Distribution and habitat preferences of two grapsid crab species in Mar Chiquita Lagoon (Province of Buenos Aires, Argentina)

Abstract

Cyrtograpsus 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. granutata, 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 desiccation, salinity, and temperature stress occurring in the upper intertidal. However, an opposite, or no such, tendency was found in the distribution of ovigerous and nonovigerous females, respectively. With increasing distance from the water edge, salinity increased and pH decreased significantiy 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 hne is significantly different from one that had been observed in water from th e 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.

Experimental studies on predation and cannibalism of the settlers of Chasmagnathus granulata and Cyrtograpsus angulatus (Brachyura: Grapsidae)

“…Pile et al, 1996 . C. granulata megalopae settle in or near the adult burrows, where various juvenile Ž . size-classes may coexist Spivak et al, 1994 . The burrows protect the settlers against fishes and birds, while the adults that live inside the burrows protect the settlers against predators including conspecific juveniles.…”

“…Olivier et al, 1972a;Reta et al, 1997 . Temperature and salinity have daily, seasonal Ž and geographic variations Anger et al, 1994;Martos and Reta, 1997;Marcovecchio et . al., 1997 .…”

“…Fernandez, 1999 , hunger of the predator Okamoto and Kurihara, 1989 , the availabilitý Ž . of alternative food Kurihara and Okamoto, 1987;Dutil et al, 1997 , Olivier et al, 1972a;Spivak et al, 1994 . Adult C. angulatus live in tidal mud Ž flats, whereas juveniles and settlers are found in crevices within polychaete Ficopoma-.…”

“…Adult C. angulatus live in tidal mud Ž flats, whereas juveniles and settlers are found in crevices within polychaete Ficopoma-. Ž tus enigmaticus AreefsB and under stones or bivalve shells Spivak et al, 1994; . Gutierrez and Iribarne, 1999 . In biogenic substrates such us polychaete reefs, settlers Ž .…”

“…C. granulata dig and inhabit semi-permanent burrows in mudflats and Ž . Spartina densiflora grasslands Boschi, 1964;Spivak et al, 1994;Iribarne et al, 1997 . Juveniles live near the conspecific adults in separate burrows, but usually juvenile and Ž adult burrows are interconnected.…”