Recent research has revealed that the nervous system setup responsible for arm movements in octopuses is divided into segments. This segmentation enables these extraordinary creatures to skillfully maneuver their eight arms and many suckers, allowing them to explore their environment, gather objects, and catch prey with great precision.
Octopuses demonstrate incredible flexibility in their arm movements, which can bend, twist, and curl in seemingly endless ways. New studies from the University of Chicago show that the circuitry within their nervous system is segmented, giving octopuses the ability to control each of their eight arms and their numerous suckers accurately while they hunt and navigate through their surroundings.
“A segmented nervous system is perfect for controlling such fluid motions,” remarked Clifton Ragsdale, PhD, a Professor of Neurobiology at UChicago and the lead author of the research. “This design likely evolved specifically in soft-bodied cephalopods with suckers to enhance their worm-like movements.”
The research titled “Neuronal segmentation in cephalopod arms” was published on January 15, 2025, in Nature Communications.
Each of an octopus’s arms contains more neurons than the entire brain of the animal. These neurons are organized in a lengthy axial nerve cord (ANC) that twists down each arm, with each curve causing a swell at each sucker.
Cassady Olson, a graduate student focusing on Computational Neuroscience and the principal investigator, set out to study the ANC’s structure and its muscle connections in the California two-spot octopus (Octopus bimaculoides), a small species indigenous to the Pacific Coast of California. While trying to analyze thin circular cross-sections of the arm under a microscope, Olson and her co-author Grace Schulz, who studies Development, Regeneration, and Stem Cell Biology, faced difficulties keeping the samples on their slides. By switching to lengthwise strips of the arms, they achieved better results, leading to an unexpected discovery.
Using cellular markers and imaging techniques, the researchers traced the structural pathways of the ANC and realized that the neuronal cell bodies were tightly grouped in columns, forming segments similar to a corrugated tube. These segments are separated by gaps called septa, allowing nerves and blood vessels to connect with nearby muscles. Additionally, nerves from different segments link to various muscle regions, suggesting a cooperative mechanism for movement.
“Modeling suggests that segmenting the control system for a long, flexible arm is the most logical approach,” Olson explained. “Likely, there is some form of communication between the segments that helps smooth out the motions.”
Nerves that emerge from the ANC also exit through these septa, forming systematic connections to the outer edges of each sucker. This arrangement indicates a spatial or topographical mapping of each sucker within the nervous system. Octopuses can flexibly control and change their suckers’ shapes, which are rich in sensory receptors that allow them to taste and smell items they come into contact with, effectively integrating tactile, gustatory, and olfactory sensations. Researchers refer to this mapping as “suckeroptopy,” believing it enhances the octopus’s intricate sensory-motor capabilities.
To investigate whether this segmented structure is present in other soft-bodied cephalopods, Olson examined longfin inshore squid (Doryteuthis pealeii), commonly found in the Atlantic Ocean. These squid have eight arms similar to those of octopuses, in addition to two tentacles. The tentacles are long and lack suckers but have clubs at their ends equipped with suckers. When hunting, squids can extend these tentacles to catch prey using the sucker-covered clubs.
By applying the same analysis approach to long strips of squid tentacles, Olson discovered that while the ANC in the non-sucker stalks was unsegmented, the clubs did exhibit segmentation similar to octopuses. This finding suggests that a segmented ANC is tailored for controlling any agile appendage with suckers in cephalopods. However, squid tentacle clubs have fewer segments per sucker, likely because they do not rely on suckers for sensory detection in the same way that octopuses do. Squids mainly depend on their eyesight while hunting in open water, whereas octopuses explore the ocean floor using their sensitive limbs.
Even though octopuses and squids diverged over 270 million years ago, their similar control mechanisms for sucker-loaded appendages, along with the differences in their non-sucker components, reveal the evolutionary process’s knack for developing optimal solutions.
“Creatures with appendages packed with suckers that perform worm-like movements require the right kind of nervous system,” Ragsdale explained. “Different cephalopods have evolved segmental structures, with variations tailored to their environmental adaptations and shaped by evolutionary forces over millions of years.”
