While fish are generally no match for predatory birds that hunt them from above, the sulphur molly, a freshwater fish species which can grow up to 4.5 inches, appear to have developed an effective defence mechanism.

When the school spots a potential predator (not necessarily birds, but any other species that may prove a threat, including humans), the school start swimming in waves that were conspicuous, repetitive and rhythmic.

And it is not just a few hundred sulphur mollies involved in this display—we're talking about a much larger number. 

According to Jens Krause at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin and Cluster of Excellence Science of Intelligence, “There are up to 4,000 fish per square metre and sometimes hundreds of thousands of fish participate in a single fish wave. Fish can repeat these waves for up to two minutes, with one wave approximately every three to four seconds.”

A video of the collective waves can be found here: https://www.eurekalert.org/multimedia/811748.

The fish’s collective action apparently thwarts the attack, and the predatory bird may switch perches or decrease the frequency of its attack. Even experimentally induced fish waves had the same effect of reducing the attack frequency.

The research team intend to continue with their investigation, so as to find out the reason behind this. Is it because the birds get confused, or do the waves tell them that their impending attack had been detected?

The findings of the research has been published in the Cell Biology journal.