ALARM SUBSTANCE
a substance produced in the round or oval alarm substance cells (previously called “clubcells") in the skin of Ostariophysi (Cypriniformes, Siluriformes) and Gonorhynchiformes, and which is released upon injury of the skin. On scenting the alarm substance members of the same species, and to a lesser extent related fishes, exhibit the fright reaction (q.v.). The dispersal of the alarm substance apparently normally acts to warn of the presence of a preying predator. The alarm pheromone is hypoxanthine-3N-oxide comprising a purine skeleton with N-O functional group and sensitive to relatively weak changes in pH. Also called alarm pheromone or Schreckstoff