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Feeding habits and selectivity of Palaemonetes argentinus were studied by examining the stomach contents of specimens from an oxbow lake throughout a year. Palaemonetes argentinus is omnivorous, feeding mainly on members of littoral benthic communities. The bulk of the stomach contents comprised algae, plant remains, oligochaetes, dipteran larvae, and a variety of organisms ranging from Rotifera to Palaemonidae. Zooplankton (rotifers, copepods, and cladocerans) were an alternative food source in winter, at low water, when macrophyte abundance had declined. In general, Oligochaeta and dipteran larvae were positively selected, Rotifera and microcrustaceans negatively, and algae indifferently, but with some variations depending on the seasons.

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.

ABSTRACT. Freshwater crabs are not commonly considered to be an important group in trophic webs, and this might be due to a lack of knowledge about their trophic roles in aquatic ecosystems. Trichodactylus borellianus is one of the most common and widely distributed freshwater crabs in the floodplains of the southern South American rivers. The main objective of the present study was to examine the trophic role of T. borellianus, in the floodplain of the Paraná River, and its relationships with the freshwater littoral community. The trophic spectrum of this species was characterized for both sexes and individuals of different sizes (adults and juveniles), throughout daily and seasonal cycles. Samples were collected from the aquatic vegetation of three shallow lakes. The diet composition and the feeding activity of T. borellianus were evaluated through the examination of the stomach contents and their degree of emptiness. This crab species consumed several plant and animal items, including amoebas, rotifers, oligochaetes, copepods, cladocerans, and insect larvae. Moreover, this species consumes filamentous and unicellular algae, diatoms, fungi, and macrophytic remains. The predatory habits varied with the season and time of day, and variations in the feeding activity of the juveniles and adults were detected and documented. The diversity of food items eaten by this crab suggests that its trophic role in the community as an omnivore and opportunistic predator provides a connection among several trophic levels from both aquatic and terrestrial communities. Keywords: omnivore, trophic habits, stomach contents, detritus, macro-invertebrates. Ecología trófica del cangrejo dulceacuícola Trichodactylus borellianus (Decapoda:Trichodactylidae) en el valle de inundación del río Paraná, sur de Sudamérica RESUMEN. Los cangrejos de agua dulce no son comúnmente considerados como un grupo importante en las redes tróficas, y esto podría ser debido a falta de conocimiento acerca de sus funciones tróficas en los ecosistemas acuáticos. Trichodactylus borellianus es uno de los cangrejos de agua dulce más comunes y ampliamente distribuido en llanuras de inundación del los ríos del sur de Sudamérica. El objetivo principal del presente estudio fue examinar el rol trófico de T. borellianus en la llanura de inundación del río de Paraná, y sus relaciones con la comunidad litoral de agua dulce. El espectro trófico de esta especie se caracterizó en ambos sexos e individuos de diferentes tamaños (adultos y juveniles), a través de los ciclos diarios y estacionales. Se tomaron muestras en la vegetación acuática de tres lagunas diferentes. La composición de la dieta y la actividad de alimentación de T. borellianus fueron evaluados mediante la observación de contenidos estomacales y grado de repleción. Esta especie consume plantas y animales, incluyendo amebas, rotíferos, oligoquetos, copépodos, cladóceros y larvas de insectos. Además, consume algas filamentosas y unicelulares, diatomeas, hongos y restos de macrófitas. Los hábitos de consumo vari...

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