According to new research, many marine animals may have independently learned to optimize their swim strokes in the same way.

A tiny cuttlefish and a huge stingray don't look like they have much in common. Genetically and morphologically, they don't. But even though their bodies are totally different shapes, it seems that both follow the same mechanical rules when they swim: Their long fins both ripple lengthwise and oscillate from side to side. And for both, the length of one undulation during swimming divided by the height of the sideways movement is always a ratio of around 20.

In fact, at least 22 marine animals have exactly the same ratio.

According to the researchers who discovered this surprisingly common ratio, these sea creatures didn't all evolve their identical swim strokes together. Instead, this way of moving evolved over and over again independently, in a phenomenon called convergent evolution. Why? Because it works.

The findings were published Tuesday in PLOS Biology.

"Physics seems to have overwhelmed chance in the evolution of these creatures," study author Neelesh Patankar of Northwestern University said. According to computational models and studies based on a robot designed to swim like these long-finned creatures, the 20 ratio gives them their maximum possible force.

For Patankar and his co-author Malcolm MacIver, the most incredible thing about the findings is that both vertebrates and invertebrates swim based on these rules -- flatworms as well as bony fish.

In each case, they say, the infinite possibilities that existed for the swimmers were constrained by the hard truth of physics.

In future studies, they hope to further quantify just how common this shared swim stroke is as a means of estimating how often physical optimization can cause convergent evolution.

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