Claudia C. Bas

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

Cyrtograpsus angulatus andChasmagnathus 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 ofC. 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 inSpartina 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 non-ovigerous females, respectively. With increasing distance from the water edge, salinity increased and pH decreased significantly inC. 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.

Hatching rhythms and dispersion of decapod crustacean larvae in a brackish coastal lagoon in Argentina

Mar Chiquita, a brackish coastal lagoon in central Argentina, is inhabited by dense populations of two intertidal grapsid crab species,Cyrtograpsus angulatus andChasmagnathus granulata. During a preliminary 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 oligohaline 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 porcellanidPachycheles 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 inC. angulatus, with almost 8000°m−3. Significantly moreC. angulatus andC. 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 ofC. granulata in low salinities (≤12‰) and at a particular sampling site may be related to local distribution patterns of the reproducing adult population. Unlike 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 like 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.

Seasonal and interpopulational variability in fecundity, egg size, and elemental composition (CHN) of eggs and larvae in a grapsoid crab, Chasmagnathus granulatus

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°45′S, 57°19′W), a highly productive estuarine coastal lagoon with strong salinity fluctuations. (2) San Antonio Bay (SA) (40°46′S, 64°50′), a physically stable but less productive coastal marine environment. Number, size, and elemental composition (CHN) of eggs and larvae differed significantly 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 effort was consistently maximal at the beginning of the season. At MC, also variability was found between two successive years. Intraspecific (both interpopulational and seasonal) variations in reproductive and developmental traits may be important for the formation of physiologically different metapopulations along the wide geographic range of C. granulatus.

Population structure of the South American Estuarine crab, Chasmagnathus granulatus (Brachyura: Varunidae) near the southern limit of its geographical distribution: comparison with northern populations

AbstractCrabs 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/m2 , 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. granulatusdid 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.

Variation in early developmental stages in two populations of an intertidal crab, Neohelice (Chasmagnathus) granulata

Duration of embryonic development, egg size, larval size at hatching, and starvation tolerance of the first zoeal stage were studied in an intertidal crab from the southwestern Atlantic, Neohelice (formerly Chasmagnathus) granulata. These reproductive traits were quantified comparing (a) two populations living in ecologically contrasting coastal habitats in Argentina, a brackish lagoon, Mar Chiquita, MC vs. an open marine habitat near San Antonio, Patagonia, SA, (b) beginning vs. end of the reproductive season, and (c) two temperatures during egg development (18 vs. 27°C). Eggs in an early stage of embryonic development were in both populations larger at the beginning than at the end of the season, and were consistently larger in the SA population. These size differences persisted through larval hatching, independent of the temperature during embryogenesis. At 18°C, eggs produced at the beginning of the season developed in both populations more rapidly than those from the end of the reproductive season, while the opposite trend was observed at 27°C. The stage duration of the zoea I was in both populations shorter at the beginning as compared to the end of the season. The nutritional flexibility of the zoea I stage was compared using as indices the point-of-reserve-saturation (PRS50) and the point-of-no-return (PNR50). The PRS50 was consistently lower in larvae from SA than in those from MC. In the MC population, this index was lower at the beginning than at the end of the season, while no significant seasonal difference was observed in larvae from SA. The PNR50 varied between temperatures of embryonic development and populations, showing also significant interactions between all three factors. The PRS50 was on average lower, and the PNR50 was higher, than values previously reported for N. granulata, suggesting a stronger nutritional flexibility in the larvae used in the present study. Our results indicate significant intraspecific variability among separate populations, seasonal variation, and carry-over effects of environmental conditions prevailing during the embryonic phase, all of which may affect the performance of the larval phase.

Adaptive Differences in Digestive Enzyme Activity in the Crab Neohelice Granulata in Relation to Sex and Habitat

Neohelice granulata (Dana, 1851) is a semi-terrestrial burrowing crab that inhabits estuaries and saltmarshes feeding on grasses or sediment depending on the microhabitat they occupy (vegetated saltmarsh or bare mudflat). Specific cellulolytic, amylolytic, and proteolytic enzyme activities were analyzed in midgut gland homogenates of males and females from each microhabitat fed in the laboratory with Spartina densiflora leaves and sediment, respectively, in order to detect sex, food and microhabitat related differences. The presence of β-1,4-glucosidase, endo-β-1,4-glucanase, α amylase, trypsin and chymotrypsin were confirmed. Specific cellulolytic activity was higher in crabs fed on leaves than in those fed on sediment or in no fed controls and variable differences between sexes were observed. Specific amylase activity of crabs fed on leaves was the lowest recorded. Trypsin and chymotrypsin specific activities were higher in saltmarsh crabs fed on leaves than in mudflat crabs fed on sediment. Different mechanisms of enzyme regulation to explain the observed differences among groups were suggested. Additionally, differences between sexes suggest different metabolic needs related to gonad maturation. It is concluded that N. granulata has the ability to adapt digestive enzyme production to support its physiological and metabolic needs based on the different food sources available at each microhabitat.

Effects of temperature and salinity on the ovarian cycle and the embryonic development of the invasive shrimp Palaemon macrodactylus.

The oriental shrimp Palaemon macrodactylus Rathbun, 1902 was detected first in Argentina in 2000 in a fully marine environment (Mar del Plata) and more recently, in two estuarine areas. Here we examined combined effects of four salinities (1, 5, 12, 23, 34 psu) and two temperatures (20°, 25°C) on survival, reproductive cycle of females and embryonic development in shrimps from the marine population of Argentina. Survival was higher than 80% in all the treatments; however, at the lowest salinity (1 psu), all females died after 12 days irrespectively of the temperature condition. Most shrimps (>80%) developed their ovaries and produced eggs at all salinity–temperature tested combinations, but all females lost their eggs after 2 days at 5 psu; a complete embryonic development occurred only at 12 and 34 psu. At the lowest temperature, the ovarian development was delayed in different proportions for individual levels of salinity. Therefore, P. macrodactylus from Mar del Plata harbor survived and completed all reproductive events at salinities ≥ 5 psu, but embryonic development was only successful at salinities ≥ 12 psu. Our results indicate that this non-native species is capable of invading estuarine habitats over the Argentinean coast but not oligohaline or freshwater environments.

Population structure of the intertidal crab Cyrtograpsus altimanus Brachyura Varunidae in a northern Patagonia mussel bed

The population structure, size at maturity, reproductive period, recruitment and habitat use were studied in a population of the grapsoid crab Cyrtograpsus altimanus in an intertidal mussel bed at San Antonio Bay, Argentina. Samples were taken from October 2000 to January 2002. Crabs were sized and sexed to estimate size–frequency distributions (SFD). Modal groups of males and females were the same. Growth, evidenced by the shift to right of SFD modal classes, was observed in spring, summer and autumn. Size at maturity of females varied seasonally. Ovigerous females were found in winter, spring and summer, but two discrete recruitment events (in spring and autumn) are proposed. Based on the maximum male and female sizes found in the mussel bed, a size constraint is proposed that forces large crabs to migrate to cobblestone adjacent habitats.