In the 1930s, the Austrian biologist Karl von Frisch, of waggle-dancing honey bee fame, performed some little-known experiments on dancing minnows. The fish weren’t dancing for fun.
Von Frisch showed that when a minnow’s skin cells are damaged, as happens during a predator attack, the fish releases a chemical into the water that triggers nearby fish to flee. Von Frisch called the chemical “Schreckstoff,” German for “scary stuff,” and yesterday, scientists described its chemical makeup.
The research team first separated the skin mucous of zebrafish into its different chemical components and then tested how each affects the behavior of a group of fish in a tank. One of the chemicals, a sugar called chondroitin sulfate, caused a dramatic fear response, as you can see in the above video.
Von Frisch had also investigated how the surrounding fish sense the alarm signal. When he removed nerves of the olfactory (smelling) system from all of the neighboring fish, they didn’t react to the Schreckstoff. In the new study, the researchers looked much more closely at how chondroitin affects the olfactory system.
They showed that chondroitin activates a particular spot of olfactory bulb, at the very front of the fish brain. Cells called “crypt neurons,” which have no known function, connect to exactly this area. And neuronal branches that begin in this area project to the habenula, an area thought to be involved in reward processing. This whole circuit, the researchers speculate, could be responsible for driving innate fear responses in many species.
The work may also answer the question of what Schreckstoff evolved from, when it seemingly has no direct benefit to the fish that releases it. This class of sugars, it turns out, help maintain fish-skin health and permeability, and are widespread in mucous and connective tissue in lots of species.