In the sulfur-soaked ponds of the state of Tabasco in Mexico lives a small silver slip of a fish, the sulfur molly. Throw a rock in it, and you might see some dancing: the water’s surface will erupt in pale, pulsating waves, spreading through the eerie blue like milk through coffee. Every few seconds, thousands of fish repeat a rapid dive to generate the wave, sometimes for up to two minutes.
Why? biologists asked. What purpose could this flashing serve?
The mollies are prey to a range 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 due to the coronavirus pandemic, analyzed hours of video captured over the course of two years of bird attacks, both real and simulated by a researcher, and believe they may have decoded the message sent by the fish.
It appears to be targeting coastal predators, they report Wednesday in Current Biology. The message reads: We see you. We are watching. Don’t try funny things.
Not every bird strike causes the eerie flashes, said David Bierbach, a biologist at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, and an author of the new paper. Kingfishers, for example, cannonball into the water and lure the mollies to flash almost every time. But kiskadees are subtle – they just dip their beaks in them. They rarely respond.
This observation gave the researchers a way to test their hypothesis that the flashing could lead to a change in the predators’ behavior. They set up perches along a sulfur stream and cameras to film hunting kiskadees. After a bird made a passage across the water, a researcher with a slingshot activated the fish’s swinging behavior, mimicking something the kiskadees routinely saw when a kingfisher was hunting next to them. Now they could compare undisturbed hunting and disturbed hunting.
While the fish wrinkled and boiled, the kiskadees sat on the perches nearby. Over the course of more than 200 hunting sessions, the researchers found that the birds waited twice as long before starting a new pass than if the water remained undisturbed. When they attacked again, they were much less successful at catching a fish than they were with standing water.
Without the researchers’ intervention, the birds caught a fish more than half the time. With the slingshot in play, it was less than a quarter of the time. When the researchers looked at kingfishers, they saw that the more often the fish flashed, the longer the birds waited, as if they were waiting, too.
That response suggests that not only does the flashing behavior make it harder for the predator to zoom in on a fish, but the birds also know that their efforts are more likely to be wasted once the waves kick in.
This is an intriguing observation, because if the fish only tried to escape the predators, they could dive deeper and stay under longer. While the low-oxygen environment of a sulfur bath means they can’t stay submerged indefinitely, they’re perfectly capable of staying submerged for longer, said Dr. Bierbach.
“They can stay under water for up to two minutes or three minutes,” he said. “But they don’t. They quickly come back to the surface and repeat their dive, very synchronously, very rhythmically.”
Synchronized behavior, such as in flocks of fireflies blinking together or flocks of birds moving through the sky together in a carefully spaced pattern, has long fascinated scientists and anyone lucky enough to have seen it. But until now, it’s been difficult to pinpoint exactly what advantage its creatures have and why it may have evolved.
The sulfur mollies seem to be one of the rare instances where the benefits of a synchronized behavior can be teased.
The birds “learn to avoid these swaying schools of fish afterwards, because the chances of getting a fish are lower when the swaying happens — and the fish aren’t eaten, which is a win-win,” said Dr. Bierbach. “For example, a signal can evolve if both parts, the sender and the receiver, benefit from this.”
There is still much to learn in the sulfur baths of Tabasco.
“Right now we’re just looking at what’s happening from above,” said Dr. Bierbach. “And now we want to go below the surface of the water, with underwater cameras.”
The researchers hope to discover how the first fish that dive may signal others, and whether their dives vary depending on the type of disturbance.
“We have to go underwater to see that,” he said.