In 1938, the Austrian ethologist Karl von Frisch made his first report on the existence of the chemical alarm signal known as Schreckstoff (fright substance) in minnows. An alarm signal is a response produced by an individual, the “sender”, reacting to a hazard that warns other animals, the receivers, of danger. This chemical alarm signal is released only when the sender incurs mechanical damage, such as when it has been caught by a predator, and is detected by the olfactory system. When this signal reaches the receivers, they perceive a greater predation risk and exhibit an antipredator response. Since populations of fish exhibiting this trait survive more successfully, the trait is maintained via natural selection. While the evolution of this signal was once a topic of great debate, recent evidence suggests schreckstoff evolved as a defense against environmental stressors such as pathogens, parasites, and UVB radiation and that it was later co-opted by predators and prey as a chemical signal. Background Chemical alarm systems have been identified in a number of different taxa, including gastropods, echinoderms, amphibians and fishes. One of the most well-studied chemical alarm signals is schreckstoff, the use of which is widespread in the superorder Ostariophysi (e.g., minnows, characins, catfishes, etc.). About 64% of all freshwater fish species and 27% of all fish species worldwide are found in the ostariophysan superorder, which highlights the widespread use and importance of this chemical alarm system in fishes. Schreckstoff in ostariophysans The production of schreckstoff has been shown to be metabolically expensive and is therefore part of a conditional strategy that can only be employed by individuals with access to sufficient resources. One putative active ingredient in schreckstoff is hypoxanthine-3N-oxide (H3NO), which may be produced in club cells which will henceforth be referred to as "alarm substance cells". The nitrogen oxide functional group was found to be the main chemical trigger of antipredator behavior in receivers. Schreckstoff is a mixture, and fragments of a glycosaminoglycan, chondroitin sulfate, are able to trigger fear responses. The precursor polysaccharide is a component of mucus, and fragments are proposed to be produced during injury. Like schreckstoff obtained from skin extract, chondroitin sulfate activates a subset of olfactory sensory neurons. Production of and responses to schreckstoff change over the course of ontogeny. For example, young brook sticklebacks (Culaea inconstans) are more likely to be caught in minnow traps that have been baited with conspecific skin