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ABSTRACT. Food fl exibility and omnivory are important features pronounced in Neotropical freshwater fi sh species, particularly for Astyanax species. Traditionally most fi sh diet resources are known to be originated in the aquatic environment, however recent studies have pointed to the importance of allochthonous sources. Besides, the colonization of macrophytes, common at several tropical reservoirs, may enhance insectivory in fi sh diet expanding or concentrating the area of available resources for feeding. Here we employed stomach contents analysis of two sympatric Astyanax species to access the importance of habitat diff erentiations as spatial complexity in two tropical reservoirs with diff erent environmental features. The NMDS analysis indicated separation in the diet of these species between reservoirs (Stress= 8.28%). Additionally, analysis of variance revealed a signifi cative diff erence in the volume of food itens ingested between the reservoirs (Anova one-way F(1, 132)=4.4446; p= 0.037). This points out the importance of environmental conditions on the composition of the diet of fi shes. This study highlighted the insectivorous feeding habit of Astyanax species and revealed diff erent feeding strategies between sympatric fi shes despite high niche overlap in both environments. Habitat heterogeneity increasing food resources availability plays an important role in the diet strategy of these Astyanax species and on their constant maintenance in the two diff erent reservoirs.KEYWORDS. Feeding resources, land-water ecotone, trophic ecology, Astyanax aff . bimaculatus, Astyanax parahybae. RESUMO. Heterogeneidade de habitat no hábito alimentar de duas espécies simpátricas e congenéricas de peixes caracídeos em dois reservatórios tropicais.A fl exibilidade alimentar e onivoria são características das espécies de peixes de água doce neotropicais, especialmente em espécies de Astyanax. Tradicionalmente a maioria dos recursos na dieta de peixes é reconhecidamente originária do ambiente aquático, porém estudos recentes têm apontado para a importância das fontes alóctones. Além disso, a colonização de macrófi tas, comuns em vários reservatórios tropicais, pode melhorar os aspectos de insetivoria na dieta dos peixes expandindo ou concentrando a área disponível para a alimentação. Aqui analisamos o conteúdo estomacal de duas espécies de Astyanax simpátricas para avaliar a importância das diferenças de habitats, como a complexidade espacial, em dois reservatórios tropicais com diferentes características ambientais. A análise NMDS indicou separação na dieta destas espécies entre reservatórios (stress = 8.28%). Além disso, a análise de variância revelou uma diferença signifi cativa no volume de itens alimentares consumidos entre os reservatórios (Anova one-way F (1. 132) = 4,4446; p = 0.037), salientando a importância das condições ambientais na composição da dieta de peixes. O estudo destacou o hábito alimentar insetívoro das espécies de Astyanax e revelou diferentes estratégias de alimentação entre esses peixes ...

Beta diversity, the spatial variation in species composition, has been related to different explanatory variables, including environmental heterogeneity, productivity and connectivity. Using a long-term time series of zooplankton data collected over 62 months in a tropical reservoir (Ribeirão das Lajes Reservoir, Rio de Janeiro State, Brazil), we tested whether beta diversity (as measured across six sites distributed along the main axis of the reservoir) was correlated with environmental heterogeneity (spatial environmental variation in a given month), chlorophyll-a concentration (a surrogate for productivity) and water level. We did not found evidence for the role of these predictors, suggesting the need to reevaluate predictions or at least to search for better surrogates of the processes that hypothetically control beta diversity variation. However, beta diversity declined over time, which is consistent with the process of biotic homogenization, a worldwide cause of concern.