Novel Epidermal Oxysterols Function as Alarm Substances in Zebrafish

Aquatic animals often use chemical cues to signal predation risk. When injured, shoaling fish skins release alarm substances that induce intense fear and a suite of anti-predator behaviors in other shoal members. However, the chemical nature of alarm substances remains largely unknown. Here we show that zebrafish alarm substance comprises 24-methyl-5α-cholestane-3α,7α,12α,24,28-pentahydroxy 28-sulfate, a novel oxysterol sulfate, and 5α-cyprinol sulfate. These compounds are present in zebrafish skin extract and, at concentrations of less than one nanomolar, each induces anti-predator behaviors and increases cortisol levels. Their mixture, at its natural ratio, replicates the skin extract in eliciting the full suite of anti-predator behavior patterns. Our findings reveal a molecular-level mechanism whereby fish signal predation danger.