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Three exposed sandy beaches were selected to compare £uctuations in density of the most abundant species, and to verify the in£uence of spatial and temporal variations on the community structure. Sampling was carried out every three months, from June 1993 to May 1995, on Fora, Prainha, and Boqueira¬ o Beaches. The ¢rst two beaches were classi¢ed by Dean's morphodynamic index () as intermediate, and the last as re£ective. Slope, beach width and median grain size were signi¢cantly di¡erent among the beaches.Two-way analysis of variance revealed signi¢cant di¡erences in species richness only among the beaches. No signi¢cant di¡erences in density of the macrofauna between beaches and season were observed. However, Prainha Beach showed a higher temporal variation of the density of the macrofauna (and higher standard deviation) than the other two beaches. Emerita brasiliensis (Crustacea: Decapoda), Excirolana braziliensis (Crustacea: Isopoda), and Pseudorchestoidea brasiliensis (Crustacea: Amphipoda) were the most abundant species. Canonical correspondence analysis calculated the in£uence of the temporal variation as 27.5%; the in£uence of the environmental variation on community structure was 20.9%. The results suggest that in spite of the long-term £uctuations in species density, the beaches did not have temporal di¡erences in the species richness and total density macrofauna during the study period.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The Crustacean Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Crustacean Biology. ABSTRACTThe population biology of Emerita brasiliensis (Decapoda: Hippidae) was studied by means of biweekly sampling from October 1990 through March 1993 at Praia da Fora Beach, Rio de Janeiro, Brazil (22?57'S, 43?10'W). Two transects were established in the swash zone, and divided into 4 strata parallel to the waterline. From each stratum, 4 replicates were taken with a 0.04-m2 sampler. Highest population densities were observed at the end of spring and during summer. The presence of ovigerous females and juveniles during the entire sampling period indicated continuous reproduction of the population. The sex ratio was 1.82 for males : 1.00 for females. Batch size varied from 610-1,093 eggs, for females 10-23 mm long, respectively. Growth and mortality rates were higher for females than for males. Longevity for 99% of the population varied approximately from 6-10 months.