Walking bichir fish may reveal how vertebrates moved onto land
Fish raised out of water develop steady walking gait, Canadian researcher finds
Fish out of water tend to flop around clumsily. But a Canadian scientist has found a way to raise fish to "walk" confidently with their heads held high — by forcing them out of the water at a young age.
The study, published online Wednesday in the journal Nature, may give us a peek into how the fishy ancestors of land animals with a backbone – including ourselves – first emerged from the water to colonize the land around 400 million years ago.
Before that time, the only vertebrates that existed were fish, and they lived in the water.
It was "not an easy place to live," said University of Ottawa physiologist Emily Standen, as it was full of other fish competing with each other for resources such as food, including many fierce predators.
Meanwhile, the land was a comparatively quiet, peaceful world of plants and insects.
"If you can get out there," Standen said, "you've got a big advantage."
Fossil evidence shows some fish made the move and claimed that advantage, evolving into the first four-legged land animals called tetrapods. But scientists still know relatively little about how the transition happened.
Standen is lead author of the new study, which is based on work she did while she was postdoctoral researcher at McGill University in Montreal. She wondered whether primitive fish might have an innate but normally untapped ability to make that move to land, and if that could reveal something about what happened 400 million years ago.
"When we dig up fossils, it's great because we get to see bones," Standen said. "But it's very difficult to understand how those bones move if you don't have a living example because sometimes animals do things in a really unique way, and we'd never guess from just bones."
Standen, working with McGill University paleontology professor Hans Larsson and student Trina Du, decided to conduct her study using a primitive African fish called a bichir.
Bichirs, which are sometimes kept as pets, have a body shape similar to the first tetrapods. They can breathe air and they can "walk" on land by propelling themselves with their fins, albeit clumsily.
Standen ordered very young bichirs from a pet supply company and decided to raise them out of water, so they would be forced to walk instead of swim.
She put them in a tank with just a few millimetres of water on the bottom. To keep the fishes' backs from drying out, she added, "I took a cue from grocery store… You know how they have those misters over the salad? We got misters and we set them up in the aquarium and we misted the fish. "
The fish were raised in the tanks for eight months, and then compared to fish that had been raised in regular tanks full of water.
They found that the fish raised on "land" had differences both in their walking ability and anatomy.
"The fish raised on land appear to be walking more effectively," Standen said. "They plant their fins closer to body mid-line when they're walking, which allows them to support themselves more effectively. And they lift their heads higher off the ground."
That reduced the friction between the fish's head and the ground.
The fish raised on land also slipped less when they planted their fins to push off while walking.
Many of those enhanced walking abilities seemed to be linked to changes in the anatomy of the fish. Among the land-raised fish, bones around their chests and shoulders changed to allow them to support more body weight on the fins.
Meanwhile, bones attached to the heads of the fish changed to allow the head to move more freely from side to side and up and down, as though the fish had something they generally don't have – a neck.
That's important on land, where most animals can't approach their food from above or below, and may need to tuck their chin in order to eat something off the ground.
Standen said she knew there would be differences in the land-raised fish, but found the nature of the anatomical changes interesting.
"I was surprised at how well it mirrored what was going on in the fossil record."
One fish that lived 375 million years ago in the Canadian Arctic, Tiktaalik, shows many traits that would have allowed it to support itself on land, such as a mobile neck, a robust ribcage, and shoulders, elbows and partial wrists on its forelimbs.
The similarities between the land-raised bichirs and fossil fish at the time of the move to land suggest that movement and behaviour of the land-raised bichirs might also reflect changes in the fossil fish as they adapted to life on land.
"It's nice to be able to compare something living that has a similar body shape to fossil to get an idea how they move," Standen said.
But she thinks the most interesting finding of the study is it suggests individual animals' ability to adapt to changes in their environment plays a role in evolution.
Standen is now busy raising more baby bichirs on land so she can look at changes not just to different bones in the their bodies, but also changes in their muscles.