New York Times¹:
"When one fish is injured, others nearby may dart, freeze, huddle, swim to the bottom or leap from the water."
"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."
"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."
"Neurons in the olfactory¹ bulb of these fish were activated"
"In zebra fish, the olfactory bulb has a close anatomical connection to a brain area called the habenula, which may also play a central role in mediating fear, Dr. Jesuthasan said."
"In humans ... it has been implicated in many different types of behavior, including stress, pain, anxiety, learning and reproduction."
¹olfactory = concerning smell
²Amanda Schaffer, 28 May 2012