Schools of mollie fish "do the wave" to evade predators
We've all seen sports spectators performing "the wave," where crowds of people stand up then sit back down to simulate a wave moving through the stadium. Well, schools of a certain fish have been found to perform a similar action, in order to keep from being eaten.
The sulphur mollie (Poecilia sulphuraria) is a small freshwater fish which gets its name from the fact that it's found mainly in hydrogen-sulphide-rich springs. Because of the water's low oxygen content, the fish tend to gather at the surface where they breath air. Needless to say, this puts them at risk of being snatched by land-based predators … and that's apparently why they do the wave.
In a recent study conducted by scientists from Germany's Leibniz Institute of Freshwater Ecology and Inland Fisheries, schools of hundreds of thousands of the mollies were observed in a spring near the Mexican city of Teapa.
It was found that when attacked from above by birds, the fish would repeatedly dive down in sequential groups, with their tail "kicks" creating rhythmic visible waves travelling across the surface of the water. The mollies would keep doing so for up to two minutes at a time – this means it wasn't simply a case of the fish just diving down once to reach deeper, safer water, then staying there.
When the scientists induced this behaviour by shooting small objects into the water, it was found that birds tended to wait twice as long before launching their next attack on the fish. Birds also changed perches above the fish more often when the waves were induced, suggesting that the behaviour caused them to seek a different hunting ground.
The researchers still haven't determined why the wave response causes a reduction in bird attacks, so further field studies are being planned. It's possible that the surface waves make the fish more difficult to see, or that the behaviour lets the birds know that they've been noticed and evasive action is being taken.