Aryn Baker is a Rome-based foreign correspondent who has spent the past 25 years writing about the intersection of climate change, conflict, migration, science, culture and politics around the world.
LA SPEZIA, Italy — One recent January afternoon, Paolo Varrella, an Italian mussel farmer with a penchant for big gambles, inspected a crateload of recently harvested oysters. He plucked one from the pile, wedged the tip of his pocketknife under the hinge, and with a slight twist and a horizontal swipe of the blade, popped open the shell to reveal the green-fringed oyster inside. He tipped the sweet, briny contents of the shell into his mouth. “Perfetto,” he declared and grinned, as if he had just won the jackpot.
In a way, he had. His hometown of La Spezia, on Italy’s Ligurian coast, is famous for mussel cultivation. But around two decades ago, Varrella broke with tradition to try his hand at oyster farming, resurrecting an industry that had been dormant for around a century. For years, Varrella’s quixotic sideline in oysters was a source of good-natured ribbing among La Spezia’s mussel farmers — why compete with France’s dominance in the oyster market when Italian mussels already offered a solid business model?
But when water temperatures spiked in the early 2020s due to climate change, the population of a local fish that preys on mussels exploded, wiping out 95% of the mussel harvest. The oysters were left unscathed. His colleague’s oyster jibes evolved into heartfelt appreciation for Varrella’s prescient bet. Without the oysters, the La Spezia shellfish cooperative would not have survived, Varella said.
These days, mussels are back in La Spezia, but oysters are also now a key pillar of the local aquaculture economy, a necessary diversification for an industry made increasingly unpredictable by climate change.
Varrella’s oysters, with their deeply cupped shells and blue-green gills prized by Michelin-starred chefs, aren’t just a culinary product that could put La Spezia on the map; they are one of the rare, farmed products that contribute more to nature than they detract. One oyster can filter up to 50 gallons of ocean water a day, removing pollution and debris. Oysters naturally build reefs that foster biodiversity and protect surrounding habitats from erosion. Farming them requires no fertilizers, feed or antibiotics; they sequester carbon as they grow. For those keeping an eye on their carbon footprint, they are a tasty, guilt-free source of protein and nutrients. “Oysters are the perfect food,” Varrella told me, with the zeal of a new convert. “Everyone should be growing oysters. Or at least eating them.”
If they did, oyster farming could help communities adapt to some of the worst impacts of climate change on our planet while cleaning up oceans and contributing a vital food source for a growing global population. But they, too, are threatened by the combination of warming waters, opportunistic disease and ocean acidification that also threaten La Spezia’s mussels. Varrella’s oysters may have withstood La Spezia’s most recent marine heat wave, but they may not survive the next. Scientists are now in a race against time to develop so-called “climate-proof” oysters that can withstand the multiple threats compounded by global warming. Oysters could help humans better adapt to climate change, but only if humans help them adapt first.
The Sea Made Solid
Like sharks and nautiluses, oysters are living fossils, their anatomy relatively unchanged in their 190 to 250 million years on this planet. When dinosaurs walked the earth, oysters dominated the oceans, sculpting the marine environment with massive reefs built on the remains of their predecessors. Excavations of ancient settlements indicate that Stone Age humans relied on oysters and other shellfish for survival, and some scientists postulate that this increase of omega-3 fatty acids in their diet helped with brain growth and the development of tools, culture and religion.
Some archaeologists hypothesize that the abundance of shellfish along the coast helped guide human migration over the North American land bridge and into the Americas. That bounty sustained human populations for millennia. In the mid-1800s, New Yorkers ate an average of 600 oysters a year, dredged from reefs so big they appeared on nautical charts. European appetites were equally voracious — and destructive. By the end of the century, wild oyster reefs had all but disappeared from the globe, having fallen victim to over-harvesting and industrial pollution. Scientists now estimate that today’s global oyster population is a mere fraction of historic numbers.
Our hunger for oysters has changed the geography of our coasts, denuding them of natural wave breaks and making them more vulnerable to sea level rise, storm surges and erosion. Had the great expanse of oyster reefs that historically protected New York Harbor south of Staten Island remained in place, instead of dredged to feed a growing population throughout the 1800s, damage from 2012’s Hurricane Sandy wouldn’t have been nearly as bad, said Sally McGee, who manages the Shellfish Growers Climate Coalition within the environmental nonprofit, The Nature Conservancy.
“Oysters could help humans better adapt to climate change, but only if humans help them adapt first.”
Groups like The Nature Conservancy are working to restore oyster reefs worldwide, from New York to Louisiana, Australia and the United Kingdom. Not for food — that would defeat the purpose — but for their other, more environmental, services. In New York City, the Billion Oyster Project has set a goal to restore 100 acres of oyster reefs by 2035.
In most cases, “restoring” means starting over. The Billion Oyster Project uses a small structure made of oyster-friendly substrate, “seeding” it with oyster larvae. The spat — young oysters — excrete a concrete-like substance to anchor themselves to the structure for life. Once mature, an oyster spawns every summer, releasing millions of gametes into surrounding waters and producing the next generation of larvae that will, in turn, settle on other shells, eventually building into house-sized reefs.
Oysters are so prolific, estimated one researcher, that if half of a female oyster’s offspring were to develop into female oysters, it would take less than five generations to repopulate the world’s oyster beds. However, rising water temperatures and increasing ocean acidity caused by climate change mean that few larvae survive in the wild, and the nascent reefs set up by oyster restoration efforts must be reseeded annually with hatchery spat to continue growing — a kind of IVF for the bivalve set.
Since its founding in 2014, the Billion Oyster Project has midwifed 150 million oysters into existence, creating a total of 19 acres of new reef across multiple locations in New York Harbor. New advances in oyster propagation are speeding up the process, and Carolyn Khoury, the program’s director of restoration, told me she is confident they will reach their goal. She said the cost of restoring the reefs is not much different from installing sea walls or other protective measures. However, as living, self-repairing systems, they are a more robust alternative to traditional construction.
“Oyster reefs are what we refer to as a nature-based solution for climate resilience,” Khoury told me. “When paired with wetland restoration, they can improve shoreline resilience to wave energy, increased storm frequency and erosion caused by climate change.”
They also provide more ecosystem services than a concrete wave barrier. Oyster reefs may not be as pretty as their coral cousins, but they are a vital habitat for spawning fish and other marine organisms. As they grow, they convert the water’s dissolved calcium and carbon into their shells, which sequester about 12 grams of carbon per oyster. The larvae that don’t survive end up as a meal for everyone else.
An oyster reef is like a giant Brita filter for the ocean, removing pollutants and purifying the water around them. They are “The Giving Tree” of the underwater world. “Everybody wants the pill for fixing climate change. Oysters are probably as close as you are going to get,” said Dan Martino, who wrote “The Oyster Book,” a history of the bivalve. “I can’t think of a single drawback to growing them.”
Martino is another recent convert to the oyster cult. Like Varrella, he started farming oysters relatively recently, around a decade ago in Martha’s Vineyard. He saw them as a solution to a looming global food crisis. Conventional land-based agriculture will not be able to meet the needs of a growing global population, he said, especially as droughts, heat and extreme weather push crops to their limits, while intensive animal farming adds to the carbon emissions causing climate change. The ocean provides an alternative. But farming fish, like farming beef and chicken, requires vast amounts of feed — usually other fish — also creates pollution, and typically requires antibiotics and other medicines to treat outbreaks of parasites and disease.
Bivalves don’t need anything. They feed on plankton filtered from the water. Fertilizers are unnecessary, and they don’t contribute significantly more carbon into the atmosphere. “They’re the most sustainable species we could farm on Earth,” Martino said. Marine conservation societies agree, rating farmed shellfish as the best choice for consumers, noting that they encourage marine biodiversity.
And they are much healthier to eat. Most bivalves have more protein than beef and are rich in omega-3 fatty acids, iron, zinc, iodine, magnesium, calcium and phosphorous. Oyster aficionados describe them as “the sea made solid.” Martino told me they are more like superfoods on the half shell: Just six oysters a day provide 100% of many of a person’s major nutritional requirements. “Farming oysters can meet our growing food demands while bettering our ecosystem, our environment, and our health,” Martino said.
“Oyster reefs are what we refer to as a nature-based solution for climate resilience … They can improve shoreline resilience to wave energy, increased storm frequency and erosion.”
Ernest Hemingway sought out oysters to help assuage his depression, writing in “A Moveable Feast” that when he was eating oysters, “with their strong taste of the sea and their faint metallic taste that the cold white wine washed away, leaving only the sea taste and the succulent texture … I lost the empty feeling and began to be happy and to make plans.”
These days, they are one of the rare treats that are both good for you and the planet. That double halo of nutrition and sustainability is driving sales: Global oyster production increased from 1.3 million tons in 1990 to 7.5 million tons in 2024 and is expected to reach 8.9 million tons by 2033, according to industry analysts. Demand continues to outpace supply. Martino said he could double production at his oyster ranch and still sell out. Rising water temperatures and increased ocean acidity caused by climate change are most likely the largest threat to the $8.5 billion-a-year industry, marine biologists who specialize in oyster aquaculture told me.
Sand In The Oyster Shell
Tim Green, a professor of fisheries and aquaculture at Vancouver Island University in Canada, has been studying oysters since 2008, when a mysterious disease all but decimated France’s oyster crop. From France, Ostreid herpesvirus 1 (OsHV-1), or oyster herpes, hitchhiked across the world’s shipping lanes on cargo carriers, ravaging New Zealand’s oyster farms, then Australia’s. In 2018, it led to mass die-offs in San Diego. “It doesn’t affect humans, but for oyster farmers, it’s devastating,” Green told me. “Ostreid herpesvirus can wipe out a farm in four days. We are talking 90% mortality.”
Meanwhile, something else is stalking oysters in the Pacific Northwest, which produces more than a quarter of America’s oysters. Farmers have reported mass die-offs for several years, usually just before the summer harvest. Summer oyster mortality, as Green and other scientists call it, could be caused by a virus similar to oyster herpes or an as-of-yet-unidentified bacteria. What Green does know is that marine heat waves — a period of unusually high ocean temperatures that can be caused by changes in ocean currents, weather events or climate patterns — trigger outbreaks of both diseases. “An elevated temperature is key,” he said. “Maybe the virus was always there, ticking along quietly, killing an oyster every once in a while, and then all of a sudden, boom, it gets hot, the oysters are stressed, and the virus kicks into overdrive.”
Scientists at the United Nations’ Intergovernmental Panel on Climate Change found that the number of marine heatwave events — in which temperatures increase 2 degrees Celsius to 5 degrees Celsius (3.6 to 9 degrees Fahrenheit) relative to historic temperatures — have doubled around the world since the 1980s, and predict that the frequency, duration, scale and intensity of future marine heatwaves will continue to increase with global warming.
Green isn’t sure if warmer waters help the pathogens proliferate or if the heat waves stress the oysters, making them more vulnerable to disease. Either way, heat can be deadly, and it’s getting worse. Heat waves in the Pacific Northwest used to be rare. Now, they sometimes happen several times a year. “It’s a ticking time bomb,” Green said.
Detonation isn’t far off, Green and other ocean scientists believe. A new study published by the United Kingdom’s University of Reading found that the world’s oceans are warming more than four times faster than they were 40 years ago. In the late 1980s, ocean warming was akin to adding a trickle of hot water to a full bathtub. Now it’s more like opening the hot water tap on full blast, Christopher Merchant, the study’s lead author, explained in a briefing for journalists. That already has enormous ramifications for ocean health — impacting everything from coral reefs to storm severity — but the study’s prediction that rising temperatures will continue to accelerate “by a significant margin,” over the next 20 years, as Merchant wrote in the study’s introduction, will fundamentally alter life underwater. The Reading professor of ocean and Earth observation elaborated in the university’s briefing: “The way to slow down that warming is to start closing off the hot tap, by cutting global carbon emissions and moving towards net-zero.”
Those carbon emissions don’t just drive up temperatures; they also increase ocean acidity. If you’ve ever dropped an egg in vinegar, you know what happens next. Oyster shells are made mostly of calcium carbonate, which starts to dissolve in an acidic environment. Mature oysters with thick shells can tolerate — to a certain extent — increased acidity, but in such an environment, juvenile larvae can’t grow strong shells fast enough (or in extremely acidic environments, any shells at all), making them vulnerable to predators and disease. “We’re getting to the point where, throughout the entire summer, it’s no longer favorable for a little oyster to make its shell,” in certain parts of the Pacific Northwest, Green told me.
“Farming oysters can meet our growing food demands while bettering our ecosystem, our environment, and our health.”
The hatcheries that provide larvae to oyster farmers found a workaround by keeping their juveniles in tanks of specially buffered water until they could grow thick enough shells to survive in the wild. Harvests improved in some cases, at least for a while. However, Green and other researchers have documented another alarming die-off among this new, coddled generation. Green suspects that raising the larvae in temperature and pH-controlled tanks reduces their resilience in other ways. “You can protect against one stress,” he said. “But how does that influence them down the line when this oyster suddenly gets exposed to elevated temperature and a disease at the same time?”
Green and his team are working on future-proofing the industry by selectively breeding to create stronger oysters. So far, he told me, they have developed a more heat-tolerant strain and another resistant to ocean acidification. They are now working on disease resistance against oyster herpes, norovirus and vibrio, he said. Unlike oyster herpes, which is harmless to humans, eating oysters infected with norovirus or some kinds of vibrio bacteria can make humans sick. Strict regulations on water testing in commercial fisheries mean such infections are rare, he noted. “The big trick now is combining all that to make a super oyster,” Green said.
It’s not the first time scientists have intervened to make a more resilient oyster. In the late 1970s, researchers selectively bred a strain of oysters that, like seedless watermelons, had three sets of chromosomes instead of two. The triploid mutation made the oyster sterile. Researchers found that when oysters spend all their energy on growth and repair instead of reproduction, the result is a tastier, faster-growing and more resilient product. “This is what’s called triploid vigor,” said Matt George, a marine biologist specializing in bivalves and now the coastal shellfish manager for the Washington Department of Fish and Wildlife.
Sterility has a market advantage. The old exhortation to only eat oysters during months with an “r” in their name — September through April — derives from the fact that oysters spawn in the summer. Just before spawning, they are gamey with gametes. Just after, they are flaccid and watery. Triploid oysters are meaty and sweet all year round. They can also be harvested sooner — after a year or two (or sometimes even sooner) instead of the three years or so needed for diploids to grow to market size — reducing the chance they will be felled by disease.
By the 2010s, the market advantages of triploid oysters had converted half of the West Coast’s commercial farmers, most of the East Coast’s, and nearly all of Europe’s. Demand for oysters on the half shell now peaks in summer, when they are best appreciated on a bed of ice served alongside a chilled rosé.
But a funny thing happened on the way to the raw bar. In 2021, an unprecedented heatwave hit the Pacific Northwest, killing at least 619 people and cooking some billion bivalves — oysters, mussels and clams — in their shells. George, the oyster specialist, had been researching triploid vigor at the time and seized the opportunity to conduct a study on heat tolerance. “Our original hypothesis was that triploidy would make oysters more climate-proof, that sterility would boost their available energy and ability to deal with large-scale climate events like this,” he said.
The hypothesis was wrong. George and his team found that triploid oysters were nearly 2.5 times more likely to die than diploids when exposed to heat stress. That’s terrible news for an industry that has grown accustomed to year-round sales as global temperatures climb. It’s also an opportunity. George suspects that the extra set of chromosomes in triploids may play a role in their stress response, even if it helps save on reproductive energy.
If scientists can figure out what causes triploids to succumb to heat stress, they can start engineering their way around it, just like they figured out how to breed sterile oysters in the first place. (To do so commercially, researchers first bred a male tetraploid — with four sets of chromosomes — and a female diploid.) “The technology is really promising,” George noted. “If we can engineer these animals to have the benefits of triploidy without the possible drawbacks of an extra separate set of chromosomes, then we are getting closer to a truly climate resilient oyster.”
Technological advances in breeding sterile, climate-resistant oysters for market may help feed a growing population on a warming planet, but they won’t do much for the wild oysters whose reefs protect our shorelines, clean our oceans and promote biodiversity.
For them, it may already be too late, concedes Green. “The oyster industry will be just fine. But for wild populations that deliver those ecosystem services, it’s a major crisis,” he told me. Groups like the Billion Oyster Project will have to augment their reefs with larvae from commercial hatcheries well into the future instead of relying on self-sustaining wild spawning, especially as temperatures and ocean acidity increase with climate change. “Without human intervention, wild oysters will start disappearing,” Green said.
For Varrella, the Italian oyster farmer, that would be a travesty. He fell in love with oysters while studying for a degree in marine biology, attracted by their ability to improve local biodiversity and clean the water around them. “We work on the ocean 365 days a year, with our hands in the water. So we can directly see the impacts of climate change. Oysters are a solution for that. With my farm, I’m doing my part. But wild oysters are just as important.”
Varrella favors triploids because the economics of a year-round product makes more sense. Martino, the oyster evangelist from Martha’s Vineyard, prefers diploids for their ability to benefit the wider ecosystem. Triploids may be better for the market, but Martino contributes to the local food chain by allowing his oysters — he buys around a million babies every year — to spawn. “I want to be the Johnny Appleseed of oysters,” he told me. “I want my oysters to make babies. It would probably destroy my farm-to-market model, but I would love to see oysters everywhere.”
And maybe, just maybe, those babies will join others in the wild, evolving into something more resilient on their own. After all, oysters survived the Ice Age, the last greenhouse era and the asteroid that killed the dinosaurs. If there are enough of them in the wild, they might just survive us humans, too.
