The Piranha Biology

The Piranhas body shape is high-backed and more or less stocky indicating that they live in slow-flowing waters. The head is large with a steep profile. A reliable differentiation of the sexes tends to be extremely difficult.

The fins in combination with the air bladder enable the fish to control all movements. They are named after the areas in which they are inserted. All fins exept the adipose fin consist of rays with fin membrane between.
When the Piranha stands still it uses the pectoral fins to neutralize the 'jet effect' that the gills produce. The dorsal fin can also be used to this task. The dorsal fin can in addition be used to put the Piranha in reverse. At high speeds the only significant fin is the tail fin, the other fins are folded close to the body.

Teeth

The teeth are hidden behind the lips and are thus barely visible on an ordinary Piranha. Because of their powerful dentition (the lower jaw in particular) Piranhas are able to cause serious injuries or death to larger animals. Even a young Piranha 15 cm (6 inches) in length has teeth that are 4 mm (1/6 inch) long and razor-sharp.

Lateral Line

The lateral line is a special sense organ situated along the sides of the fish. It can be seen as a thin line from head to tail. It detects pressure canges in the water and enables the fish to recognize art friends by their unique swim frequency. Sick fish can also be detected. And in the whitewater rivers, where the visibility is poor, the Piranhas can use the lateral line to navigate and avoid rocks and similar obstacles.

Smell

Many authors have mentioned the incredible smell possessed by sharks. Piranhas too have an extraordinary sense of smell. Wolfgang Schulte once made an experiment. Into a 200 liters (52 gallons) aquarium keeping a Piranha he put just one drop of blood. The Piranha responded to the blood with clear agitation. After several drops the Piranha swimmed about looking for the 'food'.
I myself is bound to believe this since my Piranha, Ebba, always seem to find even the tiniest piece of food lying hidden on the bottom.

Hearing

Experiments with bony fishes have resulted in the conclusion that their hearing was excellent. Since Piranhas themselves produce sound, using the air bladder as a resonator, it is assumed that they, as well, can hear well.

Eyes

The relatively large eyes of the piranha have a hard, spherical lens that does not change shape. The eye is perfectly adapted for close-up vision, and is short-sighted at rest. Of course it can be adapted for grater distances. This is achieved by the lens being displaced by the lens muscle.
The position of the eyes makes the visual field very large (se illustration above), and makes a narrow field of stereoscopic vision.
Tests have shown that fisheyes similar to those of the Piranha can distinguish between 20 different colors between 370 and 700 nm.

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