In the sulfur-infused ponds of Tabasco State in Mexico lives a tiny silver slip of a fish, the sulphur molly. Toss in a rock, and you might see a bunch of them dance: The water’s surface will erupt in pale, pulsing waves, spreading through the eerie blue like milk through coffee. Every few seconds, thousands of fish will repeat a quick diving motion to generate the wave, sometimes for up to two minutes.
Why? biologists asked. What purpose could this flashing serve?
The mollies are prey for an array of winged predators, including egrets, kingfishers and kiskadees. When birds dive to attack, the mollies flash and swirl. Scientists in Germany, unable to visit the fish because of the coronavirus pandemic, analyzed hours of video taken over the course of two years of bird attacks, both real and simulated by a researcher, and believe they may have decoded the missive being transmitted by the fish.
It seems to be aimed at predators perched on the shore, they report in Current Biology on Wednesday. The message reads: We see you. We are watching. Don’t try any funny business.
Not every bird attack triggers the uncanny flashing, said David Bierbach, a biologist at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, and an author of the new paper. Kingfishers, for instance, cannonball into the water and provoke the mollies into flashing nearly every time. But kiskadees are subtle — they dip just their beaks in. They rarely set off a response.
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