Schreckstoff As A Possible Defense Against Pathogens, Parasites and UVB Radiation
The final hypothesis posits that schreckstoff has an immune function and may be the first line of defense against pathogens, parasites and/or UVB radiation. Evidence for this hypothesis is strong. A recent comprehensive study by Chivers et al. (2007) revealed that exposure to parasites and pathogens that penetrate the skin of Ostariophysans stimulated the production of alarm cells. Additionally, increased exposure to UV radiation was correlated with an increase in alarm cell production.
The role of schreckstoff in immune response was further strengthened by the finding that skin extracts from fathead minnows inhibited the growth of Saprolegnia ferax (a water mould) in culture. In contrast, skin extracts from swordtails (Xiphophorus helleri), which are not believed to produce schreckstoff, increased S. ferax growth compared to controls. Chivers et al. (2007) were also able to demonstrate that cadmium (Cd), a heavy metal that is an immunosuppressant in vertebrates (Sanchez-Dardon et al. 1999), inhibits the production of alarm cells when fishes are infected with Saprolegnia. Furthermore, a follow up study by Halbgewachs et al. (2009) treated fathead minnows with cortisol, a well known immunosuppressant, which significantly reduced alarm cell investment in conjunction with leukocyte activity. The results of these extensive studies strongly suggest that schreckstoff’s main function is to provide immunity against a number of environmental threats aimed at the fishes’ epidermis.
If schreckstoff evolved as a defense against pathogens, parasites and UVB radiation, then the release of schreckstoff into the environment subsequently allowed for both predators and prey to exploit this system. Predators in some systems may use schreckstoff as a cue for an easy meal, either by disrupting the predation event in order to steal the prey item for themselves or by depredating the initial predator. Nearby conspecifics then exploit schreckstoff as a chemical cue, alerting them to the presence of a predator in the area.
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