as a predator cue for conspecifics evolved subsequently. Supported by more empirical studies than the kin selection hypothesis, the predator attraction hypothesis remained popular for quite some time. 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 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. Cadmium, a heavy metal and an immunosuppressant in vertebrates, inhibits the production of alarm cells when fishes are infected with Saprolegnia. Furthermore, a follow-up study 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 schreckstoff's main function is to provide immunity against a number of environmental threats aimed at the fish's 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 to steal the prey item for themselves or by preying on the initial predator. Nearby conspecifics then exploit schreckstoff as a chemical cue, alerting them to the presence of a predator in the area. Ecological considerations The most convincing research to date indicates alarm substance cells serve as an immune system response and the ecological ramifications of this substance as a chemical alarm signal developed subsequently. This finding generates a number of interesting research questions. First, as mentioned earlier, males in many ostariophysan species cease production of alarm substance cells during the breeding season, presumably so females are not inadvertently repelled from the nest when males incur mechanical damage during nest building. In light of the immune hypothesis, alarm substance