Their diet consists largely of fish, insects, worms, crustaceans, and the occasional larger animal. In contrast to their reputation, they usually feed on dead, dying, and injured vertebrates in the wild, but have been known to attack healthy animals. The fish usually feed in large schools around dusk and dawn. They locate their prey by scent or motion using a set of sensors down the sides of their bodies, the lateral line

Lateral line

The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

 system.
Red-bellied piranha do spawn around April and May during the rainy season.