When one fish gets injured, the rest of the school takes off in fear, tipped off by a mysterious substance known as “Schreckstoff” (meaning “alarm substance” in German). Credit: Mathuru et al. in Current Biology
Some schooling fish have an unusual way of warning others of predation—when they are injured, they release something that elicits a fear response in nearby fish, causing them to flee.
That “something” has been referred to by scientists as “Schreckstoff,” or the alarm substance.
Now, studying zebrafish, a team of scientists have found that it is actually a mixture of substances, one of which has been identified as chondroitin, a type of sugar commonly found in fish skin.
“Chondroitin has extremely diverse and broad functions across all vertebrates and even invertebrates,” Dr. Suresh Jesuthasan, co-author of the study and researcher at the National University of Singapore, told The Epoch Times via email.
“In fish, chondroitin is a part of the mucus which itself is multifunctional, ranging from regulating respiration to disease resistance and osmoregulation.”
The researchers tested zebrafish’s response to a variety of chemicals, and found that chondroitin is sensed by a special type of neuron called crypt cells, triggering fear responses such as darting and shifting towards the bottom of the fish tank.
Although chondroitin alone can elicit a significant fear response, the team found that it only activates one region of the olfactory bulb in the zebrafish’s brain, while crude skin extract stimulated multiple regions of the olfactory bulb, suggesting that more substances are needed to trigger a full fear response.
The researchers also observed a stronger response when chondroitin polymers are broken down into smaller fragments by enzymes, and propose that there is a simultaneous release of enzymes during injury to break down chondroitin.
According to Jesuthasan, club cells, which contain chondroitin, have been found in virtually all fish that produce Schreckstoff. Hence these cells may be the source of the components that make up Schreckstoff, including enzymes.
“Further research will be required to identify all the components that make the mixture and to assess if each of them is just as potent in eliciting fear responses,” Jesuthasan concluded.
The findings will be published in the journal Current Biology in March
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