Abundance, spatial distribution, and size structure of populations of Oreaster reticulatus (Echinodermata: Asteroidea) on sand bottoms

Populations of Oreaster reticulatus (Linnaeus, 1758) occurred on three types of carbonate sand bottom off the northeastern coast of St. Croix, U.S. Virgin Islands: (1) coarse-sand plains, (2) a medium-sand tract of callianassid shrimp mounds; (3) an isolated fine-sand patch amid a dense bed of Syringodium filiforme (Kützing). The density of O. reticulatus was lowest on sand plains and highest in the sand patch, in accordance with the degree of spatial containment. Predation was generally unimportant in limiting the populations, which were composed of primarily adults and some large juveniles. The size-frequency distribution of each population was unimodal; however, the mean size was inversely related to density, suggesting that intraspecific competition for food limits growth. A decrease in body size of larger individuals occurred at maximal densities. Resorption of the body may function as a compensatory mechanism to maintain population biomass at the carrying capacity of the habitat. Dense grass-beds [S. filiforme, Thalassia testudinum (König)] form natural boundaries about adult populations on sand bottoms, but may serve as nursery areas for early juvenile stages. The incidence and/or degree of aggregation within populations was directly related to population density. Populations were randomly distributed or minimally aggregated during the quiescent phase of the annual reproductive cycle, whereas aggregation was maximal during the reproductive period.