| Summary: | Considering th e morphology, diet and spatial distribution of Satanoperca pappaterraand Crenicichla britskii (Perciformes: Cichlidae) in the Upper Paraná River floodplain (Brazil), the following questions were investigated: (1) Could the body shape predict the use of trophic resources and habitat by C. britskiiand S. pappaterra? (2) Could the relationship between morphology and use of trophic resources and habitat be also extended to the intraspecific scale? (3) What are the most important morphological traits used to predict the variation on diet and habitat occupation within and between species? We hypothesized that intra and interspecific differences in morphological patterns imply in different forms of resource exploitation and that the ecomorphological analysis enables the identification of trophic and spatial niche segregation. Fish samplings were performed in different types of habitats (rivers, secondary channels, connected and disconnected lagoons) in the Upper Paraná River floodplain. Analyses of the stomach content was conducted to characterize the feeding patterns and twenty-two ecomorphological indices were calculated from linear morphological measurements and areas. A principal component analysis (PCA) run with these indices evidenced the formation of two significant axes, revealing in the axis 1 an ecomorphological ordination according to the type of habitat, regardless the species. The individuals of both species exploiting lotic habitats tended to have morphological traits that enable rapid progressive and retrograde movements, braking and continuous swimming, whereas individuals found in lentic and semi-lotic habitats presented morphology adapted to a greater maneuverability and stabilization in deflections. On the other hand the axis 2 evidenced a segregation related to the feeding ecology, between S. pappaterra and C. britskii. The relationship between morphology and use of spatial and feeding resource was corroborated by the Mantel test performed at inter and intraspecific levels. Therefore the hypothesis was accepted suggesting that analyses incorporating both intraspecific and interspecific morphological variations can contribute to a greater understanding about the ecological structure of fish assemblages by providing evidences on the niche characteristics of each species.
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