Cyztograpsus angulatus and Chasmagnathus granulata (Grapsidae) are the two dominant decapod crustacean species in the outer parts of Mar Chiquita Lagoon, the southernmost in a series of coastal lagoons that occur along the temperate Atlantic coasts of South America. Distribution and habitat preferences (water and sediment type) in these crab species were studied in late spring. There is evidence of ontogenetic changes in habitat selection of both species. Recruitment of C. angulatus takes place mainly in crevices of tube-building polychaete (Ficopomatus enigmaticus) 'reefs' and, to a lesser extent, also in other protected microhabitats (under stones). In the latter, mostly somewhat larger juveniles were found, suggesting that these are used as a refuge for growing individuals. Adults are most frequently found on unprotected muddy and sandy beaches. C. angulatus was found in all parts of Mar Chiquita Lagoon, including freshwater, brackish, and marine habitats. C. granulata, in contrast, was restricted to the lower parts of the lagoon where brackish water predominates and freshwater or marine conditions occur only exceptionally. It showed highest population density on 'dry mud' flats and in Spartina densiflora grassland where it can build stable burrows and where high contents of organic matter occur in the sediment. Such habitats are characterized by mixed populations of juveniles (including newly settled recruits) and adults, males and females (including a high percentage of ovigerous). Unstable ''wet mud'' as well as stony sand were found to be inhabited by chiefly adult populations, with only few ovigerous females. In ''dry mud' flats, the proportion of males increased vertically with increasing level in the intertidal zone, showing a significantly increasing trend also in their average body size. These observations may be explained by higher resistance of males, in particular of large individuals, to disiccation, salinity, and temperature stress occurring in the upper intertidal. However, an opposite, or no such, tendency was found in the distribution of ovigerous and non-ovigerous females, respectively. With increasing distance from the water edge, saliniity increased and pH decreased significantly in C. granulata burrows, whereas temperature showed no consistent tendency within the intertidal gradient. A highly significant linear relationship (r = -0.794; P < 0.001) between salinity and pH in water from crab burrows is described. This regression line is significantly different from one that had been observed in water from the lagoon, indicating consistently lower pH values at any salinity level in burrow water. This is interpreted as a result of crab and/or microbial respiration.
