A number of different
hypotheses have been proposed for the evolution of schreckstoff. The first hypothesis is that the evolution of schreckstoff has been driven by
kin selection. Support for this hypothesis would include evidence that individuals live in groups of closely related kin and that the release of chemical alarm signals increases the likelihood that related individuals will avoid predation. The second hypothesis, predator attraction, suggests the release of schreckstoff may attract additional predators which will interfere with the predation event, increasing the likelihood that the prey will escape and survive the attack. This hypothesis assumes predators will be attracted to schreckstoff and will interfere with one another either through
competition for the captured prey or through predation of one another. It additionally assumes, despite the fact that the prey has already incurred mechanical damage, it is possible for the prey to escape and recover from the attack. Testing and validating these assumptions would provide support for the predator attraction hypothesis. The third hypothesis proposes that schreckstoff has an immune function, providing protection against
pathogens,
parasites and/or UVB
radiation. For this hypothesis to be supported, a correlation between alarm substance cell production and the presence of pathogens and parasites would need to be observed. Direct evidence that schreckstoff inhibits the growth of aquatic pathogens and parasites would provide additional support for the immunity hypothesis. Another hypothesis is that schreckstoff is a breakdown product of
mucus and club cells, induced by injury. Selection for the alarm response is primarily at the level of the receiver.
Kin selection hypothesis One of the first hypotheses for the evolution of schreckstoff centered on
W.D. Hamilton’s theory of kin selection. Under the theory of kin selection, the sender of the chemical alarm signal would be willing to incur the costs of sending this signal if the benefits to related individuals were sufficiently high. In a situation where the sender of the signal is paying great costs (i.e., it releases the chemical alarm signal because it has incurred potentially mortal mechanical damage), the benefits to closely related
kin would have to be great. Under the framework of kin selection, behaviors that are seemingly detrimental to the sender are selected because they benefit individuals that are likely to share alleles by
common descent. In this way, the frequency of the sender's
alleles in the next generation is increased by their presence in successful kin. To apply kin selection theory to the evolution of schreckstoff, a number of conditions must be met. First, evidence must exist for the release of schreckstoff by the sender confers benefiting the receivers. Second, it must be shown that individuals in the order Ostariophysi associate mainly with family members. If either of these two assumptions is violated, then the kin selection hypothesis would not be supported. Some evidence exists in support of the first assumption that the release of schreckstoff confers quantifiable advantages to the receivers of this chemical signal. A laboratory experiment revealed that fathead minnows exposed to conspecific schreckstoff survived 39.5% longer than controls when placed in a tank with a predatory
northern pike (
Esox lucius). This finding suggests schreckstoff increases vigilance in receivers, resulting in a quicker
reaction time following detection of the predator. The second assumption, that individuals in the order Ostariophysi generally associate with close family members, does not appear to be supported by
empirical evidence. In
shoals of
European minnows (
Phoxinus phoxinus), no difference in relatedness was found within and between shoals, indicating individuals are not associating more closely with relatives than nonrelatives. Shoal composition has not been examined in all members of the ostariophysan order, and shoals composed entirely of family members may yet be discovered. Nevertheless, the finding that schreckstoff production is maintained in a species where the function is clearly unrelated to kin benefits provides strong evidence against kin selection as a mechanism for the evolution of schreckstoff. Fathead minnows have also been found to produce fewer
epidermal alarm substance cells (and therefore less schreckstoff) when in the presence of familiar shoalmates. The results of this study indicate one of two scenarios, neither of which is compatible with the hypothesis that schreckstoff evolved by kin selection. First, if schreckstoff evolved by kin selection, more epidermal alarm substance cells would be expected to be produced in the presence of kin than nonkin. This means familiar shoalmates in fathead minnows should be closely related kin and schreckstoff production should be increased when in shoals with familiar individuals. The study did not find this to be the case. Second, these results indicate individuals either do not associate with kin at all or production of schreckstoff varies depending on how familiar the focal fish is with the individuals with which it shoals. In conclusion, evidence does not support the hypothesis that schreckstoff evolved because it bolstered the
inclusive fitness of the sender through increased survival of kin.
Predator attractant hypothesis The predator attractant hypothesis proposes that the main purpose of schreckstoff is to attract additional predators to the area. According to this hypothesis, additional predators will interact with the initial predator, and these interactions will provide the sender with an opportunity to escape. A number of conditions must be met to support this hypothesis. First, schreckstoff must attract predators. Second, subsequent predators must disrupt the predation event, thereby increasing the
probability that the prey will escape. Third, the sender must be able to recover from the mechanical damage incurred during the predation event. A 1995 study provides support for the first condition that the release of schreckstoff must attract predators. This experiment revealed that schreckstoff extracted from the skin of
fathead minnows attracted both
northern pike (
Esox lucius) and
Colymbetes sculptilis predatory diving beetles. Additionally, a natural study showed that predatory fish were seven times more likely to strike a lure baited with a sponge soaked in fathead minnow skin extract than a sponge soaked in either water or skin extract from a nonostariophysan
convict cichlid (which presumably does not produce schreckstoff). While the previous two studies provided examples of systems in which schreckstoff acts to attract additional predators, a system was found for which this was not the case.
Spotted bass (
Micropterus punctulatus) were exposed to skin (containing schreckstoff) and muscle (control, containing no schreckstoff) extracts from five different co-occurring prey species. The spotted bass were not attracted to any of the schreckstoff treatments. This result indicates schreckstoff does not always attract relevant predators in the area. Cashner also pointed out regarding the 1995 study that northern pike are an
introduced species in much of the fathead minnow's range, so were not likely to be coevolving with these minnows during the evolution of the schreckstoff system. This system may be more ecologically relevant and little evidence suggests schreckstoff evolved as a predator attractant. In conclusion, the debate continues over whether or not the first condition for this hypothesis has been met. The second condition that needs to be met in support of the predator attraction hypothesis is that additional predators must occasionally disrupt predation events, increasing the probability that prey will escape. In the northern pike/fathead minnow system, additional northern pike may interfere with a predation event in one of two ways. First, additional northern pike of the same size interfere with a predation event by coming into contact with the main predator (biting it, etc.). Second, additional pike of larger size attracted to schreckstoff may prey on the initial predator. The probability that fathead minnows escape after being captured by a northern pike significantly increases when a second pike interferes with the predation event. The northern pike have an age-structured population biased towards younger, smaller individuals. If a younger pike attacks a fathead minnow and attracts an older, larger conspecific, then the younger pike may be at risk of cannibalism and will be inclined to release the prey to focus on escape. In regards to the second condition, additional predators do appear to disrupt predation events, increasing the probability that the sender will escape. The final condition, that individuals need to successfully recover from a predation event, appears to be satisfied. Support for this condition comes from the observation that many small fishes in natural populations exhibit scars, presumably from failed predator attempts. While the evidence that schreckstoff attracts predators is mixed, studies indicate multiple predators will interfere with each other and prey can recover from predation events when they manage to escape. The extent to which predators are attracted to a predation event depends upon the speed at which schreckstoff diffuses through its aquatic environment, which in turn depends upon water flow parameters. This hypothesis indicates schreckstoff evolved as a way of increasing the probability of survival during a predation event and its role 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==