Fish can feel another fish swim by without seeing or touching it. Tiny hair cells along their sides feel the water move.
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Look closely at the side of a fish and you may see a thin line running from head to tail. This is the lateral line. Goldfish, trout, sharks, and most other fish have one.
Along the line are tiny bumps called neuromasts. Each one is a little cluster of hair cells. Every hair cell has a bundle of tiny hairs on top. The hairs poke up into a soft cap of jelly called the cupula.
Hair cells are like the ones deep inside your own ears that help you hear and keep your balance!
When water moves, it pushes the jelly cap. The cap bends the tiny hairs, and the hair cells send a message down a nerve to the brain. This tells the fish where the water is moving.
Scroll up to the 3D model and press Make a wave, then Send a signal. In the bath, hold one hand still and move your other hand nearby. Can you feel the water push?
Blind cave fish have no working eyes, but they swim around rocks without bumping into them. They use their lateral line to feel the water around them!