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.

The grapsid crabs Chasmagnathus granulata and Cyrtograpsus angulatus are considered as key species within the benthic communities of estuaries and brackish coastal lagoons in the southwestern Atlantic region. In controlled laboratory experiments, we studied the intensity of interspecific predation as well as intra-and intercohort cannibalism in setllers in relation to refuge Ž . availability, predator characteristics species, size, sex, nutritional state , and the presence or Ž . absence of an alternative food source Artemia nauplii . In both species, the intensity of Ž . intracohort cannibalism among recently settled crabs instars I and II, AsettlersB was low, with ca. 5% mortality during 48 h experimental observation periods. Larger juveniles and adults of both species, by contrast, preyed heavily on the settlers. Predation was significantly reduced when refuges were available for the settlers. Hunger of the predators enhanced in general the predation rate. In the presence of alternative food, the consumption of settlers was significantly reduced. In C. granulata, adult females ate more settlers than the males, probably as a consequence of differences in the morphometric traits of their chelae. Cannibalism and predation by juvenile and adult crabs may play an important role in the regulation of recruitment success for both species and hence, in the structure of estuarine benthic communities. q

Crabs are among the most conspicuous and ecologically important invertebrates of the large intertidal zones that characterize estuarine and protected coastal areas in temperate regions. The habitat, population structure and breeding cycle of Chasmagnathus granulatus (Brachyura: Varunidae), a semiterrestrial burrowing crab endemic to the warm temperate coasts of the Southwestern Atlantic, were studied in San Antonio Bay (Argentina), near the southern limit of its range. San Antonio Bay has no freshwater input, winter is relatively colder, and summer warmer, than northern habitats of this species. Crabs lived both in vegetated and unvegetated zones, but density and sex ratio varied among dates and zones. The maximum observed density was 136 crabs/m 2 , the maximum carapace width (CW) was 32 mm (males) and 29.8 mm (females), ovigerous females were found only in November and January, and the smallest ovigerous female measured 17 mm CW. The population structure, spatial distribution, and recruitment pattern of C. granulatus did not differ between San Antonio Bay and northern habitats. The higher density, smaller maximum size and shorter reproductive cycle observed in San Antonio cannot be atributed to changes associated with a latitudinal cline and other factors, such as thermal amplitude and food availability, need to be studied. Hydrobiologia (2005) 537: 217-228 Ó Springer 2005

Reproductive traits at the beginning and the end of the annual reproductive season were compared between two populations of the intertidal crab Chasmagnathus granulatus living in ecologically contrasting habitats: (1) Mar Chiquita (MC) (37°45ЈS, 57°19ЈW), a highly productive estuarine coastal lagoon with strong salinity Xuctuations.(2) San Antonio Bay (SA) (40°46ЈS, 64°50Ј), a physically stable but less productive coastal marine environment. Number, size, and elemental composition (CHN) of eggs and larvae diVered signiWcantly between populations. Regardless of the season, more but smaller eggs and larvae were produced in MC, while eggs and larvae from SA revealed higher dry mass and C/N ratios indicating higher lipid content. A latitudinal temperature gradient cannot explain these patterns, suggesting that other environmental factors including salinity, quality or quantity of benthic food sources and productivity may be responsible. In both populations, fecundity and biomass per egg were higher at the beginning as compared to the end of the reproductive season. As a consequence, the reproductive eVort was consistently maximal at the beginning of the season. At MC, also variability was found between two successive years. IntraspeciWc (both interpopulational and seasonal) variations in reproductive and developmental traits may be important for the formation of physiologically diVerent metapopulations along the wide geographic range of C. granulatus.

RESUMEN.Se mencionan 243 especies de crustáceos decápodos en el Atlántico sudoccidental, entre los 25º y los 55°S, pertenecientes a 49 familias. El suborden Dendrobranchiata incluye 6 familias con 23 especies. El suborden Pleocyemata consta de 10 familias con 35 especies de Caridea, 1 familia con 2 especies de Astacidea, 1 familia con 1 especie de Palinura, 2 familias con 7 especies de Thalassinidea, 8 familias con 44 especies de Anomura y 21 familias con 131 especies de Brachyura. En la región templado cálida del Atlántico sudoccidental se han registrado 200 especies; el 73% de las mismas se extienden hacia el norte de la región en estudio ("especies tropicales"), el 27% habitan entre las latitudes de Rio de Janeiro y Tierra del Fuego. De éstas, 44 son endémicas. El número de especies tropicales disminuye con el aumento de la latitud, aunque con diferentes patrones en distintas familias; la expansión hacia el sur parece estar limitada por cambios ambientales entre los 29°-35°S, principalmente el descenso de la temperatura invernal y el efecto del Río de la Plata. La región templado-fría incluye 39 especies, 24 de las cuales habitan también el Pacífico sur. Muchas especies templado-frías (50%) alcanzan latitudes menores a 42°S en aguas profundas del Atlán-tico. Se resumen los datos referidos a la distribución geográfica, ciclos de vida e importancia económica de las especies de la región. El número de especies y familias es similar al que se encuentra en las costas del Pacífico, entre las mismas latitudes, pero existen diferencias en la composición faunística. Palabras claves: Crustacea, Decapoda, distribución geográfica, ciclos de vida, Atlántico sudoccidental. The crustacea decapoda in the southwestern Atlantic (25º-55ºS): distribution and life cyclesAbstract. A number of 243 species of decapod crustaceans, belonging to 49 families, were found in the southwestern Atlantic, between 25 and 55°S. Suborder Dendrobranchiata includes 6 families with 23 species. Suborden Pleocyemata has 10 families of Caridea with 35 species, 1 family of Astacidea with 2 species, 1 family of Palinura with 1 species, 2 families of Thalassinidea with 7 species, 8 families of Anomura with 44 species and 21 families of Brachyura with 131 species. In the warm-temperate region of the southwestern Atlantic there are 200 species; 73% of them spread to the north ("tropical species"), 27% are found between Río de Janeiro and Tierra del Fuego. From these species, 44 are endemic. The number of tropical species diminishes at higher latitudes, although different families show different patterns; environmental changes at 29º-35°S, mainly the fall in winter temperatures and the effects of La Plata River, seem to limit their southern spread. Cold-temperate region is represented by 39 species; 24 of them also inhabited the southern Pacífic ocean. Many cold-temperate species (50%) reach lower latitudes in the Atlantic (42°S or less), but in deep waters. Information on geographic distribution, life cycles, and economic importance of warm-temperate speci...

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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.

1. Zooplankton use macrophytes as day-time refuge areas when trying to escape from pelagic predators. But macrophytes can also host a diverse and abundant macroinvertebrate assemblage and zooplankton are also likely to face predacious macroinvertebrates once they enter the littoral zone. This study aimed to elucidate the role of macroinvertebrates in determining the refuge capacity of macrophytes. 2. We conducted a field enclosure experiment using plastic bags and complementary laboratory feeding trials to test how macroinvertebrates counteract the benefits to zooplankton of the macrophyte refuge. The field experiment consisted of three treatments with different macroinvertebrate assemblages: without predators (WP), low abundance and diversity (LAD) and high abundance and diversity of predators (HAD -which represents lake conditions). 3. Populations of Diaphanosoma brachyurum, Bosmina huaronensis and Moina micrura (Cladocera) and of both male and female Notodiaptomus incompositus (Copepoda, Calanoida) declined (by nearly 80%) in the presence of HAD in comparison to WP and LAD treatments. 4. Feeding trials revealed that Buenoa sp. (backswimmer), adults of Palaemonetes argentinus (grass shrimp) and Cyanallagma interruptum (damselfly) had a significant negative impact on cladocerans (D. brachyurum, B. huaronensis) and the calanoid copepod population (males, females and copepodites). These predators showed a strong predation effect ranging from 75% to 100% reductions of zooplankton populations. 5. The refuge effect offered by macrophytes to zooplankton depends on and is balanced by the predacious macroinvertebrate assemblage that plants host. The risk of confronting littoral predators is high and macroinvertebrate presence can turn the macrophytes into risky areas for zooplankton.