Octopuses have a complex motor control problem. The octopus has eight muscular arms, each of which is a hydrostat. This soft body structure does not have a rigid frame and can move with almost infinite degrees of flexibility. The arms also contain hundreds of suckers that can be shaped independently. Octopuses can control their behavior along the entire length of an arm as well as across eight arms. They also have the ability to switch between sucker movements. Scientists at the University of Chicago have discovered that the circuitry of the nervous system that controls the arm movements of octopuses has been segmented. This gives these creatures the ability to control their movement across arms and suckers in order to grasp objects and catch prey.
A octopus is seen at the USC Wrigley Marine Science Center, located on Catalina Island. University of Southern California Image Credit. Clifton Ragsdale, University of Chicago Professor said that if you want a system to control such dynamic movements then this is a great way to do it. The octopus arms have a large nervous system with an incredible number of neurons. This is more than the brain. The neurons in the axial cord (ANC) are condensed into a single large nerve that snakes up and down each arm. Each bend forms an expansion over every sucker. Researchers wanted to examine the structure and connections between the ANC, the musculature of California’s two-spot octopus arms (Octopus Bimaculoides), a species that is native to California. The researchers were attempting to examine thin circular cross sections of the arms using a microscope but samples would keep falling off. The researchers tried lengthwise stripes of the arms, and they had more success. This led them to make an unexpected discovery. Using cellular markings and imaging tools, the researchers traced the structure and connectivity of the ANC. They found that the neuronal cells were packed in columns, forming segments like a corrugated tube. The septa are gaps between these segments, through which nerves and vessels leave to reach nearby muscles. The nerves of multiple segments are connected to muscles in different areas, which suggests that the segments control each other’s movements. Cassady Ollson, an undergraduate student at University of Chicago, said that the most effective way to create a control system to manage this long and flexible arm is to break it down into segments. There must be some communication between segments. This, you would imagine, will help to smoothen out movements. The nervous system creates a topographical map or spatial representation of the suckers. Octopuses are able to move their suckers and alter the shape independently. They also have sensory receptors in the suckers that enable them to smell and taste things they touch, like having a combination of a tongue with a nose. Researchers believe that the suckeroptopy (as they named the map) facilitates the complex sensory-motor abilities. Researchers studied the soft-bodied cephalopods Doryteuthis Pealeii (Longfin Inshore Squid) that are found in the Atlantic Ocean to see if they share this structure. The squid has eight arms, each with muscle and suckers. They also have two tentacles. The tentacles are long and have no suckers. However, the club on their end has suckers. The squid uses the suckers on the clubs to grab the prey while hunting. The scientists used the same method to examine long strips of squid’s tentacles. They found that the ANC on the stalks without suckers is not segmented. However, the club at the ends are. The scientists concluded that ANCs with segments are specifically designed to control cephalopods’ dexterous appendages. The suckers on the tentacles of squids have less segments, probably because squids don’t use them for feeling the way that octopuses would. Squid hunt more in open water using their eyesight, while octopuses explore the ocean bottom with their delicate arms. The similarities in their ability to control some of the appendages using suckers, and the differences between the other parts show that evolution is always able to come up with the right solution. Professor Ragsdale explained that organisms with sucker-laden, wormlike appendages need the correct nervous system.
Different cephalopods evolved segmental structures, whose details vary depending on the environment and pressures from millions of years evolution.
This study has been published in Nature Communications.
_____
C.S. Olson et al. 2025. Nat Commun16, 443; doi: 10.1038/s41467-024-55475-5.Nat Commun 16, 443; doi: 10.1038/s41467-024-55475-5