The octopus has a motor control challenge of enormous complexity. Each of its eight arms is a muscular hydrostat, a soft-bodied structure that lacks a rigid skeleton and moves with near infinite degrees of freedom.
Moreover, the arms are packed with hundreds of suckers which can change shape independently. Even with this complexity, octopuses control behaviors effectively along the length of a single arm, across all eight arms and between suckers.
In new research, scientists at the University of Chicago found that the nervous system circuitry that controls arm movement in octopuses is segmented, giving these extraordinary creatures precise control across their arms and suckers to explore their environment, grasp objects, and capture prey.
“If you’re going to have a nervous system that’s controlling such dynamic movement, that’s a good way to set it up,” said University of Chicago’s Professor Clifton Ragsdale.
“We think it’s a feature that specifically evolved in soft-bodied cephalopods with suckers to carry out these worm-like movements.”
Each octopus arm has a massive nervous system, with more neurons combined across the eight arms than in the animal’s brain.
These neurons are concentrated in a large axial nerve cord (ANC), which snakes back and forth as it travels down the arm, every bend forming an enlargement over each sucker.
The study authors wanted to analyze the structure of the ANC and its connections to musculature in the arms of the California two-spot octopus (Octopus bimaculoides), a small species native to the Pacific Ocean off the coast of California.
They were trying to look at thin, circular cross-sections of the arms under a microscope, but the samples kept falling off the slides.
They tried lengthwise strips of the arms and had better luck, which led to an unexpected discovery.
Using cellular markers and imaging tools to trace the structure and connections from the ANC, they saw that neuronal cell bodies were packed into columns that formed segments, like a corrugated pipe.
These segments are separated by gaps called septa, where nerves and blood vessels exit to nearby muscles.
Nerves from multiple segments connect to different regions of muscles, suggesting the segments work together to control movement.
“Thinking about this from a modeling perspective, the best way to set up a control system for this very long, flexible arm would be to divide it into segments,” said Cassady Olson, a graduate student at the University of Chicago.
“There has to be some sort of communication between the segments, which you can imagine would help smooth out the movements.”
Nerves for the suckers also exited from the ANC through these septa, systematically connecting to the outer edge of each sucker.
This indicates that the nervous system sets up a spatial, or topographical, map of each sucker.
Octopuses can move and change the shape of their suckers independently.
The suckers are also packed with sensory receptors that allow the octopus to taste and smell things that they touch — like combining a hand with a tongue and a nose.
The researchers believe the suckeroptopy, as they called the map, facilitates this complex sensory-motor ability.
To see if this kind of structure is common to other soft-bodied cephalopods, the researchers also studied the longfin inshore squid (Doryteuthis pealeii), which are common in the Atlantic Ocean.
These squid have eight arms with muscles and suckers like an octopus, plus two tentacles.
The tentacles have a long stalk with no suckers, with a club at the end that does have suckers.
While hunting, the squid can shoot the tentacles out and grab prey with the sucker-equipped clubs.
Using the same process to study long strips of the squid tentacles, the scientists saw that the ANC in the stalks with no suckers are not segmented, but the clubs at the end are segmented the same way as the octopus.
This suggests that a segmented ANC is specifically built for controlling any type of dexterous, sucker-laden appendage in cephalopods.
The squid tentacle clubs have fewer segments per sucker, however, likely because they do not use the suckers for sensation the same way octopuses do.
Squid rely more on their vision to hunt in the open water, whereas octopuses prowl the ocean floor and use their sensitive arms as tools for exploration.
While octopuses and squid diverged from each other more than 270 million years ago, the commonalities in how they control parts of their appendages with suckers — and differences in the parts that don’t — show how evolution always manages to find the best solution.
“Organisms with these sucker-laden appendages that have worm-like movements need the right kind of nervous system,” Professor Ragsdale said.
“Different cephalopods have come up with a segmental structure, the details of which vary according to the demands of their environments and the pressures of hundreds of millions of years of evolution.”
The study was published in the journal Nature Communications.
Citation:
C.S. Olson et al. 2025. Neuronal segmentation in cephalopod arms. Nat Commun 16, 443; doi: 10.1038/s41467-024-55475-5
This article was first published by Sci-News on 15 January 2025. Lead Image: An octopus at the USC Wrigley Marine Science Center on Catalina Island. Image credit: University of Southern California.
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