2 different sea creatures evolved to swim like this

"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?" says Malcolm MacIver. "It is because there is a finite number of ways to really do something well." Above, a Persian carpet flatworm. (Credit: Pei Yan/Flickr)

Two kinds of sea creatures—the Persian carpet flatworm and black ghost knifefish—last shared a common ancestor 550 million years ago. But new research shows they both evolved to swim with elongated fins using the same mechanical motion.

The creatures are part of a very diverse group of aquatic animals, both vertebrate and invertebrate, that independently arrived at the same solution of how to use their fins to maximize speed.

Remarkably, researchers say this so-called “convergent” evolution happened at least eight times across three different phyla, or animal groups, supporting the belief that necessity played a larger role than chance in developing this trait.

The findings could help scientists better understand evolution and may also help pave the way for highly agile underwater vehicles.

Using computer simulations, a robotic fish, and videos of real fish, researchers studied 22 animals called median/paired fin swimmers, in a wide variety of shapes and sizes. Their elongated fins both undulate, or ripple, lengthwise and oscillate from side to side.

For all these animals’ fins, the length of one undulation during swimming divided by the mean amplitude of the sideways movement is always a ratio of around 20.

“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?” says Malcolm MacIver, associate professor of biomedical engineering and mechanical engineering at Northwestern University.

“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.”

Chance vs. physics

Evolution pushes nature toward more organized solutions and less disorder, according to the researchers who say their goal was to quantify biology and its solutions, not just make observations.

black ghost knifefish
Black ghost knifefish. (Credit: Clinton & Charles Robertson/Flickr)

“Technically, it’s chance versus physics,” says Neelesh A. Patankar, professor of mechanical engineering. “Chance offers many possibilities as to how a fish can swim, but physics and the animal’s environment put 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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Published in the journal PLOS Biology, the research is one of very few to quantify the mechanical basis (maximizing propulsive force or speed) for a case of convergent evolution, where similar features evolve in unrelated organisms as adaptations to similar lifestyles and environments, such as wings in both birds and bats.

The researchers call the recurring ratio of undulation length to sideways movement that maximizes speed—20—the “optimal specific wavelength” (OSW). In their study, they didn’t find a single exception, and they expect that the additional 1,000 species that swim in this way (median/paired fin swimmers) but that have not yet been measured also will adhere to the OSW.

“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 says. “The ratio of 20 is best.”

The National Science Foundation supported the work.

Source: Northwestern University