Fish respond to fear by secreting chemicals (pheromones) designed to warn other fish of danger. To date, some of the components of this so called “Schreckstoff” (meaning fear stuff) have been identified. These include hypoxanthine-3-N-oxide (H3NO), polysaccharides, oligosaccharides, and glycosaminoglycans (GAGs). Researchers in Singapore have now identified chondroitin sulfate, a complex sugar molecule, as one of the active ingredients in Schreckstoff.
-Stensmyr MC, Maderspacher F. Pheromones: Fish Fear Factor. Current Biology Vol 22 (6): R184-R186, 2012.