The reason for these biological similarities is, according to a research team, down to physics and the way that marine creatures need to respond to ocean currents.
Flatworms, cuttlefish, rays, and triggerfish are among the 1,000 species that are members of a group called median/paired fin swimmers. These animals move through the water by rippling a fin that runs the length of their body. When researchers from Northwestern University, as reported by The Washington Post, measured the height of the ripples and the distance between them in videos of 22 unrelated species from three different families, they found that the undulations were consistently 20 times as long as they were tall in each animal. By mapping the species in terms of how related they were to each other, the scientists determined that the motion likely developed independently at different times in this group of animals.
To explain why flatworms and rays arrived at the same swimming strokes, a science team tested a variety of ripple length-to-height ratios in a computer model and with a robotic knifefish. In these systems, the researchers found that the 20-to-1 ratio provided the maximum amount of force to propel the animal forward. Thus the swimming motion developed according to the basic laws of physics.
The findings have been published in the journal PLoS Biology. The research paper is titled “Convergent Evolution of Mechanically Optimal Locomotion in Aquatic Invertebrates and Vertebrates.”
