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Populations are dynamic in space and time. Abundance of organisms can increase or decrease, having a similar influence on the density-dependent processes and the stochastic events (Morris 1996). Moreover, animal populations have certain synchronization between their fluctuations and the different environmental factors (Ranta 1998, Ruetz et al. 2005 which have a direct influence on density, sex ratio and recruitment. The study of biotic interrelationships such as those occurring in planktonic groups or those of invertebrates living in macrophytes, requires the knowledge of change rate, response time and the possibility for a population or community to repeat these conditions through time (Poi de Neiff & Bruquetas 1989). In rivers with floodplains, such us the Paraná River, different highly dynamic habitats are recognizable, where stability and diversity are primarily conditioned by hydrology and flow of materials. The flooding pulse results in the coupling between the river and floodplain. Moreover, this basin provide shelters, foods and connectivity among different waterbodies (Bonetto & Wais 1995). Therefore, relationships among populations of aquatic organisms like crabs, are subject to the variations of the river.Trichodactylus borellianus is a common freshwater crab species of the Trichodactylidae family from South America (Lopretto 1995, Magalhães & Türkay 1996. Its natural habitat is the aquatic vegetation - The mean crab abundance among the study sites was significantly different, being lower in samples from the environment surrounding the principal channel of the Paraná River. Crab density showed two peaks: the highest occurred during late winter and early spring, and the lowest during late summer. Males were only more abundant in late winter and spring. In spring, summer and autumn, the population showed a contagious distribution. The population displayed a regular arrangement in winter. From mid-spring to mid-autumn juveniles were very frequent whereas adults predominated the rest of the year. Total crab number in the stream nearest to the main channel varied according to the level and temperature of the river. In the other sites, located further from the main channel of the Paraná River, crab numbers and vegetation biomass showed a more similar arrangement with a higher correlation.

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The crustaceans of the family Aeglidae are endemic to the southern regions of South America. Geometric morphometrics was used to assess differences in size and shape between two sympatric species of the family, A. uruguayana and A. platensis. Eleven landmarks on the dorsal region of the cephalothorax were recorded on 57 adult specimens. Interspecific and intraspecific differences in size and shape were analyzed through univariate and multivariate statistics performed on the generalized procrustes analysis aligned coordinates. Shape differences between A. uruguayana and A. platensis, and between males and females, were readily identifiable along the first and the second relative warp. MANOVA showed these differences to be significant. Intraspecific comparisons also revealed a significant sexual dimorphism in cephalothorax shape. Two-way ANOVA on centroid size did not show any significant difference between species nor between sexual.

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