Behavioral ecologists have long noticed that when a fish is injured nearby members of the same species will rapidly flee. But how this alarm signal is transmitted has remained a mystery. Researchers led by Suresh Jesuthasan of Singapore’s Duke-NUS Graduate Medical School and the Agency for Science, Technology & Research report that one component of this chemical signal is chondroitin sulfate, a polysaccharide that is part of fish skin (Curr. Biol., DOI: 10.1016/j.cub.2012.01.061). Jesuthasan’s team studied schools of zebra fish to discover this fear factor. The researchers note there are probably other molecules involved in raising an alarm, because chondroitin sulfate is common to many species of fish but fish respond strongly only to injury signals from members of their own species. The search is now on for additional molecules that trigger species-specific alarm responses, as well as receptors in the olfactory epithelium that detect the chemicals.