DISTRIBUTION PATTERNS OF TROPICAL ESTUARINE BRACHYURAN CRABS IN THE GULF OF MEXICO

Abstract

Distribution patterns are defined for 20 brachyuran crab species of Laguna de Terminos based on the range of salinity and the habitats occupied by each. Habitats include unvegetated soft substrates, macroalgae, and sea grasses. "Wide" and "restricted" distributions relate species abundance to a wide and a narrow salinity range, whereas "associated with the habitat" and "not associated with the habitat" consider the presence of a species in one or several types of habitat. Fourteen species were grouped in the restricted and associated with the habitat pattern, of which the 3 spider crabs and the 10 mud crabs were associated with submerged aquatic vegetation in salinities above 18‰, and the swimming crab Callinectes exasperatus was distributed in an adjacent limnetic lagoon on unvegetated soft substrates. The mud crab Dyspanopeus texanus was placed in the wide and associated with the habitat pattern, since it is a euryhaline species and was always found on macroalgae and sea grasses. Three other swimming crabs and two other mud crabs were found on all sorts of substrates along a wide range of salinity and were included in the wide and not associated with the habitat pattern. Six faunistic groups were formed after analysis of the abundance data, using an ordination method with axes defined by salinity and type of habitat. Four functional guilds of brachyuran crabs are proposed, based on the 3 distribution patterns, the 6 faunistic groups, and the 2 subsystems and 4 areas previously described for the lagoon. Guild A grouped 2 swimming crabs and 1 mud crab distributed along the areas of the lagoon directly influenced by river runoff, where detritus-rich unvegetated soft substrates and macroalgae are common. Guild B included: (1) the 3 spider crabs and most of the mud crabs of which seven restricted and associated with the habitat species congregated in the northcentral-northeastern subsystem of the lagoon, where the greatest heterogeneity of habitats, dense sea-grass beds and poly-euhaline environments, is common; (2) 5 restricted and associated with the habitat species mostly distributed in this subsystem were also collected in areas influenced by river runoff during the dry seasons when salinity was above 24‰; and (3) 2 wide and not associated with the habitat species were mostly found in this subsystem, but extended their distribution to areas without submerged vegetation and lower salinities. Guild C was represented by D. texanus, which was found throughout most of the lagoon on macroalgae and sea-grass beds. Guild D included Callinectes sapidus and C. similis that were distributed throughout the lagoon independently of a particular type of habitat.