Although generally described as highly predatory and primarily feeding on fish, piranha diets vary extensively, leading to their classification as omnivorous. In addition to fish (occasionally even their own species), documented food items for piranhas include other vertebrates (mammals, birds, reptiles), invertebrates (insects, crustaceans), fruits, seeds, leaves and detritus. The diet often shifts with age and size. Research on the species "Serrasalmus" aff. "brandtii" and "Pygocentrus nattereri" in Viana Lake in Maranhão, which is formed during the wet season when the Pindaré River (a tributary of the Mearim River) floods, has shown that they primarily feed on fish, but also eat vegetable matter. In another study of more than 250 "Serrasalmus rhombeus" at Ji-Paraná (Machado) River, 75% to 81% (depending on season) of the stomach content was fish, but about 10% was fruits or seeds. In a few species such as "Serrasalmus serrulatus", the dietary split may be more equal, but this is less certain as based on smaller samples: Among 24 "S. serrulatus" from flooded forests of Ji-Paraná (Machado) River, there were several with fish remains in their stomachs, but half contained masticated seeds and in most of these this was the dominant item. Piranhas will often scavenge, and some species such as "Serrasalmus elongatus" are specialized scale-eaters, feeding primarily on scales and fins of other fish. Scale- and fin-eating is more widespread among juvenile and sub-adult piranhas.