水虎鱼体形是高背和或多或少有点敦实的感觉，这表明它们生活在缓慢流动的水域。头大而且拥有陡峭的曲线。
鳍与鱼漂的组合用来控制所有的动作。各鳍的脂鳍由光线和散热之间的一道隔膜，
胸鳍、背鳍--在高速游行中起到至关重要的作用！
牙齿
水虎鱼的牙齿是隐藏在嘴唇的背后，所以一个普通的水虎鱼几乎看不出来。因为他们强大的牙列（尤其是下颌）都能够造成严重伤害或死亡对较大的动物。即使是一个15公分的幼体水虎鱼，长有差不多4毫米长锋利的牙齿。
侧线
侧线是一种特殊的感觉器官位于沿鱼的两侧。它可以被看作是一条细线从头部到尾部。它可以检测水中的压力，使鱼类通过其独特的游泳频率识别别的鱼类或者同类朋友。病鱼也可以被检测。而在白水河流，其中的能见度较差，水虎鱼可以使用侧线来导航和避免岩石和类似的障碍。
闻
许多人都提到了鲨鱼的嗅觉。水虎鱼也有非凡的意义嗅觉。沃尔夫冈·舒尔特曾做了一个实验。在200升（52加仑）的水族箱饲养水虎鱼，然后滴一滴血。水虎鱼回应血液有明确的表现形态。数滴以后，水虎鱼游动开始寻找“食物”。
听力
通过实验的结论是，他们的听力非常好。由于水虎鱼本身产生的声音，使用气囊作为谐振器能听得清楚。
眼睛
水虎鱼有比较大的眼睛，球面透镜，不改变形状。是完全适应于近距离视力，
眼睛的位置，使视场非常大（插图），并且可能能够呈现立体视觉。
试验表明，鱼眼类似于水虎鱼的370和700毫微米之间的20个不同的颜色可以区分。
The Piranha BiologyThe 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 LineThe 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.
HearingExperiments 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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