The diets and relative intestinal length of two typically herbivorous terapontid fish from contrasting high and low clarity environments were compared. Fish of both species collected from high clarity habitats were almost exclusively herbivorous, whereas conspecifics from low clarity habitats were omnivores. Relative intestinal lengths in both species were significantly shorter in low clarity environments. Pronounced contrasts in water transparency have the apparent capacity to produce dietary niche shifts in these two species that in turn induce and maintain trophic polymorphisms in a riverine environment.