Spinal secrets: What eels reveal about the origins of movement

Eels move in ways that seem to ignore the rules most vertebrates live by. They swim with flowing body waves, they can scoot across wet ground, and they often keep going even when the spinal cord is cut.

According to a new paper, two simple body signals, stretch and pressure, help explain how that is possible.

Those signals are not fancy. Stretch tells the body how much each segment bends, and pressure reports forces on the skin from water or from objects.

When these signals are fed into the spinal circuits that set a basic rhythm for movement, the eel’s whole body keeps time without micromanagement from the brain.

Secrets of eel movement

Kotaro Yasui is an assistant professor at Tohoku University’s Frontier Research Institute for Interdisciplinary Science (FRIS) and lead author of the study

“Our findings will help design highly adaptive robots capable of navigating complex and unpredictable environments,” noted Professor Yasui.

Most vertebrates walk or swim thanks to networks in the spinal cord called central pattern generator circuits.

These circuits can produce rhythmic muscle commands on their own, then shape that rhythm using feedback from the body. That is why the same overall plan can show up in animals as different as fish, salamanders, and mice.

Eels add a twist by leaning hard on two kinds of sensory input. Proprioception is internal sensing of stretch that tells a segment where it is in the cycle. Exteroception is sensing from outside the body, including water pressure on the sides of the fish or contact with a surface.

Stretch sensitive neurons like the classic lamprey edge cells have long shown how such bending signals can tune the spinal rhythm.

Flexible robot mimics eel movement

Yasui’s team built a mathematical model in which each body segment has a small local oscillator that produces a timing signal. They wired in two feedback loops, one for stretch and one for pressure, so the oscillators would be tugged into step by what the body feels.

The researchers tested the controller in two settings. First, they used computer simulations that generated body movements and force patterns. Then, they tried it on a physical eel-like robot made of flexible segments and equipped with skin-level sensors.

The same controller worked in both settings, and it produced stable swimming quickly, without central commands to lock the segments together.

The stretch loop had a special job. It sped up the time it took for a smooth traveling wave to appear from random starting positions. It also helped the body hold a strong stride once the pattern settled, even though that gain could cost a little energy because the joints work harder during sharper turns.

Spinal rhythm and sensory feedback

The team asked a simple question. If the controller swims well, can it also crawl on land when the ground is dotted with obstacles? They ran the robot through a peg field where pushing and bracing matter more than glide.

Stretch feedback made the difference. With stretch only, the robot found traction on the pegs and made steady headway.

Pressure alone did not help in that setting, which fits earlier research showing that elongated fishes such as ropefish change their kinematics across water and land in specific ways, with long, slow, high amplitude body waves on land.

The results suggest continuity rather than a clean break between swimming and crawling in these fishes.

The same spinal rhythm and sensory feedback can drive both swimming and crawling, as long as the environment provides the right reaction forces. The fish doesn’t need a separate land-based neural controller to move

What spinal injuries teach us

The most striking tests involved spinal cord transection. The group measured swimming in live American eels before and after a complete cut around the middle of the body, then matched those tests with simulation and robot runs using the same controller. 

The movement behind the injury kept a similar frequency and phase as the movement in front of it, which echoes decades old evidence that eels can recover coordinated swimming after transection as regrowth advances.

In the model, that coordination did not need signals from the brain. It emerged when the local oscillators had some built-in tendency to tick on their own, and when stretch and pressure loops were strong enough.

The body above the cut tugged the body below the cut into the same rhythm through passive motion and local sensing.

That simple recipe explains why eels can move immediately after injury while many amphibians and mammals cannot. If the posterior spinal cord cannot oscillate on its own, or if the feedback pathways are too weak, the two halves fall out of step and the body stalls.

Eel movement, robots, and evolution

Robots that move without legs need to live with uncertainty. Water currents change, floors are cluttered, and obstacles shift. Prior work has shown that proprioceptive feedback can entrain undulatory robots into reliable patterns despite turbulence and noise.

This study adds pressure sensing on the skin to that recipe and shows how a simple stretch rule helps robots use clutter for propulsion. That means eel-like machines could search pipes, navigate earthquake rubble, or inspect tanks without complex top down control.

There is also a story about evolution here. If a swimming controller can produce useful crawling when the environment offers something to push against, early vertebrates may have needed adjustments more than a full rewrite to move onto land. Flexible circuits and strong local feedback could have carried a lot of that load.

Future research directions

Biologists can now test specific predictions in the lab. For example, they can look for descending stretch pathways that echo the model’s anterior to posterior copy rule, and they can probe how pressure sensors along the skin tune rhythm in moving fish.

Engineers can search for energy savings by blending stretch rules that boost stride with rules that prevent excessive bending. They can also explore how to route around damaged links in a robot by letting neighboring segments pull each other back into rhythm.

The study is published in the journal Proceedings of the National Academy of Sciences.

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