AN EXPERIMENTAL GRADIENT ANALYSIS : HYPOSALINITY AS AN "UPSTRESS" DISTRIBUTIONAL DETERMINANT FOR CARIBBEAN PORTUNID CRABS

¹ Department of Zoology, University of Iowa, Iowa City, Iowa 52242; and Discovery Bay Marine Laboratory, P.O. Box 35, Discovery Bay, Jamaica

This study examines ecological distributions in a guild of Caribbean demersal crabs (family Portunidae) on a gradient in terrestrial influence on aquatic climate, along which the major monotonic physicochemical variable is salinity. Distributions were established by sampling in fresh lotic waters and bays with highly restricted exchange with the sea, which, for marine groups, are constantly and unpredictably climatically severe, respectively, through climatically equable waters around coral reefs. The 16 demersal portunid species collected in Jamaica (the main study area), the Florida Keys, Colombia and Curaçao all occur in undiluted seawater, but progressively fewer are found as salinity decreases. Hyposaline biotopes are virtually monopolized by members of the genus Callinectes, while Arenaeus, Portunus and Cronius spp. were found only in higher salinities. Callinectes spp. display serial replacement along the gradient; crab stages of C. maracaiboensis, C. bocourti and C. sapidus occur mainly in fresh waters, while dominance peaks occur in progressively higher salinities for C. exasperatus, C. danae, C. marginatus and C. ornatus. Acute hyposalinity tolerances of the common species were determined experimentally, and follow the same order as upstress limits and dominance peaks. The species composition of the guild changes from domination by the most to the least euryhaline species as likelihood of severe dilution decreases. In biotopes with temporarily ameliorated weather, less euryhaline species invade and may replace more euryhaline species at a rate proportional to the biotope's remoteness from the immigrants' source area. Freshwater Callinectes are catadromous, storing energy in hyposaline ecosystems, but reproducing in higher salinities.