An A*STAR researcher has found that fatally injured medaka fish release chemicals that spread fear among passing shoals1, the first study to observe such an alarm system in the Beloniformes order of fish.
Ajay Mathuru of the A*STAR Institute of Molecular and Cell Biology compares species that communicate such threats, hoping to improve our understanding of the circuitry and genetics of innate fear in vertebrates. He says: “Alarm responses are more widespread in fish than one would assume.”
Fright chemicals were discovered in fish in 1938 by Karl von Frisch, an animal-behavior scientist and Nobel laureate best known for figuring out why honey bees dance. Von Frisch noticed that maimed minnows release a substance he called Schreckstoff, meaning ‘scary stuff’ in German, which induces panic among nearby fish. For decades, researchers believed that thisSchreckstoff was produced by specialized club cells found only in the Ostariophysi superorder of freshwater fish.
Mathuru wanted to find out if other lineages could also produce the fright chemicals.
He compared the behavior of 18 medaka exposed to chemicals extracted from their wounded kin with 18 fish that had not been exposed, taking care not to disturb them. A white LED lamp was placed above the tank in a darkened room to obscure the experimental setup and the substances were gently delivered via a syringe to avoid creating ripples. “A lot of things can startle these small fish because they have many predators.”
The medaka’s response was dramatic. Within minutes of releasing the scent of distress, the fish froze, remaining static for up to 30 seconds at a time. “Becoming immobile is a good strategy because most predators target prey through motion detection,” says Mathuru. Fish exposed to the chemicals also produced higher levels of the stress hormone cortisol — although the experience of being isolated and spied on was in itself quite stressful, the data revealed.
Mathuru then went in search of the cells suspected of triggering the passive alarm system. Club cells are relatively large and dome-shaped, which makes them easy to detect on the skin surface of Ostariophysans such as zebrafish, but Mathuru could not locate them in medaka.
Their absence suggests alternative explanations about alarm-response evolution in fishes, which could reach as far back as a hundred million years ago, when zebrafish and medaka shared a common ancestor. “Another possibility is that this form of communication is fundamental for animals, and evolved more than once,” says Mathuru, who plans to conduct neuroimaging studies comparing the two species. “Both hypotheses are equally likely at this point.”