Octopuses and the Complexity of "Eight-Arm" Coordination

Octopuses are invertebrate animals belonging to the Class Cephalopoda, Order Octopoda, which stand out due to their remarkable intelligence and the extensive use of their eight arms for various complex and often autonomous functions. Possessing the most developed nervous system among invertebrates, these animals have nearly 500 million neurons divided into three main structures: the central brain, the optic lobe, and the neural network distributed throughout the arms (Carls-Diamante, 2022).

Unlike vertebrate animals, which possess a spinal cord in the center of the body and nerve fibers extending through the limbs, octopuses have about three-fifths of their neurons distributed across their arms (Carls-Diamante, 2022). These neurons cluster into highly developed ganglia that, in turn, connect to the brain via a nerve cord running through the center of each appendage. Additionally, fibrous cords connect one arm to another, forming a sort of "nerve ring" (Hale, 2025).

Given the existence of such a highly developed nervous system in each limb, a question arises: does each arm act independently, or do they all depend on commands originating from the central nervous system?

According to Gutnick (2011), the octopus brain performs a coordinating and locomotor function for the body, as observed in studies using a glass tank where the animal had to use vision to guide one of its arms toward food without the aid of chemical sensory information present in the water. The successful attempts of these animals confirmed the existence of central communication with the limbs. However, generally speaking, there is no clear evidence of an organized somatotopic map in their central nervous system like that observed in vertebrates. Stimuli generated in different regions of the basal brain can trigger reactions in all limbs, and it has not been possible to elicit similar responses in a single, specific limb. These discoveries suggest a global motor reaction by the brain, making the participation of the peripheral nervous system necessary to utilize a single arm for a specific task (Carls-Diamante, 2022).

Figure 1. Octopus vulgaris using one of its arms to reach food while being guided by its vision (Gutnick et al., 2011).

Each of the eight appendages thus demonstrates great autonomy in its movements, being capable of executing local motor responses even when isolated from central control, as has been observed in experiments. Among the noted actions, prominent examples include the gripping movement performed by arms opposite to one being pulled, as well as the recruitment of neighboring arms to investigate a stimulus applied to one of them (Hale, 2025). Furthermore, the large number of ganglia present in the limbs—and individually in each sucker—endows each one with the ability to directly process captured sensory information. This intimately links the touch and taste of these animals due to the presence of mechano- and chemoreceptors, particularly chemotactile receptors (CRs). These receptors are capable of detecting chemical compounds dissolved in water and other substances, through which octopuses can determine whether what they are holding is suitable food, without relying on vision or any other sensory organ (van Giesen et al., 2020).

In relation to the adaptations and functions of each limb, there is also the so-called "hectocotylus," the name given to the distal portion of the third right arm. Its anatomical modifications and absence of suckers allow this arm to be used for the transfer of spermatophores during reproduction, and in some species, it can detach from the rest of the male's body (Wells, 2013). This unique anatomy, combined with their vast cognitive capacity, enables the octopus to perform surprising feats with its intelligent "arms."

Thus, there are numerous records of object manipulation by these animals, both as a form of play with no apparent motive, and as tool use to modify the environment to their advantage (Godfrey-Smith, 2016). For example, the use of stones and shells by these cephalopods has been observed on several occasions, handling and moving them to construct or modify a shelter (Mather, 1994). Even human-made objects can be utilized by them. There are records of octopuses carrying coconut shells found adrift in the ocean over long distances. They rest on the concave surface and use some arms to keep it suspended while using the remaining ones to walk along the seafloor (Figure 2). This practice leaves them vulnerable during the walk, but the potential of the new object seems to outweigh the temporary risks of relocating it (Finn; Tregenza; Norman, 2009).

Figure 2. Amphioctopus marginatus walking on the seafloor while carrying two stacked coconut shells (Finn; Tregenza; Norman, 2009).

It is also worth mentioning an interesting ability developed by some species of the Order Octopoda regarding the use of their arms in camouflage and escape processes. Several species, such as A. aculeatus and A. marginatus, developed the ability to incorporate bipedal walking into their locomotion, using two limbs to walk while using the others to resemble loose objects drifting along the ocean floor, such as rolling coconuts or seaweed swaying with the movement of the water. In this way, they are able to mislead predators and move away more quickly (Amodio et al., 2021).

The species Macrotritopus defilippi, for instance, possesses the notorious ability to not only camouflage itself to mimic other animals, but also to shape its arms and body to acquire a shape and swimming pattern similar to them (Fig. 3). These octopuses use this technique to hide in plain sight, allowing them to move away from danger instead of remaining motionless to maintain the camouflage, as occurs in other species (Hanlon; Watson; Barbosa, 2010).

Fig. 3. Macrotritopus defilippi mimicking a flounder (Hanlon; Watson; Barbosa, 2010).

It is clear, then, the immense complexity of these marine invertebrates, especially regarding the use of their "eight arms" to execute diverse activities—whether in obtaining food, reproducing, escaping predators, or simply demonstrating curiosity through play. The discovery of these characteristics makes them not only unique in the animal kingdom, but also an object of great fascination for researchers who, the more they learn about them, the more they realize how much we still do not know about these incredible animals.

References:

WELLS, M. J. Octopus: Physiology and Behaviour of an Advanced Invertebrate. Dordrecht: Springer, 2013. XIV, 417 p. 

CARLS-DIAMANTE, S. Where is it like to be an octopus? Frontiers in Systems Neuroscience, v. 16, article 840022, 2022. 

HALE, M. E. Octopus as a comparative model for understanding the neural control of limb movement and limb-based behaviors. Current Opinion in Neurobiology, Apr. 2025, v. 91:102982. Epub 21 Feb 2025. 

VAN GIESEN, L.; KILIAN, P. B.; ALLARD, C. A. H.; BELLONO, N. W. Molecular basis of chemotactile sensation in octopus. Cell, 2020, v. 183, n. 3, p. 594–604.e14, 29 Oct. 2020. 

MATHER, J. A. ‘Home’ choice and modification by juvenile Octopus vulgaris (Mollusca: Cephalopoda): specialized intelligence and tool use? Journal of Zoology, 1994, v. 233, p. 359–368.

FINN, J. K.; TREGANZA, T.; NORMAN, M. D. Defensive tool use in a coconut-carrying octopus. Current Biology, 2009, v. 19, n. 23, R1069–R1070, 15 Dec. 2009. 

GODFREY-SMITH, P. Other Minds: The Octopus, the Sea, and the Deep Origins of Consciousness. New York: Farrar, Straus and Giroux (William Collins / HarperCollins ed. in other countries), 2016. 272 p.

GUTNICK, T.; BYRNE, R. A.; HOCHNER, B.; KUBA, M. Octopus vulgaris uses visual information to determine the location of its arm. Current Biology, 2011, v. 21, n. 6, p. 460–462, 22 Mar. 2011. PMID: 21396818.

AMODIO, P.; JOSEF, N.; SHASHAR, N.; FIORITO, G. Bipedal locomotion in Octopus vulgaris: a complementary observation and some preliminary considerations. Ecology and Evolution, 2021, v. 11, n. 9, p. 3679–3684, 5 Mar. 2021. PMID: 33976767. PMCID: PMC8093653.

HANLON, R. T.; WATSON, A. C.; BARBOSA, A. A “mimic octopus” in the Atlantic: flatfish mimicry and camouflage by Macrotritopus defilippi. The Biological Bulletin, 2010, v. 218, n. 1, p. 15–24, Feb. 2010.