By AMANDA SCHAFFER

Published: May 28, 2012

THE HYPOTHESIS

Fish “smell” danger, and perhaps humans do, too.

THE INVESTIGATORS

Ajay Mathuru and Suresh Jesuthasan of the Biomedical Sciences Institutes in Singapore.

When one fish is injured, others nearby may dart, freeze, huddle, swim to the bottom or leap from the water. The other fish know that their school mate has been harmed. But how?

In the 1930s, Karl von Frisch, the famous ethologist, noted this behavior in minnows. He theorized that injured fish release a substance that is transmitted by smell and causes alarm. But Dr. von Frisch never identified the chemical composition of the signal. He just called it schreckstoff, or “scary stuff.”

Schreckstoff is a long-standing biological mystery, but now researchers may have solved a piece of it. In a study published in February in Current Biology, Suresh Jesuthasan, a neuroscientist at the Biomedical Sciences Institutes in Singapore, and his colleagues isolated sugar molecules called chondroitins from the outer mucus of zebra fish.

They found that when these molecules are broken into fragments, as they might be when the fish’s skin is injured, and added to water, they prompt alarm behavior in other fish. At low concentrations, the fish were “mildly perturbed,” Dr. Jesuthasan said. At high concentrations, they stopped darting altogether and froze in place for an hour or longer.

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