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These fish work together by the hundreds of thousands to make waves [phys.org]:

December 22, 2021

These fish work together by the hundreds of thousands to make waves

In the sports arena, spectators sometimes create a spectacle known as a wave, as successive groups stand up in unison to yell with arms in the air. Now, researchers reporting in Current Biology on December 22 have shown that small freshwater fish known as sulphur mollies do a similar thing, and for life or death reasons. The collective wave action produced by hundreds of thousands of fish working together helps to protect them from predatory birds.

"The surprises came once we realized how many fish can act together in such repeated waves," said Jens Krause of 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 meter 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."

When you're in the vicinity of these unusual fish, found in sulphuric springs that are toxic to most fish, this behavior is hard to miss. That's because the mollies do the same thing in response to a person nearby.

"At first we didn't quite understand what the fish were actually doing," said David Bierbach, co-first author along with Carolina Doran and Juliane Lukas, also at the Leibniz Institute of Freshwater Ecology and Inland Fisheries and Cluster of Excellence Science of Intelligence. "Once we realized that these are waves, we were wondering what their function might be."

Journal Reference:
Redirecting, (DOI: 10.1016/j.cub.2021.11.068 [doi.org])

Original Submission