but would simultaneously occupy dens as close as 15 cm apart. Enrichment of areas with artificial dens increased local density, indicating that dens were limiting; however, with suitable dens widely spaced in the lake, it is doubtful that den defense interactions affected density. Predatory fishes appear to limit octopus density off the coast of Eleuthera.