In many fish species, an injured individual will release a substance researchers call Schreckstoff — or scary stuff — causing the rest of the school to swim away to safety.
This flight en masse from possible danger was first documented in the 1930s, and researchers have since puzzled over both the chemical identity of the alarm compound and the evolution of a signal that offers no clear benefit to the sender.
Researchers from Singapore and Switzerland have identified that the signal released from the skin of an injured fish does not consist of a single compound, as previously believed, but rather a mixture. Examining zebrafish, they found that the components of Schreckstoff include glycosaminoglycan (GAG) chondroitin, which can trigger a fear response in the group. Their findings are published today in Current Biology.
From the point of view of the school, the evolutionary advantages are obvious. Detecting the alarm substance, the group is alerted to a potential predator or harm and can swim to safety. “Get away and get down,” is how one of the authors, zoologist Suresh Jesuthasan of the National University of Singapore, describes the response, noting that zebrafish tend to seek safety at the bottom of the tank.
But how might this response serve the unfortunate fish being torn apart by a predator? Co-author Rainer Friedrich, a biologist at the Friedrisch Miescher Institute for Biomedical Research in Basel, Switzerland, says that one possible explanation is that the release of the substance may be a “passive event not influenced by the sender”. The evolution of the signal may have developed on the receiver side.
“A substance comes out when an animal is injured,” says Jesuthasan. “If other members of the species can detect it and respond, then they will survive.”
The chemical alarm could be useful for people as well as fish. Being able to isolate the compounds that make up Schreckstoff could help with commercial fishing or even conservation efforts, says Grant Brown, a biologist at Concordia University in Montreal, Canada.
The lamprey eel, for instance, regularly infests the St. Lawrence River. Jesuthasan suggests that if researchers manage to isolate the species’ alarm signal, it could be used as a chemical repellent to control swarms.
And Brown notes that the cues that serve to warn fish within species also attract predators to a feeding site. He suggests that chemical alarms could be used in commercial fishing to attract large species to foraging locations.
However, the uses of alarm chemicals, whether for species’ survival or human purposes, depends on the health of the aquatic environment. The acidification of waterways, one of the main consequences of industrial pollution and greenhouse-gas emissions, “might diminish the ability of fish to communicate through signalling,” says the study’s lead author, neuroscientist Ajay Mathuru of the Biomedical Sciences Institute in Singapore.
Image courtesy of Red Sea Holidays.