Is shreckstoff really a predator attractant?

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Schreckstoff, the alarm substance produced by some fish when they are in fear, is used to indicate alarm to members of the same species shoaling nearby.

However, many scientists believe that predators too might use schreckstoff to help them find vulnerable prey.

Evolutionary biologist Mollie Cashner of the University of Southern Mississippi has recently been running experiments using the North American Spotted bass, Micropterus punctulatus, to try and find out whether this hypothesis is really true for this species.

The alarm substance is found in the skin of ostariophysan fish, as well as some other species including the Gonorhynichiformes, and is produced by alarm substance cells, previously called club cells. The schreckstoff is actually a pheromone based on hypoxanthine-3N-oxide with a purine skeleton.

To determine whether Spotted bass hone in on schreckstoff released by their prey, Cashner produced extracts from the skin from a number of natural prey species, including Blacktail shiner, Cyprinella venusta, Bullhead minnow, Pimephales vigilax, Western mosquitofish, Gambusia affinis, Blackspotted topminnow, Fundulus olivaceous and Bluegills, Lepomis macrochirus.

Some of the extracts were based on the muscle tissue from the prey fish and contained no schreckstoff, while others were extracted from the skin, were the pheromone is produced. By exposing the bass to each extract and monitoring the response, Cashner was able to determine the effect they had upon the predators.

Interestingly, the schreckstoff-containing skin extract didn't appear to have any increase in foraging behaviour in the bass from any of the five prey species, but there did appear to be an increase in the feeding response from the non-schreckstoff containing muscle extract from Cyprinella venusta.

It appears in the case of the Spotted bass, that it relies on stealth to hunt and isn't using pheromones as previously thought.

For more details read the paper: Are Spotted bass, Micropterus punctulatus, attracted to schreckstoff? A test of the predator attraction hypothesis. Copeia: Vol. 2004, No. 3, pp 592-598.