Elongated, eel-like fish have fascinated biologists for decades due to their extraordinary locomotor capabilities. Unlike many vertebrates, eels and their relatives demonstrate not only graceful swimming but also the remarkable ability to crawl over uneven terrain. Even more astonishing is their capacity to continue coordinated movement after severe spinal cord injuries that would cause paralysis in most other animals. Despite these intriguing observations, the underlying neural mechanisms that enable such resilient and versatile movement have remained largely elusive to scientists. This gap in understanding is largely due to the complexity of studying how multiple sensory feedback systems work in tandem within these animals.
Recent investigations have centered on the role of central pattern generators (CPGs), specialized neural networks distributed along the spinal cord which generate rhythmic motor patterns fundamental to locomotion. Prior research hypothesized that sensory inputs from skin pressure and muscle stretch feedback influence these CPGs, tuning their activity to allow adaptive, undulating movement. However, the sheer intricacy of these multisensory interactions has made experimental studies in living eels extraordinarily challenging, limiting mechanistic insights into the modulation and integration of sensory signals by the spinal neural circuitry.
A groundbreaking multidisciplinary team, featuring experts from EPFL’s School of Engineering, Tohoku University in Japan, and the University of Ottawa in Canada, has now addressed this puzzle by developing an advanced mathematical model that integrates both stretch and pressure sensory feedback mechanisms within the eel’s motor control circuit. Published in the prestigious journal Proceedings of the National Academy of Sciences, this model conceptualizes each body segment of the eel as housing a CPG-like neural circuit capable of autonomously generating rhythmic movements that are finely regulated by multisensory input. This approach represents a critical step forward in linking peripheral sensory information with localized neural network dynamics to explain the robust motor behaviors witnessed in eels.
To validate their theoretical framework, the researchers engaged in innovative experiments using amphibious eel-like robots engineered in EPFL’s BioRobotics Laboratory. These biomimetic robots replicate the anguilliform undulations and flexible body mechanics characteristic of real eels, permitting controlled testing of the computational model’s predictions. In aquatic trials, the simulations confirmed rapid emergence of stable swimming patterns, with stretch feedback identified as the paramount sensory influence stabilizing locomotion swiftly after movement initiation. This finding underscores the importance of proprioceptive feedback in tuning the rhythmic outputs of spinal circuits during dynamic aquatic environments.
Remarkably, the same neural control circuitry that orchestrates swimming also facilitated terrestrial crawling behaviors when the robotic model was tested on land. The amphibious robot successfully navigated complex terrain and obstacles, relying heavily on stretch feedback to generate sufficient thrust by pushing against irregular surfaces. This cross-environment versatility highlights a shared neural architecture supporting fundamentally different modes of locomotion without necessitating distinct circuits for swimming versus crawling. It suggests that the evolutionary leap from aquatic to terrestrial movement in vertebrates could have involved repurposing pre-existing neural modules rather than evolving entirely novel control systems.
Auke Ijspeert, head of the BioRobotics Lab at EPFL, reflects on the broader evolutionary implications: the discovery that a single kind of neural circuit can underlie both swimming and crawling challenges the traditional notion that terrestrial locomotion required radically new neural substrates. Instead, ancestral aquatic CPG networks might have been co-opted and adapted for land-based motor control, illuminating a possible neurobiological pathway underpinning one of the most profound transitions in vertebrate evolution. This insight enriches our understanding of how complex motor behaviors can evolve through modification of existing physiological mechanisms.
The team further leveraged their model and robotic platform to investigate the remarkable resilience of eel locomotion following spinal cord transection — a severe injury severing the nerve pathways typically essential for movement coordination. Their simulations revealed that distributed neural circuits possessing inherent spontaneous rhythmic activity, when coupled with continual stretch and pressure feedback, can sustain coordinated swimming even when communication across the spinal cord is interrupted. This decentralized control paradigm contrasts sharply with the brain-dependent locomotion observed in many vertebrates and sheds light on potential intrinsic recovery mechanisms that allow eels to circumvent paralysis.
From an engineering perspective, the study’s implications extend beyond biological curiosity, pointing toward the development of a new generation of resilient robots capable of navigating unpredictable and hazardous environments. Robots equipped with decentralized motor control systems inspired by eel spinal circuits and multisensory integration could maintain movement despite partial damage or sensory loss. This robustness is particularly valuable for search and rescue operations in disaster zones, where complex terrains and mechanical damage often hamper traditional robotic platforms. Incorporating multisensory feedback loops, as demonstrated in this research, establishes a blueprint for autonomous machines that are not only mechanically versatile but also neurologically adaptable.
Importantly, this research also holds promise for biomedical advances, especially concerning spinal cord injuries. Understanding how eels accomplish movement without a fully intact spinal cord challenges prevailing views about neural dependence on centralized control from the brain. It invites new avenues in designing neuroprosthetics and rehabilitation protocols that emphasize restoring or mimicking decentralized sensory-motor integration, perhaps enabling patients with paralysis to regain some degree of autonomous movement. The insights gained from eel locomotion offer a compelling biological model for engineering decentralized, brain-independent motor function.
These findings herald a paradigm shift in both neuroscience and robotics, emphasizing the profound capacity of distributed neural circuits and sensory feedback to generate complex, adaptive motor behaviors. By bridging theoretical modeling with robotics and biological validation, this research exemplifies the power of interdisciplinary investigation to unravel longstanding mysteries of animal movement. It sets the stage for future studies to further dissect how specific sensory modalities interact with neural networks and to refine robotic designs that harness these principles for real-world applications.
Ultimately, the work from EPFL’s BioRobotics Laboratory and collaborators not only deepens our fundamental understanding of vertebrate motor control but also inspires innovative technological solutions. The integration of multisensory feedback within decentralized neural architectures offers a scalable, resilient framework adaptable to diverse domains—from soft robotics and bioinspired engineering to neurorehabilitation. As research proceeds, the translation of these insights into functional robotic systems and medical interventions will continue to push the boundaries of what autonomous machines and human patients alike can achieve.
This convergence of biology, mathematics, and engineering epitomizes the forefront of modern science. It highlights how investigating the nuanced mechanics of an humble eel’s movement can ripple outward, influencing fields as varied as evolutionary biology, robotics design, and clinical neuroscience. The ability to mimic such elegant natural control systems in synthetic devices promises not only robust and versatile robots but also opens pathways to understanding life’s most complex motor functions.
Subject of Research: Neural control circuits underlying locomotion in elongate fish, specifically the integration of multisensory feedback in central pattern generators enabling robust swimming and terrestrial crawling.
Article Title: Multisensory feedback makes swimming circuits robust against spinal transection and enables terrestrial crawling in elongate fish
News Publication Date: 18-Aug-2025
Web References:
https://www.pnas.org/doi/10.1073/pnas.2422248122
http://dx.doi.org/10.1073/pnas.2422248122
Image Credits: The EPFL BioRob Lab’s amphibious eel-like robot. 2025 BIOROB EPFL CC BY SA
Keywords: Biomechanics, Robotic designs, Vertebrates, Evolutionary biology, Central nervous system
