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.

Mar Chiquita, a brackish coastal lagoon in central Argentina, is inhabited by dense populations of two intertidal grapsid crab species, Cyrtograpsus angulatus and Chasrnagnathus granulata. During a prehminary one-year study and a subsequent intensive sampling programme (November-December 1992), the physical properties and the occurrence of decapod crustacean larvae in the surface water of the lagoon were investigated. The lagoon is characterized by highly variable physical conditions, with oligohahne waters frequently predominating over extended periods. The adjacent coastal waters show a complex pattern of semidiurnal tides that often do not influence the lagoon, due to the existence of a sandbar across its entrance. Besides frequently occurring larvae (exclusively freshly hatched zoeae and a few megalopae) of the two dominating crab species, those of three other brachyurans [Plathyxanthus crenulatus, Uca uruguayensis, Pinnixa patagonica) and of one anomuran (the porcellanid Pachycheles haigae) were also found occasionally. Caridean shrimp (Palaemonetes argentinus) larvae occurred in a moderate number of samples, with a maximum density of 800. m -3. The highest larval abundance was recorded in C. angulatus, with almost 8000. m -3. Significantly more C. angulatus and C. granulata zoeae occurred at night than during daylight conditions, and more larvae (statistically significant only in the former species) during ebb (outflowing) than during flood (inflowing) tides. In consequence, most crab zoeae were observed during nocturnal ebb, the least with diurnal flood tides. Our data suggest that crab larvae do not develop in the lagoon, where the adult populations live, but exhibit an export strategy, probably based upon exogenously coordinated egg hatching rhythms. Zoeal development must take place in coastal marine waters, from where the megalopa eventually returns for settlement and metamorphosis in the lagoon. Significantly higher larval frequency of C. granulata in low salinities (--< 12%o) and at a particular sampling site may be related to local distribution patterns of the reproducing adult population. Unhke crab larvae, those of shrimp (P. argentinus) are retained inside the lagoon, where they develop from hatching through metamorphosis. They significantly prefer low salinity and occur at the lagoon surface more often at night. These patterns cannot be explained by larval release rhythms hke those in brachyuran crabs, but may reflect diel vertical migrations to the bottom. It is concluded that osmotic stress as well as predation pressure exerted by visually directed predators (small species or life-cycle stages of estuarine fishes) may be the principal selection factors for the evolution of hatching and migration rhythms in decapod larvae, and that these are characteristics of export or retention mechanisms, respectively.

The semiterrestrial crab Neohelice (=Chasmag-nathus) granulata (Dana 1851) is a predominant species in brackish salt marshes, mangroves and estuaries. Its larvae are exported towards coastal marine waters. In order to estimate the limits of salinity tolerance constraining larval retention in estuarine habitats, we exposed in laboratory experiments freshly hatched zoeae to six diVerent salinities (5-32‰). At 5‰, the larvae survived for a maximum of 2 weeks, reaching only exceptionally the second zoeal stage, while 38% survived to the megalopa stage at 10‰. Shortest development and negligible mortality occurred at all higher salt concentrations. These observations show that the larvae of N. granulata can tolerate a retention in the mesohaline reaches of estuaries, with a lower limit of ca. 10-15‰. Maximum survival at 25‰ suggests that polyhaline conditions rather than an export to oceanic waters are optimal for successful larval development of this species. In another experiment, we tested the capability of the last zoeal stage (IV) for reimmigration from coastal marine into brackish waters. Stepwise reductions of salinity during this stage allowed for moulting to the megalopa at 4-10‰. Although survival was at these conditions reduced and development delayed, these results suggest that already the zoea-IV stage is able to initiate the reimmigration into estuaries. After further salinity reduction, megalopae survived in this experiment for up to >3 weeks in freshwater, without moulting to juvenile crabs. In a similar experiment starting from the megalopa stage, successful metamorphosis occurred at 4-10‰, and juvenile growth continued in freshwater. Although these juvenile crabs showed signiWcantly enhanced mortality and smaller carapace width compared to a seawater control, our results show that the late larval and early juvenile stages of N. granulata are well adapted for successful recruitment in brackish and even limnetic habitats.

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Larvae uf an estudrine grapsid crdb Chasmagnuthus granul(lt(l l)ana 1851, tram temperate and subtropical regions at South America, were reared in seawater [32"/,,,,) at five different cnnstant lemperatures (12, 15, 18, 21, 2.1 ~ Conlplele larval dewflopment fronl hdtching IZoea [) to metamorphosis (Crab l) occurred in a range Irom I5 to 24 :C 1 tighest surviwll (60".;, to the first juvenile stagel was observed at 18 ~C, while all ldrvae reared at 12 ~C died before metamorphosis. The duration at development (D) decreased with increasing temperature (T). This relationship is described for r larval sta.qes as o poweK function (linear regressions alter logarithmic tr~msh)rntationof both Dand T) The temperature-dependenceoItheinstantdneousdevelopnlentalrate(D') is compared among larval stages and temperatures using the Q,,, coefficient (van't Hall's equation). Throu~h aii four zoeal stages, this Index tends to mcredse dunng development and to decrease with increasing T (comparing langes 12-18, 15-21, 18-24 ~C). In lhe Megalnpa, low" Q::, values were found in the range from 15 to 24 ~C. In another series of experiments, larvae were reared at constant lB ~C, and their dry weight (W) and respiratory response Io changes in T were measured in all successive stages during the intermoult period (stage C) of the moulting cycle. Both individual and weight-specific respiration (R, OOJ increased exponentially with increasing 7~ At each temperature, R incredsed significantly during growth and development through successive larval stages. No significantly dilferent QO, values were found in the first three zoeal stdges, while a significant decrease with increasing W occurred in the Zoea IV and Megalopa. As m the temperature-dependence of D, the respiratory response to changes in temperature (Q,.,,) depends on both the temperalure range and the developmental stage, however, with different patterns. In the zoeal stages, the respiratory Q~,, was minimum (l.7-2.2) at low temperatures (12-18 ~ but maximum (2.2-3.0} at 18-24 ~ The Megaiopa, in contrast, showed a stronger metabolic response in the lower than in the upper temperature range (Q~o = 2.8 and 1.7. respechvely). We interpret this pattern as an adaptation to a sequence of temperature conditions that should typically be encountered by C. granulata larvae during their ontogenetic migrations: hatching in and subsequent export from shallow estuarine lagoons, zoeal development in coastal marine waters, which are on average cooler, return in the Megalopa stage to warm lagoons. We thus propose that high metabolic: sensitivity to changes in temperature may serve as a signal slimulating larval migration, so that the zoeae should tend to leave warm estuaries and lagoons, whereas the Megalopa should avoid remaining in the cooler marine waters and initiate its migration towards shallow coastal lagoons.