Researchers studying 22 wildly diverse aquatic creatures -- some who last shared a common ancestor half a billion years ago -- have discovered that they evolved to swim with the same stroke to maximize their speed. This mode of swimming emerged multiple times independently across both invertebrate and bony swimmers alike, according to findings published in PLOS Biology last month.
“Why do you see the same traits, such as the camera-lens eye or wings, in animals that are so different and have no common ancestor with that trait?” Northwestern University’s Malcolm MacIver says in a news release. “It is because there is a finite number of ways to really do something well. In our study, we have quantified how an unusual group of swimming animals optimizes force and, therefore, speed.” The black ghost knifefish (pictured above) and the Persian carpet flatworm, for example, shared a common ancestor 550 million years ago.
So, the Northwestern team led by MacIver and Neelesh Patankar used computer models, a robotic knifefish called Ghostbot, and videos of (real) fish to study 22 so-called “median/paired fin” swimmers. Unlike trout and salmon -- who swim by moving their tails and the rear half of their bodies -- these swimmers have elongated fins that run the length of their body. The fins undulate lengthwise (creating ripples) and also oscillate side to side. This fluttery, wavy, widespread form of locomotion is used by animals ranging from cuttlefish to triggerfish to rays. Watch this very cool compilation video below to see what I’m talking about:
The team discovered that the relationship between the length of the undulation and the sideways movement is fixed: Specifically, the length of one undulation divided by the average amplitude of the sideways movement is always a ratio of 20. They call this recurring ratio, the optimal specific wavelength. “Chance does play a role in these animals -- they don’t all adhere exactly to the optimal number 20 -- but there is a point where variability can become deadly, that swimming with the wrong mechanics means you waste energy and won’t survive,” MacIver explains. “The ratio of 20 is best.”
In this remarkable example of convergent evolution, animals from three phyla -- flatworms, mollusks, and chordates -- independently arrived at the same speed-maximizing solution. Furthermore, those 22 critters belong to eight different clades, or groups of organisms that stem from one common ancestor. That means this pattern of swimming emerged eight times.
“Technically, it’s chance versus physics,” Patankar adds. “Chance offers many possibilities as to how a fish can swim, but physics and the animal’s environment puts constraints on these possibilities. In this case, the selection pressure is very high, pushing the animal to one particular solution, and necessity triumphs.”
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