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

Great star corals in the grip of disease have been saved with probiotics — beneficial bacteria that attack or displace invading pathogens or possibly trigger immune responses to them. What’s causing this deadly disease remains unidentified. But researchers at the Smithsonian Marine Station in Fort Pierce, Fla., were able to successfully halt progression of the disease’s symptoms, the team reports June 5 in Frontiers in Marine Science. The condition is called stony coral tissue loss disease and is characterized by white lesions that lead to the loss of polyps — tiny soft-bodied organisms similar to sea anemones — blanketing coral. Eventually, nothing but the white coral skeleton is left behind. The disease emerged in Florida in 2014 and has spread rampantly throughout the Florida Keys and the Caribbean. A great star coral (M. cavernosa) colony is infected with stony coral tissue loss disease on the coral reef in Fort Lauderdale. The lesion, where the white band of tissue occurs, typically moves across the coral, killing coral tissue along the way. Kelly Pitts/Smithsonian Researchers suspect that the disease is bacterial in nature. Antibiotic treatments can offer a quick fix, but these drugs do not prevent reinfection and carry the risk of the mysterious pathogen building resistance against them. So, in late 2020, the Smithsonian group tried for a more sustainable solution, giving probiotics to 30 infected great star coral colonies. The helpful microbes came from corals tested in the lab that showed resistance to the disease. “We noticed that one of the coral fragments would not get infected … so one of the first things we did was try to culture the microbes that are on this coral,” says microbiologist Blake Ushijima, who developed the probiotic used in the team’s experiment. “These microbes produce antibacterial compounds … and one had a high level of activity against bacteria from diseased corals,” acting as a “pro” biotic, by somehow neutralizing pathogens. The identified microbe, a bacterium called McH1-7, became the active ingredient in a paste delivered by divers to several infected colonies. They covered these colonies with plastic bags to immerse them in the probiotic solution, injecting the paste into the bags using a syringe. They also applied the paste directly to other colonies, slathering lesions caused by the disease. A probiotic paste of McH1-7 is applied to the disease lesion of a great star coral (M. cavernosa) colony infected with stony coral tissue loss disease. The paste was then smoothed flat with a gloved hand so that all apparently infected tissue was covered by the lesion-specific treatment.Kelly Pitts/Smithsonian For two and a half years, the team monitored the corals’ health. The probiotics slowed or stopped the disease from spreading in all eight colonies treated inside bags. On average, the disease’s ugly advance was held to only 7 percent of tissue, compared with an aggressive 30 percent on untreated colonies. The paste put directly on the coral had no beneficial effect. The results are encouraging, but coauthor Valerie Paul cautions against declaring the probiotic a cure. She doubts the practicality of swimming around with heavily weighted plastic bags and putting them on corals. And, she points out, the study was limited to one species of coral, when the disease plagues over 30. Sponsor Message Still, Ushijima considers the study a proof of concept. “The idea of coral probiotics has been thrown around for decades, but no one has directly shown their effects on disease in the wild,” he says. “I think it’s very exciting because it’s actually opening the door to a new field.”

Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent every weekday. Probiotics are everywhere, claiming to help us poop, restore gut health, and more. They can also be used to help threatened coral reefs. A bacterial probiotic has helped slow the spread of stony coral tissue loss disease (SCTLD) in wild corals in Florida that were already infected with the disease. The findings are detailed in a study published June 5 in the journal Frontiers in Marine Science and show that applying this new probiotic treatment across coral colines helped prevent further tissue loss. What is stony coral tissue loss disease (SCTLD)? SCTLD first emerged in Florida in 2014. In the 11 years since, it has rapidly spread throughout the Caribbean. This mysterious ailment has been confirmed in at least 20 other countries and territories. Other coral pathogens typically target specific species. SCTLD infects more than 30 different species of stony corals, including pillar corals and brain corals. The disease causes the soft tissue in the corals to slough off, leaving behind white patches of exposed skeleton. The disease can devastate an entire coral colony in only a few weeks to months.  A great star coral (Montastraea cavernosa) colony infected with stony coral tissue lossdisease (SCTLD) on the coral reef in Fort Lauderdale, FL. The lesion, where the white band of tissue occurs, typically moves across the coral, killing coral tissue along the way. CREDIT: KellyPitts, Smithsonian. The exact cause of SCTLD is still unknown, but it appears to be linked to some kind of harmful bacteria. Currently, the most common treatment for SCTLD is using a paste that contains the antibiotic amoxicillin on diseased corals. However, antibiotics are not a silver bullet. This amoxicillin balm can temporarily halt SCTLD’s spread, but it needs to be frequently reapplied to the lesions on the corals. This takes time and resources, while increasing the likelihood that the microbes causing SCTLD might develop resistance to amoxicillin and related antibiotics. “Antibiotics do not stop future outbreaks,” Valerie Paul, a study co-author and the head scientist at the Smithsonian Marine Station at Fort Pierce, Florida, said in a statement. “The disease can quickly come back, even on the same coral colonies that have been treated.” Finding the right probiotic Paul and her colleagues have spent over six years investigating whether beneficial microorganisms (aka probiotics) could be a longer lasting alternative to combat this pathogen. Just like humans, corals are host to communities known as microbiomes that are bustling with all different types of bacteria. Some of these miniscule organisms produce antioxidants and vitamins that can help keep their coral hosts healthy.  First, the team looked at the microbiomes of corals that are impervious to SCTLD to try and harvest probiotics from these disease-resistant species. In theory, these could be used to strengthen the microbiomes of susceptible corals.  They tested over 200 strains of bacteria from disease-resistant corals and published a study in 2023 about the probiotic Pseudoalteromonas sp. McH1-7 (or McH1-7 for short). Taken from the great star coral (Montastraea cavernosa), this probiotic produces several antibacterial compounds. Having such a stacked antibacterial toolbox made McH1-7 an ideal candidate to combat a pathogen like SCTLD. They initially tested McH1-7 on live pieces of M. cavernosa and found that the probiotic reliably prevented the spread of SCTLD in the lab. After these successful lab tests, the wild ocean called next. Testing in the ocean The team conducted several field tests on a shallow reef near Fort Lauderdale, focusing on 40 M. cavernosa colonies that showed signs of SCTLD. Some of the corals in these colonies received a paste containing the probiotic McH1-7 that was applied directly to the disease lesions. They treated the other corals with a solution of seawater containing McH1-7 and covered them using weighted plastic bags. The probiotics were administered inside the bag in order to cover the entire coral colony.   “This created a little mini-aquarium that kept the probiotics around each coral colony,” Paul said. For two and a half years, they monitored the colonies, taking multiple rounds of tissue and mucus samples to see how the corals’ microbiomes were changing over time. They found that  the McH1-7 probiotic successfully slowed the spread of SCTLD when it was delivered to the entire colony using the bag and solution method. According to the samples, the probiotic was effective without dominating the corals’ natural microbes.  Kelly Pitts, a research technician with the Smithsonian Marine Station at Ft. Pierce, Floridaand co-lead author of the study treats great star coral (Montaststraea cavernosa) colonies infected with SCTLD with probiotic strain McH1-7 by covering the coral colony in a plastic bag, injecting a probiotic bacteria solution into the bag and leaving the bag for two hours to allow for the bacteria to colonize on the coral. CREDIT: Hunter Noren. Fighting nature with nature While using this probiotic appears to be an effective treatment for SCTLD among the reefs of northern Florida, additional work is needed to see how it could work in other regions. Similar tests on reefs in the Florida Keys have been conducted, with mixed preliminary results, likely due to regional differences in SCTLD. The team believes that probiotics still could become a crucial tool for combatting SCTLD across the Caribbean, especially as scientists fine tune how to administer them. Importantly, these beneficial bacteria support what corals already do naturally.  “Corals are naturally rich with bacteria and it’s not surprising that the bacterial composition is important for their health,” Paul said. “We’re trying to figure out which bacteria can make these vibrant microbiomes even stronger.”

A Deadly Disease Is Eating Away at Caribbean Corals and Wreaking Havoc on Reefs. Could Probiotics Be the Solution? New research suggests the probiotic McH1-7 could help stop the spread of stony coral tissue loss disease among wild corals near Fort Lauderdale, Florida Scientists determined the most effective method of halting the disease was covering a coral colony with a weighted plastic bag, then injecting a seawater solution that contains the probiotic. They left the colony covered for two hours to allow the probiotic bacteria to colonize the coral. Hunter Noren Probiotics can be good for human health. Now, new research suggests they might also help protect coral reefs. A bacterial probiotic helped slow the advance of stony coral tissue loss disease—a fast-spreading and deadly condition—among wild corals in Florida, researchers report today in a new study published in the journal Frontiers in Marine Science. The probiotic may be a good alternative to antibiotics like amoxicillin, which temporarily curb the spread of the disease but must be reapplied frequently. In addition, scientists fear stony coral tissue loss disease may one day become resistant to these antibiotic treatments—just as “superbugs” that infect humans are building resistance to our own drugs. Antibiotics are meant to kill microorganisms, but probiotics are beneficial living microbes. The idea is that a probiotic can be incorporated into corals’ natural microbiomes, ideally offering them longer-lasting protection. First discovered in Florida in 2014, stony coral tissue loss disease attacks the soft tissue of more than 30 different species of coral. Without treatment, the disease eventually kills the corals, and their soft tissue falls off, revealing the white calcium carbonate skeleton below. In just weeks or months, it can devastate a whole colony. Stony coral tissue loss disease can be spread by fish that eat coral, as well as by boaters and divers who do not disinfect their gear. The condition has since expanded its range beyond Florida to reefs throughout the Caribbean. Several years ago, researchers looking at the great star coral (Montastraea cavernosa) discovered a probiotic called Pseudoalteromonas sp. strain McH1-7. Laboratory tests showed McH1-7 stopped or slowed the progression of stony coral tissue loss disease in infected corals. It also helped prevent the disease from spreading to healthy corals. But that was in the lab. Would McH1-7 be similarly effective in the ocean? Researchers were eager to find out, so they set up an experiment on a shallow reef off the coast of Fort Lauderdale. Study co-author Kelly Pitts, a research technician with the Smithsonian Marine Station, applies a paste containing the probiotic directly onto the disease lesion of an infected coral. Hunter Noren Experimenting with wild corals For the study, the scientists focused on 40 great star coral colonies that were showing symptoms of stony coral tissue loss disease. In one experimental condition, the researchers made a paste that contained McH1-7 and applied it directly onto the disease lesions. For comparison, they also applied the same paste, minus the probiotic, to some corals. In another condition, they covered infected coral colonies with weighted plastic bags, then filled the bags with seawater solutions made with and without McH1-7. They left the corals covered for two hours. “This created a little mini-aquarium that kept the probiotics around each coral colony,” says study co-author Valerie Paul, head scientist at the Smithsonian Marine Station at Fort Pierce, Florida, in a statement. The scientists completed all the treatments within the first 4.5 months of the project. Then, they returned periodically to gather tissue and mucus samples from the corals to measure changes to their microbiomes. Over the next 2.5 years, they took photos from a variety of different angles, which they then used to create 3D models that could track the disease’s progression. In the end, the results suggest covering the corals with plastic bags filled with the probiotic seawater solution was the most effective method. More than two years post-treatment, the colonies that received the probiotic bag had lost just 7 percent of their tissue, while colonies in the control bag condition faced 35 percent tissue loss. Scientists applied a probiotic paste directly to disease lesions on some corals. Kelly Pitts The probiotic paste, by contrast, appears to have made the situation worse: The corals that had the probiotic paste applied directly to their lesions lost more tissue than those treated with the control paste, which did not contain McH1-7. “We do not really know what is going on with the probiotic paste treatment,” Paul tells Smithsonian magazine in an email. But she has a few theories. It’s possible the high concentrations of McH1-7 contributed to localized hypoxia, or low-oxygen conditions that further harmed the already stressed corals, she says. Or, the probiotic could have changed the microbiome at the lesion site in some negative way. Another possibility is that McH1-7 produces antibiotics or other substances that were harmful at high concentrations. Amanda Alker, a marine microbiologist at the University of Rhode Island who was not involved with the study, wonders if this finding suggests McH1-7 is beneficial at specific dosages—a question future laboratory research might be able to answer, she tells Smithsonian magazine in an email. She’s also curious to know which specific molecular components of the probiotic are responsible for the increased tissue loss when applied as a paste. More broadly, Alker would like to see additional experiments validating the bag treatment method, but she says this “inventive” technique seems promising. “Their approach is a safer solution than antibiotic treatment methods that have been deployed to combat [stony coral tissue loss disease] in the field so far,” she says. “Further, this is a practical solution that could be implemented widely because it doesn’t require highly specialized equipment and has the ability to be used with any type of microbial solution.” Looking ahead to save reefs Probiotics are likely not a silver bullet for protecting corals. For one, researchers still don’t know exactly what causes stony coral tissue loss disease, which makes it difficult to determine how or why the probiotic works, Paul says. In addition, since the disease has spread to many different parts of the Caribbean, it might be challenging to use the bag treatment technique on all affected colonies. “We would need to develop better methods of deploying the probiotic through time release formulations or other ways to scale up treatments,” Paul says. “Right now, having divers swim around underwater with weighted bags is not a very scalable method.” The researchers have also conducted similar experiments on infected corals located farther south, in the Florida Keys. However, these tests have produced mixed results, probably because of regional differences in stony coral tissue loss disease. This is another hurdle scientists will likely need to overcome if they hope to expand the use of probiotics. “We probably need to develop different probiotics for different coral species and different regions of the Caribbean,” Paul says. Researchers returned to gather samples of tissues and mucus to see how the corals' microbiomes had changed. Hunter Noren Even so, scientists are heartened by the results of the experiments conducted near Fort Lauderdale. With more research, the findings suggest probiotics could be a promising tool for combatting the disease elsewhere. “Coral probiotics is a challenging field, because there are hundreds of different types of bacteria that associate with corals, and there are limitless experiments that need to be performed,” Amy Apprill, a marine chemist at Woods Hole Oceanographic Institution who was not involved with the research, tells Smithsonian magazine in an email. “These researchers made a major advance with their study by demonstrating the utility of whole colony treatment as well as the specific probiotic tested.” Apprill adds that, while antibiotics have been widely used to control stony coral tissue loss disease, scientists haven’t conducted much research to see how these treatments are affecting the plants and creatures that live nearby. “Using a naturally occurring bacterium for disease treatment may result in lessened impacts to other members of the coral reef ecosystem,” she says. Amid rising ocean temperatures, scientists expect to find even more diseased coral colonies in the future. Warmer waters may also allow other pathogens to thrive and proliferate. Against that backdrop, Apprill adds, probiotics and the different methods of applying them will be “major allies” in the fight to save coral reefs. Paul is also optimistic. Through research and field studies, she’s confident researchers will be able to develop interventions that can “help corals better survive changing environments and respond better to diseases and bleaching,” she says. Get the latest stories in your inbox every weekday.

Fruit flies don't naturally enjoy the taste of cocaine. Credit: Deposit Photos Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent every weekday. In a world first, scientists at the University of Utah have engineered fruit flies susceptible to cocaine addiction. But as strange as it sounds, there are potentially life-saving reasons for genetically altering the insects to crave the drug. The novel biological model could help addiction treatment therapies development and expedite research timelines. The findings are detailed in the Journal of Neuroscience. As surprising as it may sound, humans have a lot in common with fruit flies. In fact, we share around 70–75 percent of the same genes responsible for various diseases, as well as many of the same vital organs. Researchers have relied on the insects for genetic studies for years, especially for investigating the biological roots of certain addictions like cocaine abuse. This is due in large part to the fruit fly’s quick life cycle and its comparatively simple genetic makeup. But while scientists have administered the drug to the bugs in the past, there’s always been a small problem. “Flies don’t like cocaine one bit,” Adrian Rothenfluh, the study’s senior author and an associate professor of psychiatry, said in a statement. Even when previously introduced to cocaine, Rothenfluh’s team noted that the insects routinely opted for pure sugar water over sugar water laced with cocaine. Study first author Travis Philyaw theorized the reason may reside in a fly’s sense of taste that is found on their legs. “Insects are evolutionarily primed to avoid plant toxins, and cocaine is a plant toxin,” Philyaw explained. “They have taste receptors on their ‘arms’—their tarsal segments—so they can put their hand in something before it goes in their mouth, and decide, ‘I’m not going to touch that.'” After confirming that cocaine activates a fruit fly’s bitter-sensing taste receptors, Rothenfluh and Philyaw switched off those nerves. Once deactivated, there was little to stop the flies from developing a cocaine habit. These modified flies were subsequently introduced to sugar water infused with a low concentration of cocaine. Within 16 hours, the insects indicated a preference for the drug-laced drink. “At low doses, they start running around, just like people,” said Rothenfluh. “At very high doses, they get incapacitated, which is also true in people.” Now that researchers know how to breed the modified fruit flies, they can more easily study how cocaine addiction evolves in the body. Not only that, but they can do so on a much faster timeline by analyzing hundreds of genes at a time. “We can scale research so quickly in flies,” said Philyaw. “We can identify risk genes that might be difficult to uncover in more complex organisms, and then we pass that information to researchers who work with mammalian models.” From there, scientists can identify treatment targets that help link to human therapy options. “We can really start to understand the mechanisms of cocaine choice, and the more you understand about the mechanism, the more you have a chance to find a therapeutic that might act on that mechanism,” explained Rothenfluh.

Satellite view of coral reefs in New CaledoniaShutterstock/BEST-BACKGR​OUNDS There might be an upside to the loss of coral reefs. Their decline would mean oceans can absorb up to 5 per cent more carbon dioxide by 2100, researchers estimate, slowing the build up of this greenhouse gas in Earth’s atmosphere. “It is a beneficial effect if you only care about the concentration of CO2 in the atmosphere,” says Lester Kwiatkowski at Sorbonne University in Paris, France. But the decline of corals will also reduce biodiversity, harm fisheries and leave many coasts more exposed to rising seas, he says. Advertisement How much the world will warm depends mainly on the level of CO2 in the atmosphere. So far the land and oceans have been soaking up around half of the extra CO2 we have emitted. Any factors that increase or decrease these so-called land or ocean carbon sinks could therefore have a significant impact on future warming. It is often assumed that corals remove CO2 from seawater as they grow their calcium carbonate skeletons. In fact, the process, also known as calcification, is a net source of CO2. “You’re taking inorganic carbon in the ocean, generally in the form of carbonate and bicarbonate ions, turning it into calcium carbonate and that process releases CO2 into the seawater, some of which will be lost to the atmosphere,” says Kwiatkowski. Unmissable news about our planet delivered straight to your inbox every month. Sign up to newsletter This means that if reef formation around the world slows or even reverses, less CO2 will be released by reefs and the oceans will be able to absorb more of this greenhouse gas from the atmosphere – a factor not currently included in climate models. Observations suggest coral reef calcification is already declining as rising seawater temperatures cause mass coral bleaching and die-offs. The higher level of CO2 is also making oceans more acidic, which can make it harder to build carbonate skeletons and even lead to their dissolution. Kwiatkowski and his team took published estimates of how corals will be affected by warming and ocean acidification and used a computer model to work out how this might change the ocean sink in various emission scenarios. They conclude that the oceans could take up between 1 and 5 per cent more carbon by 2100, and up to 13 per cent more by 2300. This doesn’t take account of other factors that can cause reef decline such as overfishing and the spread of coral diseases, says Kwiatkowski, so might even be an underestimate. On the other hand, the work assumes that corals aren’t able to adapt or acclimatise, says Chris Jury at the University of Hawai’i at Manoa, who wasn’t involved in the study. “If the worst-case or even medium-case scenario in this study comes to pass, it means the near-total destruction of coral reefs globally,” says Jury. “I think that with consideration of realistic levels of adaptation and acclimatisation by corals and other reef organisms, the authors might come to different conclusions under a low to moderate level of climate change.” If Kwiatkowski’s team is correct, it means that the amount of emitted CO2 that will lead to a given level of warming – the so-called carbon budget – is a little larger than currently thought. “I think we would like our budgets to be as accurate as possible, even if we’re blowing through them,” says Kwiatkowski. Journal reference:PNAS DOI: 10.1073/pnas.2501562122 Topics:

From August 23rd - September 14th, 2023 (Kodiak, Alaska to Seward, Alaska), NOAA Ocean Exploration conducted Seascape Alaska 5: Gulf of Alaska Remotely Operated Vehicle Exploration and Mapping (EX2306), a remotely operated vehicle (ROV) and mapping expedition to the Gulf of Alaska on NOAA Ship Okeanos Explorer. Operations during this 23-day expedition included the completion of 19 successful remotely operated vehicle (ROV) dives, which were conducted in water depths ranging from 253.1 m to 4261.5 m for approximately 87 hours of bottom time and resulted in the collection of 383 samples. EX2306 also collected more than 28,000 sq. km of seafloor bathymetry and associated water column data using an EM 304 multibeam sonar. These images were captured on dives that were included in the source data for the How Little We’ve Seen: A Visual Coverage Estimate of the Deep Seafloor paper. They are good general reference imagery for the type of deep ocean observations captured by ROVs.(Image Courtesy of NOAA Ocean Exploration)NewsletterSign up for our email newsletter for the latest science newsKey Takeaways on Deep Ocean Exploration: We have visually explored less than 0.001 percent of the deep sea floor. To put that in perspective, 66 percent of the planet is deep ocean, and 99.999 percent of that ocean is unknown to us.Like ecosystems on land, the sea has a complex food web. Most of life in the sea depends on detritus, mostly phytoplankton, falling down from the surface, something called “marine snow.”Organisms that live in shallow water absorb carbon dioxide and take that with them when they sink to the bottom, often to be buried in deep-sea sediment. This is known as a carbon sink. It’s important to know the rates at which this happens, because this partially offsets the carbon we’re adding to the atmosphere. It’s been said many times that we know more about the moon than our own ocean. But is it really true that we’ve explored only a tiny portion of the sea?Katy Croff Bell wondered about this, too. Bell is an oceanographer and the founder of the Ocean Discovery League. She knew that Woods Hole Oceanographic Institution and others have been operating deep-sea submersibles like Alvin for decades, and there are facilities in 20 or so places around the world doing deep-sea research. But how much of the sea floor have these projects actually explored visually, not just mapped or sampled?Mapping the Deep OceanBell started looking up dive data and doing some math. “I stayed up way too late and came up with a very, very tiny number,” she recalls. She didn’t believe her own results and got everyone she could think of to double-check her math. But the results held. Over the next four years, she and her team compiled a database of dives from organizations and individuals around the world, and the data support her initial estimate. The number is indeed tiny. It turns out that we have visually explored less than 0.001 percent of the deep sea floor. To put that in perspective, 66 percent of the planet is deep ocean, and 99.999 percent of that ocean is unknown to us. Bell and her team published their findings in May 2025 in the journal Science Advances.Why Deep Sea Exploration MattersFrom July 14 - July 25, 2023, NOAA Ocean Exploration and partners conducted the third in a series of Seascape Alaska expeditions on NOAA Ship Okeanos Explorer. Over the course of 12 days at sea, the team conducted 6 full remotely operated vehicle (ROV) dives, mapped nearly 16,000 square kilometers (6,180 square miles), and collected a variety of biological and geological samples. When combined with numerous biological and geological observations, data from the Seascape Alaska 3: Aleutians Remotely Operated Vehicle Exploration and Mapping expedition will help to establish a baseline assessment of the ocean environment, increase understanding of marine life and habitats to inform management decisions, and increase public awareness of ocean issues. These images were captured on dives that were included in the source data for the How Little We’ve Seen: A Visual Coverage Estimate of the Deep Seafloor paper. They are good general reference imagery for the type of deep ocean observations captured by ROVs. (Image Courtesy of NOAA Ocean Exploration)About 26 percent of the ocean has been mapped with multi-beam sonar, explains Bell, and that gives us an idea of the shape of the ocean floor. But that’s like looking at a topographical map of an area you’re planning to hike. You know where the hills and valleys are, but you have no idea what kind of plants and animals you’re likely to encounter. If you want to understand the deep ocean, you need to get down there and see what kind of rocks and sediment are there, learn about the corals and sponges and other animals living there, she says. Samples of ocean life are helpful, but they do not give anything like a full picture of the life-forms in the deep sea, and more importantly, they tell you little about the complex ecosystems they’re a part of. But when you put mapping and sampling together with visual data, plus data about temperature, depths, and salinity, Bell says, you start to build a picture of what a given ocean habitat is like, and eventually, the role of that habitat in the global ocean system.The Deep-Sea "Snow" That Provides LifeFrom August 23rd - September 14th, 2023 (Kodiak, Alaska to Seward, Alaska), NOAA Ocean Exploration conducted Seascape Alaska 5: Gulf of Alaska Remotely Operated Vehicle Exploration and Mapping (EX2306), a remotely operated vehicle (ROV) and mapping expedition to the Gulf of Alaska on NOAA Ship Okeanos Explorer. Operations during this 23-day expedition included the completion of 19 successful remotely operated vehicle (ROV) dives, which were conducted in water depths ranging from 253.1 m to 4261.5 m for approximately 87 hours of bottom time and resulted in the collection of 383 samples. EX2306 also collected more than 28,000 sq. km of seafloor bathymetry and associated water column data using an EM 304 multibeam sonar. These images were captured on dives that were included in the source data for the How Little We’ve Seen: A Visual Coverage Estimate of the Deep Seafloor paper. They are good general reference imagery for the type of deep ocean observations captured by ROVs. (Image Courtesy of NOAA Ocean Exploration)Like ecosystems on land, the sea has a complex food web. Most of life in the sea depends on detritus, mostly phytoplankton, falling down from the surface, something called “marine snow,” explains James Douglass, an ecologist at Florida Gulf Coast University who studies life on the sea bed. This snow of nutrients is eaten by what are called suspension feeders, including filter feeders, such as sponges and corals, which have tentacles or basket-like appendages to trap the snow. Then other organisms, such as crabs and worms, feed on these creatures. The crabs and worms, in turn, are eaten by fish. Deposit feeders, such as the sea pig, a type of sea cucumber that “trundles across the bottom eating mud all day,” add to the already huge variety of life, Douglass says. The types of organisms you have in the deep sea depend on how deep it is, whether the sea floor is rocky or muddy, how quickly currents bring food, and whether there are underwater hot springs or cold seeps, or other sources of extra energy, says Douglass. So yes, it’s a complicated world down there, and there’s an awful lot we don’t yet know.Deep-Sea Ecosystems and Climate Change From July 14 - July 25, 2023, NOAA Ocean Exploration and partners conducted the third in a series of Seascape Alaska expeditions on NOAA Ship Okeanos Explorer. Over the course of 12 days at sea, the team conducted 6 full remotely operated vehicle (ROV) dives, mapped nearly 16,000 square kilometers (6,180 square miles), and collected a variety of biological and geological samples. When combined with numerous biological and geological observations, data from the Seascape Alaska 3: Aleutians Remotely Operated Vehicle Exploration and Mapping expedition will help to establish a baseline assessment of the ocean environment, increase understanding of marine life and habitats to inform management decisions, and increase public awareness of ocean issues. These images were captured on dives that were included in the source data for the How Little We’ve Seen: A Visual Coverage Estimate of the Deep Seafloor paper. They are good general reference imagery for the type of deep ocean observations captured by ROVs. (Image Courtesy of NOAA Ocean Exploration)Learning about ocean ecosystems is extremely valuable as basic science. But it has a more urgent purpose as well. Though we often think of the land and the sea as two completely separate places, they are intertwined in many significant ways. The ocean has absorbed 90 percent of the excess heat and 30 percent of the carbon dioxide released into the atmosphere by humans, says Bell. “But we don’t really have a good understanding of how this is going to impact deep-sea ecosystems, and those ecosystems play a vital role in the process of carbon sequestration,” she says.When it comes to climate change, the deep sea has a lot to teach us. In parts of the deep sea, Douglass explains, nothing disturbs the layers of sediment that are deposited slowly over the course of thousands, even millions of years. Geologists can interpret the layers and study the fossils preserved in them to get an understanding of what the conditions of the planet were like in the distant past, similar to the way climatologists study Antarctic ice cores. “We've learned things about how the ocean ecosystem changes when climate changes. We've learned that some worrying things can happen under certain climate conditions in the deep ocean,” Douglass says. “For example, the ocean can become less oxygenated, which would be a catastrophic threat to deep-sea life.”The Deep Ocean and Climate RegulationAnd, of course, there’s carbon dioxide. “The deep sea is not just a passive record of what happened to the climate; it’s involved in regulating climate,” Douglass says. Organisms that live in shallow water absorb carbon dioxide and take that with them when they sink to the bottom, often to be buried in deep-sea sediment. This is known as a carbon sink. Douglass says it’s very important to know the rates at which this happens, because this partially offsets the carbon we’re adding to the atmosphere. “Deep-sea carbon storage is a huge element in our understanding of the planet's ability to regulate climate,” he adds.If we are to truly understand the way the entire planet works, we need to understand the deep sea and its complex ecosystems as well as life on land and in the shallows. And to do that, Bell says, we need to get down there and look.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:Science Advances. How little we’ve seen: A visual coverage estimate of the deep seafloorAvery Hurt is a freelance science journalist. In addition to writing for Discover, she writes regularly for a variety of outlets, both print and online, including National Geographic, Science News Explores, Medscape, and WebMD. She’s the author of Bullet With Your Name on It: What You Will Probably Die From and What You Can Do About It, Clerisy Press 2007, as well as several books for young readers. Avery got her start in journalism while attending university, writing for the school newspaper and editing the student non-fiction magazine. Though she writes about all areas of science, she is particularly interested in neuroscience, the science of consciousness, and AI–interests she developed while earning a degree in philosophy.1 free article leftWant More? Get unlimited access for as low as $1.99/monthSubscribeAlready a subscriber?Register or Log In1 free articleSubscribeWant more?Keep reading for as low as $1.99!SubscribeAlready a subscriber?Register or Log In