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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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Background: In inland water systems, it is important to characterize the trophic links in order to identify the 'trophic species' and, from the studies of functional diversity, understand the dynamics of matter and energy in these environments. The aim of this study is to analyze the natural diet of Trichodactylus kensleyi of subtropical rainforest streams and corroborate the temporal variation in the trophic activity during day hours. Results: A total of 15 major taxonomic groups were recognized in gut contents. The index of relative importance identified the following main prey items in decreasing order of importance: vegetal remains, oligochaetes, chironomid larvae, and algae. A significant difference was found in the amount of full stomachs during day hours showing a less trophic activity at midday and afternoon. The index of relative importance values evidenced the consumption of different prey according to day moments. Results of the gut content indicate that T. kensleyi is an omnivorous crab like other trichodactylid species. Opportunistic behavior is revealed by the ingestion of organisms abundant in streams such as oligochaetes and chironomid larvae. The consumption of allochthonous plant debris shows the importance of this crab as shredder in subtropical streams. However, the effective assimilation of plant matter is yet unknown in trichodactylid crabs. Conclusions: This research provides knowledge that complements previous studies about trophic relationships of trichodactylid crabs and supported the importance of T. kensleyi in the transference of energy and matter from benthic community and riparian sources to superior trophic levels using both macro-and microfauna.

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The relative growth of a number of morphological dimensions of the South American freshwater crab Trichodactylus borellianus (Trichodactylidae) were compared and related to sexual dimorphism. Crabs were collected from ponds in the Middle Paraná River in Argentina. A regression model with segmented relationship was used to test for relative growth between these measurements where breakpoints infer the body size at which crabs reach sexual maturity. In both sexes the carapace width and the length, height, and thickness of the right and left chelae were measured, as well as the male pleopod length and the female abdomen width. All of these measurements were found to show positive allometry with the exception of the male pleopod length and the left chelae, which did not show a breakpoint. In females the breakpoint for the abdomen width inferred a morphological sexual maturity at carapace width 6.9 mm. In males the break point for the pleopod length was at carapace width 6.6 mm, with that for the chelae measurements was between carapace widths 6.4 and 6.9 mm. The relative growth pattern in Trichodactylus borellianus was found to be similar to that recorded for other species of the family Trichodactylidae.

Williner, V. 2010. Foregut ossicles morphology and feeding of the freshwater anomuran crab Aegla uruguayana (Decapoda, Aeglidae). -Acta Zoologica (Stockholm) 91: 408-415.The acquisition and processing of food is critical to animal survival and reproductive success. This work describes the foregut ossicles of Aegla uruguayana, which have been proposed to impart trophic characteristics. In addition, stomach contents were analysed using Index of Relative Importance and Weighted Result Index to characterize the diet. The Pearre index was applied to analyse trophic selectivity. We found A. uruguayana has a morphological foregut typical of macrophage organisms. Stomach contents included items typical of omnivorous, generalist and opportunistic feeding modes. Vegetal remains included algae (filamentous, unicellular and colonial morphotypes), insect larvae, oligochaetes, microcrustaceans (copepods and cladocerans), mites, tardigrades, juveniles of A. uruguayana and rotifers. Morphological descriptions of the foregut can reveal feeding habits and provide data on the possible trophic profile of a species, while guiding the selection of appropriate methodology for subsequent analysis. Our stomach content data corroborated the foregut description, but the presence of small prey suggested A. uruguayana utilizes both predation and detritivory trophic strategies.