Raising fish on land seems like the sort of idea you’d get while recovering from general anesthesia. But for three McGill University researchers, it made perfect sense. How else would you find out what behavioral and physiological changes might have taken place when fish first made the move from sea to land over 400 million years ago?
"I used to look at fins and their motion, and I always thought it was so interesting and complex," says Emily Standen, lead author of a study published in Nature today, and an evolutionary biomechanics researcher who now works at the University of Ottawa. "And then I thought, wow, how does that change from a fin to something that might work on land? That’s how this project started."
To find out exactly what might have happened when aquatic animals first moved to land, Standen and her colleagues took 111 juvenile Polypterus senegalus — a fish species that goes by the common name Senegal bichir, or "dinosaur eel" — and raised them for eight months in a terrestrial environment. This environment consisted of mesh flooring covered in pebbles and just 3 millimeters of water — a precaution that, combined with water misters, prevented the fish from drying out. The researchers also formed a control group using 38 fish growing up in their usual aquatic environment.
"There’s anecdotal evidence that they move on land."
"These fish have functional lungs and can breathe air," explains Standen. Dinosaur eels also have gills, but they breathe at the surface regularly to increase their oxygen supply. They also occasionally use their fins to walk on land. "There’s anecdotal evidence that they move on land from ephemeral pond to ephemeral pond [when they dry up]," Standen says, "but they don’t do it voluntarily." Still, that was more than enough to attempt to raise these young fish on land.
"We used high-speed video to analyze their movements at the end of the eight-month period," Standen says. Because of time restraints, this part of the analysis was carried out in 20 of the terrestrial fish and 10 of the aquatic fish. The researchers also killed and scanned individuals from both groups to look at how living on land had affected their skeletons.
As it turns out, growing up on land really does change how a fish walks.
Growing up on land really does change how a fish walks
"Fish raised on land walk with a more effective gait," Standen says. "They plant their legs closer to the body’s midline, they lift their heads higher, and they slip less during that walking cycle." The fish raised on land were also generally more consistent in the way they walked than their aquatic counterparts. But their behavior wasn’t the only thing that changed. For example, "the bones in the pectoral girdle — the bones that support the fins — changed their shape," she says. "And their clavicles became elongated." Most changes that occurred were ones that gave their heads and fins more room to move around.
"It’s an important change, because if you think of a terrestrial lifestyle," Standen explains, "you actually need a neck on land because you’re stuck on this plane, and you need to have more head motion that’s independent from the body."
"they lift their heads higher, and they slip less."
Yet, the experimental nature of this study does limit some conclusions the researchers can draw. For one thing, the dinosaur eel isn’t directly related to the fish species that first walked on land, such as the Eusthenopteron. "We don’t have a living ancestor of the fish we’re interested in," Standen explains. Instead the scientists went with the next best thing: a species that’s physically similar to the fossils from that period. "It’s a great model because it’s elongate, has armor scales, and has ventral lateral pectoral fins‚ which means its fins are at the side, close to the belly of the fish."
Despite this limitation, Standen says it’s exciting to think that we can use experiments on living animals to make inferences about what might have happened to physically similar animals during large scale evolutionary transitions. "It seems quite clear that environmentally induced changes may have facilitated their transition to land," Standen says. "Selective pressures then acted on these changes, and they became fixed in the genome over very long periods of time."
Standen is now repeating the experiment with the hope of identifying changes in the fish’s muscles. She would also like to look at what happens when the fish are kept on land for longer periods. "It’s quite possible that larger changes would occur if we kept them on land for longer," she says. "The big dream is to do this over several generations."