Why These Mexican Fish Do the Wave

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.

This observation gave the researchers a way to test their hypothesis that the flashing could result in a change in the predators’ behavior. They set up perches along a sulfur stream as well as cameras to film hunting kiskadees. After a bird had made one pass over the water, a researcher with a slingshot triggered the waving behavior in the fish, imitating something the kiskadees saw routinely when a kingfisher was hunting alongside them. Now they could compare undisturbed hunting and disturbed hunting.

As the fish rippled and seethed, the kiskadees sat on the perches nearby. Over the course of more than 200 hunting sessions, the researchers saw that the birds waited twice as long before starting another pass than when the water remained undisturbed. When they did attack again, they were much less successful at catching a fish than with still waters.

Without the interference of the researchers, 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 watched kingfishers, they saw that the more times the fish flashed, the longer the birds waited, as if they were waiting it out, too.

That response suggests that not only does the flashing behavior make it harder for the predator to zero in on a fish, but the birds also know that their efforts are more likely to be wasted once the waves begin.

This is an intriguing observation, because if the fish were merely trying to escape from the predators, they could dive deeper and stay down longer. Though the low-oxygen environment of a sulfur pool means they can’t stay under indefinitely, they are perfectly capable of more extended stays below, Dr. Bierbach said.

“They can stay for up to two minutes or three minutes under the water,” he said. “But they don’t do it. They come quickly back up to the surface and repeat their diving, very synchronously, very rhythmically.”

Synchronized behavior, like in swarms of fireflies that blink in unison or flocks of birds that move together in a carefully spaced pattern across the sky, has long fascinated scientists and anyone else who is lucky enough to have seen it. But so far, it has proved difficult to pinpoint exactly what benefit the creatures receive from it and why it might have evolved.

The sulphur mollies seem to be one of the rare cases where the benefits of a synchronized behavior can be teased out.

The birds learn “to avoid these waving fish shoals afterward because the chance of getting a fish is lower if the waving is happening — and the fish don’t get eaten, which is a win-win situation,” Dr. Bierbach said. “This is how a signal can evolve, if both parts, the sender and the receiver, have a benefit from that.”

Much remains to be learned at Tabasco’s sulfur pools.

“At the moment we just look from above at what happens,” Dr. Bierbach said. “And now we want to go below the water’s surface, with underwater cameras.”

The researchers hope to discover how the first fish that dive are able to signal others, and if their dives vary depending on the type of disturbance.

“We have to go underwater to see that,” he said.

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