Piranhas are born with a set of teeth. Baby piranhas dine on small crustaceans and vegetation. The piranha's teeth are partially hidden behind their lips; the only exposed parts are their jagged points. Piranha teeth fit perfectly between one another when their mouths close. Native tribes in South America use the teeth of piranhas for weaponry and cutting tools. When devouring their prey, sometimes piranhas accidentally eviscerate their own lips.
All piranhas have lateral lines, a sense organ used for detecting pressure changes in the water. Piranhas are able to distinguish fish in their packs from other animals due to the water pressure. Lateral lines also help piranhas navigate through whitewater rivers, where rocks and objects may not be visible to the fish's naked eye. This organ stretches along both sides of the piranha. Humans are able to see the lateral line underneath the fish's scales; the line runs between the piranha's head and fins.
Marine biologists can use the coloration of a piranha's scales to determine its native geographic range. Environmental conditions, swimming through dark waters, stress and sunlight have an effect on a piranha's scale color. A piranha's scale does not produce the color itself; the scale's color derives from small, pigment-containing cells embedded in the fish's skin. When light hits the piranha's chromatophores, the fish's scales become lighter. When piranhas age, their scales tend to grow darker. Various piranha species are able to recognize each other due to the color of their scales.
The shape of a piranha's head depends on the fish's genus. Pygocentrus piranhas have convex-shaped heads, which gives them the appearance of a bulldog. These piranhas have powerful lower jaws, giving them the ability to chew through meat with ease. The Serrasalmus piranha genus features species with more concave-shaped heads. These fish also dine on vegetation and fruits, so Serrasalums piranhas do not require jaws as powerful as the Pygocentrus piranhas. The eyes of each piranhas is large compared to the rest of its body. Piranha eye lenses do not change shape; these fish are short-sighted.