BUT I WOULD WALK 500 MILES, AND I WOULD WALK 500 MORE
How are amphibious centipedes able to walk and swim? How are they able to coordinate their bodies as they transition from land to water or vice-versa? To better answer these fascinating questions, the Fulcrum sat down with Emily Standen, a U of O professor within the department of biology.
In 2019, she worked in collaboration with researchers from Tohoku University located in Japan on a study titled, “Decoding the Essential Interplay Between Central and Peripheral Control in Adaptive Locomotion of Amphibious Centipedes” to learn more about the underlying mechanism involved in the locomotion of these invertebrates.
At their cores, amphibious animals are those that are adapted to live in both terrestrial and aquatic ecosystems. Examples that commonly come to mind are animals like salamanders or frogs. Not many think of amphibious centipedes, including Standen prior to doing this work.
For those that are terrified of the idea of swimming centipedes, not to worry, to date only a few species of the genus Scolopendra are known to have an amphibious lifestyle, with a keen ability to both walk and swim and these do not exist in North America.
“I really had to adjust my feelings on centipedes, and aquatic ones at that, to work with these animals, I’m happy to say, although I am still glad I won’t stumble upon them in my backyard, I have a new found appreciation for their incredible capacity to move.” she said.
Three components of neural control
For background, “there’s a basic understanding that there are three components to neural control, the brain that provides a top-down sending motor signals to the body. There are central pattern generators (CGPs), which are groups of neurons that exist along the spinal cord, capable of producing rhythmic signals to the muscles. And then there’s sensory feedback, receptors and neurons that bring information from the world back to the CGPs and to the brain.”
Standen is interested in the interplay between all three of these control components.
She continued, “amphibious centipedes are an excellent model because moving through water and over land provide very different sensory inputs to the locomotor system allowing researchers to modify the sensory signals provided to the system. In addition, the segmented body plan of a centipede allows a researcher to lift the exoskeleton of a single segment, cut the descending nerve cord coming from the brain and essentially block any top down signal from reaching the posterior segments.”
In this way one can compare the behaviour of the anterior segments that are receiving a signal from the brain, with those posterior to the cut that are not receiving signal from the brain. How those segments differ in their behaviour provides insight into what leg and body movements are controlled by sensory feedback and CPGs compared top down brain control.
How do their movements differ on land and water?
Through observations, they were able to identify that when amphibious centipedes are walking at relatively slow speeds on land their body stays relatively straight, and they have a tendency to pass waves of leg motion down their body which propels them forward.
As opposed to swimming where they fold their legs along their body, meaning their legs aren’t used at all and are able to pass a wave down their body similar to the way a snake moves or a fish swims. This movement could also be described as an undulation.
To understand how they’re controlling these movements researchers “removed the ganglion of a segmented partway along the body-severing the spinal cord and removing any signal from the brain to the posterior section of the animals. What’s amazing is that body segment posterior of the severed cord showed normal leg movements. Essentially, with no signal from the brain they were able to walk normally,” explained Standen.
She continued, “If you put that transected animal in water, the anterior segments that have a connection with the brain fold their legs and the body undulates as in normal swimming, but posterior of the cut, the segments look paralyzed, No leg folding, no body oscillation.”
Meaning that the front part of the body can perform the undulation normally seen in swimming because it still gets a message from the brain, but the back end can’t. Although segments were able to walk normally with no signal from the brain Standen stresses that the brain signal is still important for eliciting the swimming behaviour.
Standen and her colleagues, were able to conclude that sensory feedback from each limb as it connected with the ground in the terrestrial environment provides enough stimulation to the local segment CPG to elicit a motor signal to move the leg without need of a top down signal from the brain. In essence the CPG was driving the leg locally. In swimming, the forces felt by the legs are not enough to trigger any sort of motion. In addition, because the limbs remain outstretched and the body still, it can be hypothesized that leg position and body undulation in swimming are driven by a top down signal from the brain.
How do they switch between land and water?
Undoubtedly, this begs the question: how do centipedes flip between manoeuvering their bodies for walking and for swimming?
Standen responded, “the transition between walking and swimming is also explained by the sensory feedback felt by each leg. As a centipede walks into water, the front legs lose contact with the ground, the lack of contact removes the sensory feedback sent to the CPG and the resulting motor signal sent to oscillate the leg stops. The leg stops moving. Shortly after, the brain sends a top down signal to fold the legs and start a body oscillation for swimming.”
She added, “the same process occurs in reverse when the animal leaves the water. A swimming animal with all legs tucked and body oscillating hits land, those first legs experience the force feedback of contact with the ground. This contact is enough to trigger the CPG to send a walking signal to the leg in question. The walking signal trumps the swimming signal being sent by the brain at that local segment. In this way, the legs in contact with ground walk, while those still in the water swim. In this way the legs open up and start walking one by one, in a very elegant way.”
In addition, there is local communication from one segment to the next posterior segment. The posterior segment copies the anterior segment such that it is in a walking configuration by the time it hits land.
This data suggests that controlling so many legs is actually not so difficult. Meaning, that the animal isn’t thinking, “legs at the number seven position need to move and then after the number eight legs need to move.” Rather, the sensory feedback from the legs to the CPGs are doing it automatically based on environmental conditions.
For more information on the fascinating work being done in Standen’s lab regarding evolutionary and comparative biomechanics visit her website here.