One of the many advantages of using zebrafish to study fear is that, like other similar fish, they produce a special alarm substance in their skin when they are injured. This substance is produced by cells called club cells, and the purpose of the substance is to signal to other members of the school that a fish has been injured. When the skin of a fish is damaged by a predator, for example, the alarm substance is released, and other fish can smell it. The "smell of danger" causes the other fish to be more cautious and to behave as if they are afraid. When they sense this alarm substance, the other zebrafish swim in tight groups to increase protection. They also swim more erratically (in a zig–zag pattern), to both decrease the likelihood of being eaten and to stir up the sediments (pieces of leaves, sand, or earth on the ocean floor) to make the water cloudy. Sometimes the zebrafish also freeze in place, decreasing the likelihood that the predator will see them.

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