Welcome to the “Fish Gym” in the Shubin-Di Santo Lab

A Polypterus "works out" on the treadmill in Neil Shubin and Valentina Di Santo's MBL Whitman Center lab. Credit: Valentina Di Santo

This story is part of a year-long series commemorating the 10-Year Anniversary of the MBL-UChicago affiliation.

10th anniversary of the MBL UCHicago affiliation logo

“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 (MBL).

“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 un-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.

Neil Shubin with his fish treadmill for training fish to walk in his Whitman lab (Credit Diana Kenney)
Neil Shubin with the treadmill flow tank for videotaping walking fish in his Whitman Center lab. Credit: Diana Kenney

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 lowest speeds where 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 race track. 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.”

Valentina Di Santo examining a flow tank at Harvard University. Credit: Perry Leenhouts
Valentina Di Santo examining a flow tank at Harvard University. Credit: Perry Leenhouts
Valentina di Santo makes an experimental substrate in Rowe. Credit: Diana Kenney
Di Santo glues together a pebbly substrate for training fish on different terrains. Credit: Diana Kenney

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.

They could even bring the fossil fish Tiktaalik back to life.

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. 

Unfortunately, fossils can only tell us so much. But, if Iida can take the principles he will develop making a robotic Polypterus and apply them to what we know about Tiktaalik, he could create a robotic version of this extinct fish, allowing scientists like Di Santo and Shubin to examine how it might have moved. A realistic robotic Tiktaalik could reveal a lot about why fish decided to take to the land.

“Fishes are so much like us,” Di Santo said. She explained that both fish and people try to save energy when they can -- “move smarter, not harder.” This quest to save energy is so powerful that it might have laid the foundation for the evolutionary success of land-dwelling vertebrates: One day, a fish grew tired of swimming and decided to take a walk along the seafloor, kicking off one of the most important events in the history of our planet. Soon, thanks to Di Santo and Shubin’s research, we may better understand why that change came to be.

The fusion of diverse scientific disciplines -- physiology, paleontology, robotics -- lies at the heart of Di Santo and Shubin’s research. They decided to collaborate at MBL “because of the opportunity to work with so many different people - embryologists, neurobiologists, marine biologists” who can inform their research, said Di Santo. By combining many different fields, they hope to take their work in surprising new directions. 

“The MBL is an excellent environment to have new ideas and to collaborate,” she said.

Shubin has been very active at the MBL during and since its affiliation with the University of Chicago in 2013. In addition to serving in leadership roles, including co-interim director of the MBL in 2017-2018, Shubin has seeded several collaborations in the MBL Whitman Center, including the present project with Di Santo. Shubin is the Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy and Special Advisor to the Provost for the Life Sciences at the University of Chicago,