POPULATION-DYNAMICS OF PINNIXA-PATAGONIENSIS RATHBUN, 1918 (BRACHYURA-PINNOTHERIDAE) A SYMBIOTIC CRAB OF SERGIO-MIRIM (THALASSINIDEA-CALLIANASSIDAE) IN CASSINO BEACH, SOUTHERN BRAZIL

Pinnotherid crabs are typically found associated with many benthic inv ertebrates. On sandy beaches of southern Brazil, Pinnixa patagoniensis RATHBUN, 1918 inhabits burrows of the ghost shrimp Sergio mirim (RODR IGUES, 1971). The population dynamics of this crab were studied from m onthly samples taken between March 1989 and November 1991 near the Pat os Lagoon outlet, southern Brazil (ca. 32 degrees 13'S; 52 degrees 15' W). There was no correlation between the number and size of the crabs and the size of their respective hosts. Mean densities and incidence r eached 3.1 indiv..opening-(1) (range: 0 to 29) and 83.9%, respectively . Sex-ratios were not significantly different from 1:1 for the differe nt size classes and sampling dates. Ovigerous females were found betwe en October and March, showing an average fecundity of 1529 eggs per fe male. The annual recruitment pattern was bimodal, with a minor peak in summer and a fall peak that accounted for almost 70% of the populatio n maintenance. Males and females both reached a maximum carapace width of 11.5 mm and had a life span of nearly two years as estimated from fitted vow BERTALANFFY growth curve parameters.