When you picture a fish that spends its whole life constantly swimming at high speed, you probably see a tuna or marlin – something that cruises miles and miles each day through the open ocean. These fish generally have crescent-shaped tails with a narrow caudal peduncle – the part of the fish tail just before the tail fin – built for efficient swimming 24/7. However, fish living in fast-flowing rivers also need to swim fast constantly to be successful. The main difference from their deep-sea counterparts is that these riverine fish are swimming as hard as they can to go nowhere; they are essentially on a giant water treadmill. Living in torrential rapids, these fish need specialized traits to help them swim all day just to stay in the same spot, a behavior known as station-holding, to avoid getting washed downstream. Are the adaptations for swimming constantly in a flowing river to station-hold like those for roaming the relatively calm open ocean at speed?
To find out, a group of scientists from Northern Arizona University, USA, led by Daniel Kimball in Alice Gibb's lab, looked at the tail anatomy of three species of minnow from the southwestern USA to compare with that of tuna and other open ocean cruisers. These closely related minnows – bonytail, humpback chub and roundtail chub, listed in descending order of flow preference – are some of the species that traverse the world-famous rapids of the Colorado River as it passes through the Grand Canyon. Building on previous work, Kimball and colleagues found that not only does the bonytail have spines for muscle attachment on its vertebral column, which insert at sharper angles than for your typical fish, but the angle of these spines gets shallower from head to tail. This makes its caudal peduncle much thinner and more streamlined than those of most other fish, independently evolving the same shape as the tuna tail, despite being completely unrelated. Meanwhile, the slower-water preferring roundtail chub had more obtuse vertebral spines with a thicker, more typical tail shape.
Tails are more than just bones, though. How does the bonytail's tail muscles and connective tissues compare with those of tuna and other oceanic speedy species that prefer slower moving water? Kimball and colleagues tried stretching the muscles that move the minnows’ tails to find out how much force they can withstand. They also measured how much collagen, a tough connective tissue, was found in these muscles by examining them under a microscope. The team found that the tail muscles of the bonytail resisted tearing better than those of the other species, with roundtail chub having the least tear-resistant muscles. This means that compared with its lazier, though still relatively active relatives, the bonytail can safely transmit a lot more force through its tail to propel itself through fast-flowing rapids. Additionally, the team found that the muscles powering the bonytail's tail had more collagen than its other muscles, which was not seen in the other species. This suggests their tail muscles may act like the springy tendons of tuna tails, storing elastic energy on each tail movement, allowing bonytails to swim efficiently at high speeds.
While there may not be any freshwater tuna, bonytails come as close as possible thanks to convergent evolution allowing both species to arrive at the same solution for high performance swimming. However, instead of swimming non-stop to cross entire oceans, bonytails swim non-stop just to stay still in fast-flowing rivers. Like a cross-country runner on a treadmill, the bonytail could cruise through the oceans with tunas if they wanted to, but prefer the scenic landscape of the Grand Canyon.