Tiny fish do 'the wave' to scare off predatory birds
Coordinated behaviour by thousands of small fish in a unique environment helps protect them
Small freshwater fish found in sulphur springs in Mexico take part in coordinated action similar to fans in a stadium doing 'the wave.' Its purpose is to confuse and discourage predatory birds.
Sulphur mollies are about 25 millimetres in length, and can survive in sulphur springs in Mexico that are toxic to many other kinds of wildlife. Because there is limited oxygen in the sulphurous water, they will congregate at the surface in large numbers for access to oxygen.
There can be as many as four thousand fish per square metre, creating what Jens Krause, a researcher at the Leibniz Institute of Freshwater Ecology and Inland Fisheries at Humboldt University in Berlin described as a "fish carpet."
Sulphur mollies in high density seen from below the surface:
Ordinarily this would make them a tasty target for predatory birds.The wave seems to be a defensive behaviour that has evolved that takes advantage of their density. Krause and his colleagues recently published their study of how its coordinated and how predators respond to it.
When the fish are disturbed by the presence of a predatory bird, they engage in the coordinated wave. As many as hundreds of thousands take part.
When some fish detect a potential threat, they dive, which triggers the fish around them to dive as well. The diving behaviour propagates as a visible wave through all the fish. The fish return to the surface three or four seconds later, and may then repeat this behaviour, so the visible wave continues for up to several minutes.
Sulphur mollies doing 'the wave':
This wave creates the illusion that something is travelling across the water. This seems to confuse attacking birds such as kingfishers, herons and egrets, which often stop or abandon their attacks.
Krause has found that the wave can be a very successful strategy. It makes predation less likely and birds who do want to try their luck a second time wait twice as long to attack again.
A kingfisher attack:
Produced and written by Mark Crawley