Take a look at a fish’s back, and you’ll be looking at an evolutionary snap shot. As fish have evolved, their dorsal fins have become more complex. Salmon have the simplest fins, with one soft structure on their backs. But as you look further up the evolutionary tree, fish have added a second spiny fore-fin, until you arrive at perch-like fishes, where the two fins are fused. Knowing that the soft part of the fin was the evolutionarily conserved section, Eliot Drucker and George Lauder wondered whether sunfish used the ancient portion of the fin while swimming, and if they did, how much the was fin contributing to the fishes overall manoeuvrability.
According to Drucker, ‘vortices are the hallmark of fluid force production’, so he watched the eddy patterns the fish left behind as they swam upstream. Using a digital imaging technique, he was able to map the way that...
