Call it the fish version of instant messaging. When a fish is injured, it secretes a compound that makes other fish dart away (as seen in the latter half of the sped up video above, when the red light flashes). The substance, named Schreckstoff (German for "scary stuff"), protects the entire community of fish, but no one knew how it worked. Now they do, thanks to an analysis of fish mucus reported today in Current Biology. The key ingredient in Schreckstoff is a sugar called chondroitin sulfate, which is found in abundance in fish skin. When the skin is torn, enzymes break the compound down into sugar fragments that activate an unusual class of sensory neurons known as crypt cells in other fish. And the fish take off.
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