ABSTRACT. Diverse reproductive strategies may be adopted by different species of Macrobrachium prawns, and even among different populations of the same species. The present study evaluated the influence of differences in the reproductive strategies of two geographically isolated populations of Macrobachium amazonicum, upon female fecundity, reproductive output and the chemical content of prawns and eggs. One prawn population from Pará only completes its life cycle in brackish water, whereas another from Mato Grosso do Sul only inhabits freshwater. Pará female prawns exhibited a larger average size and weight and produced more eggs than females from Mato Grosso do Sul. However, the Mato Grosso do Sul population produced eggs that were larger in volume than those of females from the other population. Furthermore, eggs produced by Pará prawns were composed primarily of water (56%), whereas those produced in Mato Grosso do Sul were composed mostly of organic matter (80%). This difference in the eggs' chemical compositions did not apply to the chemical compositions of the females, as individuals from both sites were composed primarily of water. Mato Grosso do Sul females invested a higher amount of energy in brood formation (14% of their wet weight) than individuals from Pará (only 10%). It is possible that M. amazonicum populations show a higher degree of plasticity in their reproductive activity due to habitat conditions and genetic differences. Although the brackish population produces larger individuals, and exhibits higher fecundity, the freshwater population exhibited a higher reproductive investment. These results suggest a high reproductive capacity to adapt to different environmental conditions for this species, which should be considered in the context of aquaculture activities.Keywords: Macrobrachium amazonicum, reproductive potential, ecological plasticity, egg production, Brazil.Variabilidad reproductiva del camarón del río Amazonas, Macrobrachium amazonicum (Caridea, Palaemonidae): influencia del ciclo de vida en la fecundidad RESUMEN. Diversas estrategias reproductivas pueden ser adoptadas por distintas especies de camarones del género Macrobrachium, e incluso entre diferentes poblaciones de la misma especie. En el presente estudio se evaluó la influencia de las diferencias en las estrategias reproductivas de dos poblaciones de Macrobachium amazonicum, geográficamente aisladas, sobre la fecundidad, rendimiento reproductivo y composición química de los camarones y sus huevos. Una población de camarones de agua dulce de Pará sólo completa su ciclo de vida en agua salobre mientras, que otra de Mato Grosso do Sul sólo habita en aguas dulces. Las hembras de la población de Pará presentaron, en promedio, mayores tamaños y mayor peso y producen más huevos que las hembras de Mato Grosso do Sul. Sin embargo, la población de Mato Grosso do Sul produjo huevos que eran más grandes en volumen que aquellos de las hembras de la otra población. Aún más, los huevos producidos por hembras de la población de Pará contení...

The purpose of this study was to determine the shell occupation pattern of Pagurus exilis using the percentage of shell types that were occupied and the morphometric relationship between hermits and occupied shells. Specimens were collected monthly from July 2001 to June 2003. A total of 1737 individuals were collected, occupying 19 gastropod shell species. Buccinanops gradatum (82%) was the most occupied shell, followed by Natica isabelleana (6.4%), Stramonita haemastoma (3.2%), and Cymatium parthenopeum (2.2%). There was a differential shell utilization pattern between sexes; males occupied larger and heavier shells while the non-ovigerous females occupied the lightest ones. Pagurus exilis occupied more bare shells instead of incrusted ones. Buccinanops gradatum was the most occupied shell species as a function of its availability in the survey, and this occupation is strongly associated with the higher internal volume presented by these shells.

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Shell preferences, as shown by laboratory choice experiments, are important determinants of shell utilization under natural conditions. Size and shell species preferences of the hermit crab Pagurus exilis were determined and compared for the most occupied shell types (Olivancillaria urceus, Natica isabelleana and Buccinanops gradatum) in the Caraguatatuba region, Brazil, and for the two most occupied species (B. gradatum and N. isabelleana) in Mar del Plata, Argentina. All experiments were conducted under laboratory conditions, using glass aquaria where the hermit crabs were placed naked with a large number of shells of appropriate sizes. Animals often must compromise among different sets of demands when selecting resources (Yoshino et al. 1999). In hermit crabs particularly, shell selection is not by chance but based on adequacy and availability of resources (Reese 1962, Conover 1978, and is affected by both shell size and species (Abrams 1978, Conover 1978, Blackstone 1985, Lively 1988, Siu and Lee 1992, Ohmori et al. 1995, Hahn 1998, Rodrigues et al. 2000, Mantelatto and Dominciano 2002, Mantelatto and Meireles 2004.Moreover, the fitness of a particular shell may differ between hermit crab species, reflecting several selection pressures, which associated with different habitats, act in different ways on each crab species (Bertness 1981) in different areas. Although the shell selection process has been well investigated (Vance 1972, Elwood et al. 1979, Bertness 1980, Wilber 1990, Hazlett 1992, Ohmori et al. 1995, Wada et al. 1997, Garcia and Mantelatto 2001, Meireles and Mantelatto 2005, there is a scarcity of studies about the shell preferences of two different populations of the same species from different locations distantly removed from one another. Comparative studies of species known to occur over wide geographical