Why does crucian carp have thin black lines?

That's the side line of the fish. The lateral line of fish body is used as a sensory organ to find bait, enemies and the relationship between individuals of the same species, and it is also a sensory organ for them to judge whether they are close to various obstacles such as rocks and shore walls. The color on the fish's back gradually becomes lighter from the back to the abdomen, and it is used as a protective color to disguise and protect itself.

Extended data:

Lateral line system is a unique skin sensory organ of aquatic vertebrates, which is tubular or grooved and buried in the skull or under the skin on the side of the body. The lateral line tube passes through the skull and scales with a series of lateral line holes and is connected with the outside world. The lateral tube is filled with mucus, and the sensilla is immersed in mucus.

The lateral line can feel the low-frequency vibration, has the positioning function of controlling the flow direction, and can also help visually determine the position of near and far objects.

Lateral lines can feel low-frequency vibration, have the directional function of controlling fluidity, and can also help visually determine the position of distant objects, so they have important biological significance in fish life.

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