BULLETIN OF MARINE SCIENCE, 44(1): 251-262,1989 DECAPOD AND STOMATOPOD COMMUNITIES OF SEAGRASS-COVERED MUD BANKS IN FLORIDA BAY: INTER- AND INTRA-BANK HETEROGENEITY WITH SPECIAL REFERENCE TO ISOLA TED SUBENVIRONMENTS JeffG. Holmquist, George V. N. Powell and Susan M. Sogard ABSTRACT Using throw trap sampling, we compared the decapod and stomatopod fauna of the grass- covered mud banks in five major subenvironments of Florida Bay and tested for zonation in these fauna across individual banks. Fauna was primarily Gulf-Carolinean, despite the presence of a rich Antillean community just outside the bay. The two subenvironments adjacent to open ocean had high species richness, but banks became increasingly depauperate toward the interior of the bay, with the innermost study site supporting a virtual monoculture of the grass shrimp Thor j/oridanus. Species richness was similar across bank tops and exposed and sheltered sides. For the majority of species and sites, the exposed sides had the lowest abundance of the three transects. We suggest that restricted circulation may limit larval recruitment to inner portions of the bay (particularly for those species whose adult populations within the bay frequently fell to virtually nil) and that physical parameters, especially salinity, partially mediate the community structure of organisms that do gain access to the bay's isolated areas. Florida Bay's unique system of web-like mud banks (Holmquist et aI., 1989, fig. 1) divide the bay into shallow basins less than 3 m deep. The shallow banks are covered with dense meadows of Thalassia testudinum, Halodule wrightii, Syringodium filiforme, with Thalassia by far the most abundant (Zieman et aI., 1989). Despite comprising only 25% of Florida Bay's area, the banks exert a considerable influence on the bay ecosystem in terms of sediment accumulation (Enos and Perkins, 1979), seagrass standing crop and productivity (Zieman et aI., 1989), and restriction of circulation (Ginsburg, 1956; Fleece, 1962; Gorsline, 1963; Taft and Harbaugh, 1964; Scholl, 1966; Turney and Perkins, 1972; Enos and Perkins, 1979; Davies, 1980; Costello et aI., 1986). Because the banks are shallow, the demersal community faces predation from wading birds (Recher and Recher, 1968) and extremes of turbulence, temperature, and desiccation not encountered in deeper seagrass beds (Holmquist et aI., 1989), but this stress may be mitigated by the greater bank seagrass cover. Florida Bay's marine fauna is of considerable interest as the bay functions as an interface for Antillean, Carolinean, and Gulf ecological provinces. Despite the key location, the marine invertebrates have been little surveyed; the Crustacea are no exception. Tabb et ai. (1962b) and Brook (1978) provided a study of the extreme northwestern bay, and M. Robblee and J. Ti1mant are currently pursuing spatial and temporal aspects of decapod community ecology in Johnson Key Basin. Besides this work, there are investigations of commercial species (pink shrimp, stone crab, spiny lobster) populations (Higman, 1952; Idyll et aI., 1964; 1966; Tabb et aI., 1962a; Allen and Costello, 1966; Costello and Allen, 1966; Davis and Dodrill, 1979; Bert et aI., 1986; Costello et aI., 1986; Bert and Stevely, unpublished I; Davis and Dodrill, 1989) and three species lists from small portions , Bert, T. M. and J. M. Stevely. Population characteristics of the stone crab, Menippe mercenaria. in Florida Bay and the Florida Kcys. Unpublished. 251