Fish diets can vary in food quality, quantity and size. The variation can be caused by several factors, including season and the ontogenic phase of the individual (McCormick 1998). We studied the ontogenic changes in feeding habits of two freshwater fishes, <span name="style_italic">Agonostomus monticola </span>and <span name="style_italic">Brycon behreae, </span>from the Térraba River basin, South Pacific of Costa Rica. Both populations were omnivorous, but displayed ontogenic shifts in terms of quantity and quality of the food items consumed. As it grew, <span name="style_italic">A. monticola </span>modified its diet from insectivorous towards a higher consumption of vegetables, which was accompanied by an increase in relative length of the intestine. While remaining dependent on vegetation as staple food, <span name="style_italic">B. behreae </span>diversified its diet in two ways. Initially, from soft plant parts to seeds, leaves, and fruits. Secondly, prey items changed from insects into a more carnivore diet (fish and shrimp). These findings for both species stress the importance of protecting riparian vegetation in these tropical ecosystems. Rev. Biol. Trop. 57 (Suppl. 1): 285-290. Epub 2009 November 30.diversified its diet in two ways. Initially, from soft plant parts to seeds, leaves, and fruits. Secondly, prey items changed from insects into a more carnivore diet (fish and shrimp). These findings for both species stress the importance of protecting riparian vegetation in these tropical ecosystems. Rev. Biol. Trop. 57 (Suppl. 1): 285-290. Epub 2009 November 30.

cambios ontogénicos; ecología de la alimentación; peces de agua dulce; Agonostomus; Brycon; Costa Rica