| Abstract: We analyzed the ontogenetic shifts in digestive tract morphology and diets of the young of Iheringichthys labrosus, Hypophthalmus edentatus and Plagioscion squamosissimus, sampled in the Itaipu Reservoir, Brazil-Paraguay.We described the dental structures, the gill rakers and the digestive tract, and analyzed the diet of the young fish. We observed teeth in the jaws and pharynx in young of the three species. In H. edentatus, the gill rakers developed more rapidly, and were longer and more numerous on the first arch, related to their planktivorous feeding habit. I. labrosus and P. squamosissimus had long gill rakers only on the first arch, they were short and thick on the rest. The stomach was defined only in P. squamosissimus, with pyloric caeca. Their diets were mainly zooplankton. I. labrosus fed particularly on cladocerans and rotifers; H. edentatus consumed essentially cladocerans; and P. squamosissimus fed basically on copepods. We observed greater similarity in diet between the young of I. labrosus and H. edentatus, especially for the first length classes. The diet of I. labrosus became more diversified at the end of the larval period, indicating a transition in the feeding habit and habitat for this species. P. squamosissimus showed a very different diet from the others species. The differences in oral anatomy, allied to the morphology of the digestive tract, visual acuity, swimming hability, way of foraging, and especially mouth position, form and size, were determining factors in the diets of these species. |