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 (Wisenden and Smith 1998). One putative active ingredient in schreckstoff is hypoxanthine-3N-oxide (H3NO), which may be produced in what will henceforth be referred to as alarm substance cells (Argentini 1976; Pfeiffer 1982). Brown et al. (2000) identified the nitrogen oxide functional group to be the main chemical trigger of antipredator behavior in receivers. Mathuru et al. (2012) found that schreckstoff is a mixture, and that 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 stickleback (Culaea inconstans) are more likely to be caught in minnow traps that have been baited with conspecific skin extracts than adults (Chivers and Smith 1994). This result indicates that young brook sticklebacks do not make the association between schreckstoff and the potential presence of a predator as readily as adult brook sticklebacks. Whether this association strengthens over time as a result of learning or physiological development remains unclear (Chivers and Smith 1994).
In addition to changes across ontogeny, the degree to which schreckstoff is produced varies within the breeding season. Male fathead minnows (Pimephales promelas) cease production of schreckstoff during the breeding season, but still exhibit antipredator behaviors in response to schreckstoff during this time (Smith 1973). It has been postulated that schreckstoff production is halted at this time because male fathead minnows will often incur mechanical damage while building their nests. It would be detrimental to a male to produce schreckstoff while building a nest, as it would inadvertently repel females, thereby decreasing the likelihood of obtaining a mate. By ceasing schreckstoff production during the breeding season, males circumvent this problem. The cessation of alarm substance cell production appears to be controlled by androgens (Smith 1973).
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