Schreckstoff

Schreckstoff (German for “alarm substance”) is an alarm pheromone released by specialized skin cells when damaged in certain fish, mostly smaller schooling fish such as gobies, darters, and minnows. To be more precise, the pheromone is produced by goblet or club (Kolbenzellen) cells located in the skin. Release of the chemical occurs when mechanical injury—mostly from a bite—occurs to the fish. The actual chemistry of the pheromone is not known because pure pheromone has not been isolated for chemical testing.

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Karl von Frisch was the first to detect the presence of Schreckstoff substance in 1938. He noticed that when he placed an injured minnow (Phoxinus phoxinus) in a tank with other fish of the same species, the schools dispersed, the fish exhibited frantic behavior, then schooled up, and gradually swam away from the source. One might think that all schooling fishes would exhibit this trait, but that is not the case. It has been proven that the presence of Schreckstoff substance is taxonomically distributed. The Ostariophysian family was thought to be the only order of fish that used the alarm pheromone (Pfeiffer, 1963) until it was shown that the pheromone does exist in two separate species of darters, but in a different system. (Smith, 1979) Moreover, this system seems to be widespread throughout the Percidae family.

Through experimentation, it has been shown that different species can pick up on each other’s version of Schreckstoff substance, so that if one fish of a different species is attacked, other species in the area know to get out of the area. There has also been fish that once used the pheromone, but lost the ability to produce and detect it for obvious reasons. For example, the piranha could not use this pheromone because it would interfere with their diet, which consists of similar species. If they were to attack a fish and Schreckstoff substance was released, it would not do the piranha any good to go into frenzy and swim away.

It has been hypothesized that Schreckstoff substance evolved for three reasons. First, it came about as a way to ensure the survival of conspecifics (members of the same species) and similar species. This pheromone not only warns conspecifics, but also nearby fish of related species of the presence of a predator. While it may seem pointless to some that fish are warning other fish of the inherent danger of a predator while they are being attacked, but it serves as a way of making sure that that particular fish’s genes go on in its offspring. Secondly, it has been hypothesized that Schreckstoff substance is a way to prevent fish from eating their own species, more specifically their own offspring. If a fish bit into a member of the same species, it would have the same effect on them as if it were released from a fish under attack from a predator. Thirdly, it has been recently hypothesized that predator fish have begun to be able to pick up Schreckstoff substance, attracting them to injured fish and hopefully an easy meal. Experimentation has shown that as the number of predators in the area around an injured fish goes up, so does the handling time (the time it takes from first strike until the time of ingestion). This allows the prey more time to free itself and possibly escape.

The Schreckstoff system is specific to the superorder Ostariphysi and is not found in darters or other Percids (Smith 1992). There are also four non-exclusive major adaptive hypotheses to explain the evolution of the sender's portion of this system: kin selection or parental care, cooperative altruism with familiar shoal mates, attraction of secondary predators, and toxicity of the Schreckstoff chemicals (predation deterent).

References

• Pfeiffer, W. 1963. The fright reaction of North American fish. Can J Zool 41:69-77.
• Smith, RJF. 1979. Alarm reaction of Iowa and johnny darters (Etheostoma, Percidae, Pisces) to chemicals from injured conspecifics. Can J Zool 57:1278-1282.
• Smith, R.J.F. (1992) Alarm signals in fishes. Reviews in Fish Biology and Fisheries 2:33-63.