Britannica Sources used in this answer

How do alarm substances warn ostariophysan fish schools of danger?

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knifefish — ostariophysan Knifefish, or featherback (Xenomystus nigri)

In ostariophysan fish, alarm substances serve as a critical warning system for schools of fish. These substances are secreted by specialized cells located in the epidermis and are released into the water only when the skin of a fish is injured, such as during a predator attack.1

When other members of the school perceive these chemicals through their nasal organs, they exhibit a rapid "fright reaction," often retreating and appearing frightened. This mechanism acts as a form of insurance for the individual and is particularly valuable for species that exhibit social behavior, as it effectively alerts nearby school members to the presence of danger. This chemical signaling system is found in nearly all tested ostariophysan species and has contributed significantly to their biological success.1