Editor’s note: This story is part of a year-long series commemorating the 10-year anniversary of the Marine Biological Laboratory's affiliation with the University of Chicago.
“Inelegant” is a generous way to describe how Polypterus moves on land.
A small, brown fish with a wide, flat head and strong pectoral fins, Polypterus wriggled powerfully when Valentina Di Santo, a fish physiologist from Stockholm University, placed it on a mesh mat on the floor of Rowe Laboratory at the Marine Biological Laboratory.
“This one’s a jumper,” said Di Santo, a Whitman Center scientist at MBL this summer. She watched as the Polypterus writhed, periodically launching itself into the air in twists and spins. After some time, though, the fish calmed down and began to do something very non-fish-like. Contorting from side to side, Polypterus swung one pectoral fin forward, then the other, slowly dragging its body across the ground like a clumsy salamander.
In a rudimentary way, it was starting to walk.
Walking is among the most important moments in vertebrate evolution. The first fish that crawled onto land diversified quickly to exploit an abundance of resources, giving rise to everything from amphibians and reptiles to birds and mammals.
This oft-repeated story, however, obscures the real moment of innovation. Walking “really happened long, long before the first fish ever thought about going onto land,” said Di Santo. Before fishes walked on land, they were able to walk underwater.
The first fish to climb onto shore needed strong fins to support its weight. It’s likely that before land walking, fish would already have to be adapted to walking, or “proto-walking” along the seafloor. In all likelihood, this underwater walking behavior is the origin of the transition to land.
Taking a stroll in prehistoric times
Understanding how walking began is key to understanding our ancient past. Di Santo, along with paleontologist Neil Shubin of the University of Chicago, are collaborating at the MBL to answer a simple yet vexing question: Why would something that could swim ever choose to walk?
Di Santo and Shubin believe the answer lies in efficiency. Fishes are inherently unstable underwater, especially at the lowest speeds at which they need to move their fins to control posture. Swimming slowly requires a large amount of energy. Imagine swimming like riding a bike: It’s much less stable at slow speeds. Being able to stroll along the seafloor might be like getting off the bike and walking.
At the front of their Whitman Center lab is a transparent, three-foot-long tank shaped like a racetrack. Right where the starting line would be, there is a treadmill with a high-speed camera pointing at it from beneath. This contraption is a hybrid flow tank/treadmill that allows Di Santo and Shubin to watch exactly what happens when a fish starts walking.
When the experiment begins, with the tank full of water and a fish at the starting line, the treadmill and flowing water start moving at the same speed.
“We expect that fish may walk rather than swim at the lowest speeds,” Di Santo said. As the water flow velocity increases, though, there will be a point where the fish may choose to lift up in the water column and swim instead. All the while, the high-speed camera will film how the fish moves its fins, and an oxygen meter will measure how much energy the fish is expending as it transitions from walking to swimming.
“We’re trying to understand the rules of walking,” Di Santo said–which fins the fish uses, how it moves its body, when it chooses to swim and to walk, whether there is a transitional period of hybrid ‘skipping’ locomotion. The scientists will test at least 11 different species in this device, including sharks, rays, gobies, and lungfish, trying to understand what defines each of their walking styles. Polypterus will be tested in this tank, too, but only after some special training.
This training happens next to the flow tank, in what Di Santo called a “Polypterus gym.” Some of the fish here live in tanks with no water, just mist coming down from pipes.
Amazingly, Polypterus do fine without water to swim in. They have lungs, so they can breathe air just fine as long as the misters keep them moist. Without being able to swim, though, they have to walk.
Di Santo and Shubin hope to test how these fish change their walking style, and the efficiency of it, as they get more and more familiar with life on land. In some of the tanks, they have even installed pebbly hills for the fish to climb to see if uneven terrain might drive change in their walking style. The fish will remain in this enclosure for between three months and a year.
“There’s a chance that after so long out of water,” Di Santo said, “these fish may start to move more efficiently out of water.”
How to build a robotic fish
This audacious idea–training Polypterus to behave like a salamander–is not even the final step of this project. While Di Santo is running the flow tank experiments, Shubin will be doing anatomical studies to figure out the exact anatomy of these fishes’ fins, both in normal Polypterus and those in the terrestrial tanks. Then, armed with Di Santo and Shubin’s data, roboticist Fumiya Iida of the University of Cambridge will try to construct a robotic Polypterus.
If Iida can make a robot Polypterus that walks and swims like the real thing, he can try creating robot versions of other walking fish and compare them to their real-life counterparts.
Tiktaalik was a fish that lived 375 million years ago. It was long and crocodile-shaped, spending most of its life in water but occasionally walking on land. When Shubin co-discovered the fossil of Tiktaalik in the Canadian Arctic in 2004, it was hailed as the “missing link” between sea and land vertebrates, our ancestor.
—Prof. Chuan He
