Extra-sensory perception The nine-armed octopus and the oddities of the cephalopod nervous system A mix of autonomous and top-down control manage the octopus's limbs. Kenna Hughes-Castleberry – Jun 7, 2025 8:00 am | 19 Credit: Nikos Stavrinidis / 500px Credit: Nikos Stavrinidis / 500px Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only   Learn more With their quick-change camouflage and high level of intelligence, it’s not surprising that the public and scientific experts alike are fascinated by octopuses. Their abilities to recognize faces, solve puzzles, and learn behaviors from other octopuses make these animals a captivating study. To perform these processes and others, like crawling or exploring, octopuses rely on their complex nervous system, one that has become a focus for neuroscientists. With about 500 million neurons—around the same number as dogs—octopuses’ nervous systems are the most complex of any invertebrate. But, unlike vertebrate organisms, the octopus’s nervous system is also decentralized, with around 350 million neurons, or 66 percent of it, located in its eight arms. “This means each arm is capable of independently processing sensory input, initiating movement, and even executing complex behaviors—without direct instructions from the brain,” explains Galit Pelled, a professor of Mechanical Engineering, Radiology, and Neuroscience at Michigan State University who studies octopus neuroscience. “In essence, the arms have their own ‘mini-brains.’” A decentralized nervous system is one factor that helps octopuses adapt to changes, such as injury or predation, as seen in the case of an Octopus vulgaris, or common octopus, that was observed with nine arms by researchers at the ECOBAR lab at the Institute of Marine Research in Spain between 2021 and 2022. By studying outliers like this cephalopod, researchers can gain insight into how the animal’s detailed scaffolding of nerves changes and regrows over time, uncovering more about how octopuses have evolved over millennia in our oceans. Brains, brains, and more brains Because each arm of an octopus contains its own bundle of neurons, the limbs can operate semi-independently from the central brain, enabling faster responses since signals don’t always need to travel back and forth between the brain and the arms. In fact, Pelled and her team recently discovered that “neural signals recorded in the octopus arm can predict movement type within 100 milliseconds of stimulation, without central brain involvement.” She notes that “that level of localized autonomy is unprecedented in vertebrate systems.” Though each limb moves on its own, the movements of the octopus’s body are smooth and conducted with a coordinated elegance that allows the animal to exhibit some of the broadest range of behaviors, adapting on the fly to changes in its surroundings. “That means the octopus can react quickly to its environment, especially when exploring, hunting, or defending itself,” Pelled says. “For example, one arm can grab food while another is feeling around a rock, without needing permission from the brain. This setup also makes the octopus more resilient. If one arm is injured, the others still work just fine. And because so much decision-making happens at the arms, the central brain is freed up to focus on the bigger picture—like navigating or learning new tasks.” As if each limb weren’t already buzzing with neural activity, things get even more intricate when researchers zoom in further—to the nerves within each individual sucker, a ring of muscular tissue, which octopuses use to sense and taste their surroundings. “There is a sucker ganglion, or nerve center, located in the stalk of every sucker. For some species of octopuses, that’s over a thousand ganglia,” says Cassady Olson, a graduate student at the University of Chicago who works with Cliff Ragsdale, a leading expert in octopus neuroscience. Given that each sucker has its own nerve centers—connected by a long axial nerve cord running down the limb—and each arm has hundreds of suckers, things get complicated very quickly, as researchers have historically struggled to study this peripheral nervous system, as it’s called, within the octopus’s body. “The large size of the brain makes it both really exciting to study and really challenging,” says Z. Yan Wang, an assistant professor of biology and psychology at the University of Washington. “Many of the tools available for neuroscience have to be adjusted or customized specifically for octopuses and other cephalopods because of their unique body plans.” While each limb acts independently, signals are transmitted back to the octopus’s central nervous system. The octopus’ brain sits between its eyes at the front of its mantle, or head, couched between its two optic lobes, large bean-shaped neural organs that help octopuses see the world around them. These optic lobes are just two of the over 30 lobes experts study within the animal’s centralized brain, as each lobe helps the octopus process its environment. This elaborate neural architecture is critical given the octopus’s dual role in the ecosystem as both predator and prey. Without natural defenses like a hard shell, octopuses have evolved a highly adaptable nervous system that allows them to rapidly process information and adjust as needed, helping their chances of survival. Some similarities remain While the octopus’s decentralized nervous system makes it a unique evolutionary example, it does have some structures similar to or analogous to the human nervous system. “The octopus has a central brain mass located between its eyes, and an axial nerve cord running down each arm (similar to a spinal cord),” says Wang. “The octopus has many sensory systems that we are familiar with, such as vision, touch (somatosensation), chemosensation, and gravity sensing.” Neuroscientists have homed in on these similarities to understand how these structures may have evolved across the different branches in the tree of life. As the most recent common ancestor for humans and octopuses lived around 750 million years ago, experts believe that many similarities, from similar camera-like eyes to maps of neural activities, evolved separately in a process known as convergent evolution. While these similarities shed light on evolution's independent paths, they also offer valuable insights for fields like soft robotics and regenerative medicine. Occasionally, unique individuals—like an octopus with an unexpected number of limbs—can provide even deeper clues into how this remarkable nervous system functions and adapts. Nine arms, no problem In 2021, researchers from the Institute of Marine Research in Spain used an underwater camera to follow a male Octopus vulgaris, or common octopus. On its left side, three arms were intact, while the others were reduced to uneven, stumpy lengths, sharply bitten off at varying points. Although the researchers didn’t witness the injury itself, they observed that the front right arm—known as R1—was regenerating unusually, splitting into two separate limbs and giving the octopus a total of nine arms. “In this individual, we believe this condition was a result of abnormal regeneration [a genetic mutation] after an encounter with a predator,” explains Sam Soule, one of the researchers and the first author on the corresponding paper recently published in Animals. The researchers named the octopus Salvador due to its bifurcated arm coiling up on itself like the two upturned ends of Salvador Dali’s moustache. For two years, the team studied the cephalopod’s behavior and found that it used its bifurcated arm less when doing “riskier” movements such as exploring or grabbing food, which would force the animal to stretch its arm out and expose it to further injury. “One of the conclusions of our research is that the octopus likely retains a long-term memory of the original injury, as it tends to use the bifurcated arms for less risky tasks compared to the others,” elaborates Jorge Hernández Urcera, a lead author of the study. “This idea of lasting memory brought to mind Dalí’s famous painting The Persistence of Memory, which ultimately became the title of the paper we published on monitoring this particular octopus.” While the octopus acted more protective of its extra limb, its nervous system had adapted to using the extra appendage, as the octopus was observed, after some time recovering from its injuries, using its ninth arm for probing its environment. “That nine-armed octopus is a perfect example of just how adaptable these animals are,” Pelled adds. “Most animals would struggle with an unusual body part, but not the octopus. In this case, the octopus had a bifurcated (split) arm and still used it effectively, just like any other arm. That tells us the nervous system didn’t treat it as a mistake—it figured out how to make it work.” Kenna Hughes-Castleberry is the science communicator at JILA (a joint physics research institute between the National Institute of Standards and Technology and the University of Colorado Boulder) and a freelance science journalist. Her main writing focuses are quantum physics, quantum technology, deep technology, social media, and the diversity of people in these fields, particularly women and people from minority ethnic and racial groups. Follow her on LinkedIn or visit her website. 19 Comments

During an excavation, amidst the Patagonian winds and hard rock, a fossil began to turn green. It was an unexpected reaction: the adhesive applied to protect the bones, fragile after millions of years beneath the ice, had interacted with plant matter trapped in the rock’s cracks. This greenish hue earned the fossil the nickname Fiona, like the ogre from Shrek.But Fionais much more than a ogre-themed name. It is the first complete ichthyosaur ever excavated in Chile and, even more remarkably, the only known pregnant female from the Hauterivian — a stage of the Early Cretaceous dating back 131 million years. Her skeleton, discovered at the edge of the Tyndall Glacier in Torres del Paine National Park — an area increasingly exposed by glacial retreat — belongs to the species Myobradypterygius hauthali, originally described in Argentina from fragmentary remains.The discovery, led by Judith Pardo-Pérez, a researcher at the University of Magallanes and the Cabo de Hornos International Center (CHIC), and published in the Journal of Vertebrate Paleontology, offers an unprecedented glimpse into ancient marine life — from how these majestic reptiles reproduced to how they adapted to oceans vastly different from those of today.An Ichthyosaur Maternity Ward in Patagonia(Image Courtesy of Irene Viscor)So far, 88 ichthyosaurs have been found on the Tyndall Glacier. Most of them are adults and newborns. Two key facts stand out: food was abundant, and no other predators were competing with them.Fiona, who measures nearly 13 feet long, is still encased in five blocks of rock. Despite the challenge, she was transported to a local clinic, where CT scans allowed researchers to study her skull and body. Her species was identified thanks to one of her fins. “There’s no other like it in the world,” says Pardo-Pérez. The limbs were remarkably elongated, suggesting this animal was built for long-distance swimming.Inside her, there were more surprises. One of them was her stomach contents, which revealed what may have been her last meal: tiny fish vertebrae. But the most striking find was a fetus, about 20 inches long, already in a position to be born.“We believe these animals came to Magallanes — the southern tip of Chilean Patagonia — from time to time to give birth, because it was a safe refuge,” Pardo-Pérez says. “We don't know how long they stayed, but we do know that mortality was high during the first few days of life.”One of the big unanswered questions is where they went next, as there are no records of Myobradypterygius hauthali, apart from a piece of fin found in Argentina. The most abundant remains come from southern Germany, but those date back to the Jurassic period, meaning they’re older.Palaeontologist Erin Maxwell suggests, “In many modern ecosystems, species migrate to higher latitudes during the summer to take advantage of seasonally abundant resources and then move to lower latitudes in winter to avoid harsh conditions,” she explains. “We believe Mesozoic marine reptiles may have followed similar seasonal patterns.”Sea Dragon GraveyardThe environment where Fiona was discovered — dubbed the "sea dragon graveyard" — also has much to reveal.According to geologist Matthew Malkowski of the University of Texas at Austin, the Hauterivian age is particularly intriguing because it coincided with major planetary changes: the breakup of continents, intense volcanic episodes, and phenomena known as "oceanic anoxic events," during which vast areas of the ocean were depleted of dissolved oxygen for hundreds of thousands of years.One such poorly understood event, the Pharaonic Anoxic Event, occurred around 131 million years ago, near the end of the Hauterivian, and still raises questions about its true impact on marine life. “We don't have a firm grasp of how significant these events were for marine vertebrates, and geological records like that of the Tyndall Glacier allow us to explore the relationship between life, the environment, and Earth’s past conditions,” Malkowski notes.Evolution of IchthyosaursReconstruction of Fiona. (Image Courtesy of Mauricio Álvarez)Don't be misled by their body shape. “Ichthyosaurs are not related to dolphins,” clarifies Pardo-Pérez. Although their hydrodynamic silhouettes may look nearly identical, the former were marine reptiles, while the latter are mammals. This resemblance results from a phenomenon known as convergent evolution: when species from different lineages develop similar anatomical features to adapt to the same environment.Ichthyosaurs evolved from terrestrial reptiles that, in response to ecological and climatic changes, began spending more time in the water until they fully adapted to a marine lifestyle. However, they retained traces of their land-dwelling ancestry, such as a pair of hind flippers — absent in dolphins — passed down from their walking forebears. They lived and thrived in prehistoric oceans for about 180 million years, giving them ample time to refine a highly specialized body: their forelimbs and hindlimbs transformed into flippers; they developed a crescent-shaped tail for propulsion, a dorsal fin for stability, and a streamlined body to reduce drag in the water. Remarkably, like whales and dolphins, “ichthyosaurs had a thick layer of blubber as insulation to maintain a higher body temperature than the surrounding seawater and gave birth to live young, which meant they didn’t need to leave the water to reproduce,” explains Maxwell.Whales and dolphins also descend from land-dwelling ancestors, but their transition happened over a comparatively short evolutionary timespan, especially when measured against the long reign of the ichthyosaurs. “Their evolution hasn't had as much time as that of ichthyosaurs,” notes Pardo-Pérez. “And yet, they look so similar. That’s the wonderful thing about evolution.”Read More: Did a Swimming Reptile Predate the Dinosaurs?Fossils on the Verge of DisappearanceOne of the key factors behind the remarkable preservation of the fossils found in the Tyndall Glacier is the way they were buried. According to Malkowski, Fiona and her contemporaries were either trapped or swiftly covered by underwater landslides and turbidity currents — geological processes that led to their sudden entombment.But the good fortune that protected them for millions of years may now be running out. As the glacier retreats, exposing fossils that were once unreachable, those same remains are now vulnerable to wind, rain, and freeze-thaw cycles, which crack the surrounding rock. As vegetation takes hold, roots accelerate erosion and eventually conceal the fossils once again.“While climate change has allowed these fossils to be studied, continued warming will also eventually lead to their loss,” Maxwell warns. In Fiona’s story, scientists find not only a record of ancient life, but also a warning etched in stone and bone: what time reveals, climate can reclaim.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:María de los Ángeles Orfila is a science journalist based in Montevideo, Uruguay, focusing on long-form storytelling. Her work has appeared in Discover Magazine, Science, National Geographic, among other outlets, and in leading Uruguayan publications such as El País and El Observador. She was a fellow in the 2023 Sharon Dunwoody Mentoring Program by The Open Notebook and often explores the intersections of science, culture, and Latin American identity.