Scientists have known about the existence of "schreckstoff" (which means "scary stuff" in German) for decades — it's a substance released by an injured fish that warns the rest of the school of danger. But the nature of this alarm signal was a mystery until now.
Researchers, publishing in the journal Current Biology, have figured out the chemistry of this scary stuff. Ajay Mathuru and his colleagues isolated the key ingredient in schreckstoff, a sugarlike molecule called glycosaminoglycan (GAG) chondroitin. It's found in abundance in fish skin, and when a fish is injured, fragments diffuse through the water and trigger an alarm response in other fish.
Mathuru and his colleagues studied schreckstoff extracted from zebrafish, a commonly studied fish related to minnows and catfishes. When they broke down the GAG chondroitin in schreckstoff with enzymes or otherwise rendered it inert, zebrafish lost their fearful reaction to the substance.
The researchers also found that schreckstoff activates a particular region of the zebrafish olfactory bulb, part of the brain that processes odors. This region contains anatomically unusual sensory neurons known as crypt cells, which the authors suggest are specifically dedicated to sensing schreckstoff. These neurons seem to only respond to schreckstoff and not to other odors.
These findings also help explain how this sugary alarm signal may have evolved, even though the signal does not benefit the wounded fish releasing it. Evolution would favor fish that recognize chemicals associated with injury. Fish able to pick up the scent of schreckstoff are more likely to avoid predators, survive, and reproduce.
The next puzzle for these researchers is sorting out the different varieties of "scary stuff" released by different fish species. Some fish can sense alarm cues from closely related species but not those from more distant relatives. Mathuru and colleagues are now working on testing whether the schreckstoff emitted by zebrafish will cause the same scary reaction in other species of fish.