When one fish in a shoal gets injured it gives off a mysterious substance that causes the rest of the shoal to flee in fear. This substance is known as "Schreckstoff" which means "scary stuff" in German. Now, researchers have discovered what this "scary stuff" is made of.
Part of the chemical makeup of the substance is a special type of sugar, known as chondroitin sulphate, that is in abundance in fish skin. When a fish is wounded the sugar compound breaks down – probably triggered by enzymes released upon injury – and alarms nearby fish.
"Our results provide a solution to a 70-year-old puzzle: the nature of this alarm signal," says Suresh Jesuthasan of A*Star's Neuroscience Research Partnership and the Duke/National University of Singapore Graduate Medical School in Singapore.
Researchers used the Zebra danio (Danio rerio) to conduct their study, which shows that the sugar fragments and registers in a particular part of the fish's brain; a region in the scent-processing olfactory bulb that includes an enigmatic class of sensory neurons known as crypt cells.
It is suggested that those neurons might be specialised in detecting the sugary alarm cue.
"This region of the olfactory bulb has unique projections to higher centres of the brain," Jesuthasan says, "so, there may be a special circuit mediating aspects of the innate fear response."
The findings have identified a new class of odourants for fish, and helps to explain the evolution of this "danger" signal – despite that the signal appears to offer no benefit to the injured fish sending the signal.
The sugar fragments are naturally released when an injury occurs, irregardless of whether or not anyone can smell them. Those fish that are able to pick up the scent are more likely to survive and go on to reproduce.
However, there are still unanswered questions. For instance, some fish species will sense alarm cues from closely related species but respond less intensely to cues from more distant relatives. This suggests that chondroitin fragments may occur in a variety of "flavours".
The researchers are also interested in determining if the fragments that elicit fear in Zebra danio will garner an alarm response from other species too.
Ultimately, the discovery could lead to new insight into the nature of fear itself.
"These findings underscore the usefulness of the zebrafish as a system to investigate the neural basis of innate fear," Jesuthasan said. "Given the transparency of larval fish, it is possible to see at single-cell resolution what happens throughout the brain when the organism detects danger. This will allow us in the future to ask how neurons, as a group, regulate behaviour and emotional responses."
For further information, see the paper: Ajay S. Mathuru, Caroline Kibat, Wei Fun Cheong, Guanghou Shui, Markus R. Wenk, Rainer W. Friedrich, Suresh Jesuthasan. Chondroitin Fragments Are Odorants that Trigger Fear Behavior in Fish. Current Biology, 2012; DOI: 10.1016/j.cub.2012.01.061
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