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 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