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A small piece of silverfish called the sulfur molly lives in the sulfur-infused ponds of the State of Tabasco, Mexico. Throw it on a rock and you can see a band dancing: the surface of the water will burst in pale, pulsating waves, spreading like milk through coffee through the eerie blue. Every few seconds, thousands of fish repeat a rapid dive action to create the wave, sometimes for up to two minutes.
Why? biologists asked. What purpose can this flashing serve?
Mollies are prey for a number of winged predators such as egrets, kingfishers and kiskades. When the birds dive to attack, the moles glow and spin. Scientists in Germany have studied hours of video, both real and simulated by a researcher, taken during two years of bird attacks that were unable to visit fish due to the coronavirus pandemic, and they believe they may have decrypted the message the birds were conveying. fish.
It seems to target predators perched on the shore, they Report Wednesday in Current Biology. The message reads: See you soon. We are watching. Don’t try a funny job.
Not every bird attack triggers an uncanny glow, said David Bierbach, biologist at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries and author of the new paper. The kingfisher, for example, throws a cannonball into the water and almost always provokes the mollies to sparkle. But kiskades are cunning – they just stick their beaks in. They rarely respond.
This observation gave the researchers a way to test their hypothesis that blinking might induce a change in the behavior of predators. They perched along a sulfur stream and set up cameras to film predatory kiskades. Once a bird had crossed the water, a slingshot researcher triggered the swaying behavior in the fish, imitating what kiskades routinely see when a kingfisher hunts alongside them. They can now compare undisturbed hunting with disturbed hunting.
The kiskades sat on nearby perches as the fish swam and boiled. Over 200 hunting sessions, the researchers found that the birds waited twice as long as when the water was left intact before starting another transition. When they attacked again, they were much less successful at catching fish than in still waters.
Without the intervention of researchers, the birds caught more than half a fish. When the slingshot was in play, it was less than a quarter of the time. When the researchers watched the kingfisher, they found that the brighter the fish, the longer the birds waited, as if they were waiting for it too.
This response suggests that the shimmering behavior not only makes it harder for the hunter to focus on a fish, but also that the birds know their efforts are more likely to be wasted when the waves start.
This is an interesting observation, because if the fish were just trying to avoid predators, they could dive deeper and stay down longer. Dr. While the low oxygen environment of a sulfur pool means they can’t stay below it indefinitely, they have an excellent capacity to stay below for longer, Bierbach said.
“They can stay underwater for up to two or three minutes,” he said. “But they don’t. They come to the surface quickly and repeat their dives very synchronized, very rhythmically.”
Synchronous behavior, such as fireflies flashing at the same time or flocks of birds moving together in a carefully spaced pattern across the sky, fascinated scientists and anyone lucky enough to see it. But until now, it has been difficult to pinpoint exactly what benefits living things derive from this and why they might have evolved.
Sulfur moles seem to be one of those rare occasions where the benefits of a synchronized behavior can be overlooked.
Dr. “Birds learn to avoid these swaying flocks of fish later on because they have a lower chance of catching fish if they’re wobbly — and the fish aren’t eaten, which is a win-win situation,” Bierbach said. “If both parts of the sender and receiver benefit from this, it’s a signal. can evolve.”
There is much to learn in the sulfur pools of Tabasco.
Dr. “Right now we’re just looking at what’s going on from above,” Bierbach said. Now we want to go under the surface of the water with underwater cameras,” he said.
The researchers hope to discover how the first fish to dive might be able to signal to others and whether their dives vary with the type of affliction.
“We have to go underwater to see this,” he said.
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