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.
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.
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.
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.