No one has yet figured out what exactly makes sea stars so suddenly fall apart. Some studies point towards bacteria and viruses, while others have found no such link. It’s possible sea stars are wasting away for various reasons that show up as a similar set of symptoms. The mass die-off of sea stars in the eastern Pacific has been linked to warming seas and could have been a stress response to fast environmental change.

It was further inflamed by what was arguably the biggest ocean heatwave on record. Nicknamed the Blob, it occurred when a persistent high-pressure zone pinned a thousand-mile-wide body of unusually warm water against North America’s west coast from Alaska to California and stayed there from 2014 to 2016. Sea temperatures were as much as five degrees Fahrenheit higher than average. Subsequent analysis confirmed the Blob was not related to natural climatic variations but was a direct result of the climate crisis. The heated sea unleashed chaos in oceanic ecosystems, causing food webs to collapse; starving millions of fish, seabirds, sea lions, and whales; and killing off kelp forests. The heatwave also made sea star wasting disease worse, and by the time the Blob dispersed, there were very few sunflower sea stars left.

For now, the sea star wasting disease has eased in the Pacific,^g but there are no signs of sunflower sea stars recovering in the wild. Fearful for the future of the species, conservationists have stepped in. Staff at the Nature Conservancy teamed up with echinoderm expert Jason Hodin at the University of Washington, who they hoped could figure out how to rear sunflower sea stars in captivity. In 2019, they located one of the last known populations of the species in the Salish Sea, gathered up a few dozen healthy-looking adults, and took them to the Friday Harbor Laboratories on the shores of San Juan Island off the coast of Washington State.

Almost nothing was known about the biology of the species. Hodin had a lot of experience rearing and studying other echinoderms, but with sunflower sea stars he was starting more or less from scratch. Much of the information in existing scientific literature turned out to be wrong. For instance, it assumed these sea stars spawn in summer months. But when Hodin brought them into his laboratory in the summer of 2019, nothing happened. Not until the following winter were the adult sea stars ready to spawn. Hodin’s team carefully collected the sea stars’ sperm and eggs, mixed them together, then filled beakers and jars with the resulting larvae, which look nothing like sea stars and more like odd-shaped sea slugs.

For two months, Hodin waited patiently for the sea star larvae to reach the most dramatic moment in their lives, metamorphosis. He watched as star shapes began to assemble inside the transparent bodies of the larvae. Then came the time when the larvae sank to the bottom, cast off their younger incarnation, and transformed into five-armed sea stars, two one-hundredths of an inch across.

Still, Hodin and his team faced one of their biggest challenges. They had to work out what to feed the juvenile sea stars. It’s common for young sea stars to graze on microscopic algae, but not sunflower sea stars. After a lot of trial and error, and uneaten food offerings, the tiny sea stars finally started nibbling on even tinier baby sea urchins. As it turns out, sunflower sea stars have a good appetite for urchins throughout much of their lives.

Now the aquarium tanks at the Friday Harbor Labs contain dozens of enormous sunflower sea stars. They’ve answered another important question about their biology: to reach maturity takes them three years. The next step is to see how these captive-reared animals fare in more natural conditions. Hodin’s team will deploy cages in select spots off the coast and place sea stars inside, giving them their first taste of wild waters.

The story of sunflower sea stars warns of a frightening new reality for the ocean. Changes can sweep in so quickly that a species could be lost before biologists have a chance to learn about its life and work out a plan to save it. Hodin and the Nature Conservancy may have got to sunflower sea stars just in time, but the future of the species remains uncertain. There’s no telling if or when the wasting disease might return. But there is at least an inkling of good news coming from Hodin’s laboratory studies. Preliminary research is showing that captive-reared sea stars seem to cope well and keep growing at higher temperatures. This bodes well for the possible future release of sunflower sea stars back into an ocean noticeably warmer than it was when the species was last widespread and abundant. There are no immediate plans for a sea star release programme. But if remnant populations don’t recover by themselves, rearing more in aquariums maintains the possibility of one day returning the species to the wild.

Plans to set free captive-reared animals often provoke questions of ethics and concerns of messing with natural systems. It’s true that plenty of ecological disasters have been triggered by people moving species around the planet, although most of these were species released a long way outside their natural range, such as the giant African land snails and Florida rosy wolfsnails brought to the South Pacific. On the contrary, many breeding programmes, with great care and consideration, have saved species from almost certain extinction. Were it not for captive-breeding efforts around the world, there wouldn’t be scimitar-horned oryx in the deserts of central Africa or candy-striped endemic tree snails crawling through the forests of Polynesia or giant tortoises chewing on prickly pear cactus trees on Española Island in the Galápagos.

I see no good reason not to try to do the same for ocean-going species that are in most urgent need of help and can feasibly be reared in captivity.^h Frankly, too much is already changing in the Anthropocene ocean to be precious about keeping some notional version of wild nature untouched by human hands. Millions of captive-reared salmon and trout are released into the ocean every year to boost fisheries. Why not do the same for endangered species? How glorious it would be to see giant sunflower sea stars hunting once again alongside sea otters, offering even more hope for the future of kelp forests.

A different seaweed forest exists far from shore. It doesn’t grow upwards from the seabed but forms a detached canopy floating at the surface of the Sargasso Sea, a 1.5-million-square-mile region of the Atlantic Ocean to the east of Bermuda, the only sea on earth that has no coastlines, bounded instead by rotating currents. Inside this gyre lies a network of islands composed of golden Sargassum seaweeds,ⁱ their tangled mats of serrated leaves suspended by small, round bladders filled with gas. Like an upside-down rain forest, this unique habitat (the only one of its kind) creates shelter and food for a rich ecosystem of animal life. Sea spiders, sea slugs, and snails creep through the canopy, and sea anemones fix to the weeds like flowers. Sargassum crabs and sargassum shrimp live exclusively in this drifting forest, as does the sargassum anglerfish, which is brilliantly camouflaged to match its weedy habitat and uses its modified fins to clamber through the undergrowth and sneak up on prey.

In 2011, Sargassum suddenly started appearing where it hadn’t been seen before. Colossal quantities began washing up on coastlines in Brazil and Mexico, along the US Gulf Coast, and on numerous Caribbean islands. Every summer since then, more Sargassum has piled up on beaches, creating a dangerous public nuisance that’s disastrous for tourism industries, ruining beachgoers’ vacations. Decomposing piles of Sargassum release toxic hydrogen sulphide gas that stinks of rotten eggs, and there are worries too about pathogenic bacteria in the seaweed flotsam. Clean-up efforts are costing millions of dollars every year.

Coastal ecosystems are also being harmed because thick mats of Sargassum drifting into coastlines block the sunlight needed by seagrass meadows and coral reefs to flourish. Sea turtle hatchlings are emerging from their sandy nests on beaches only to encounter impenetrable mountains of stranded Sargassum in their path to the sea. Tragically, the palm-size turtles are getting trapped by the very same seaweeds that should offer them sanctuary when they swim out to the floating forest.

The source of this scourge has been spotted from space. Satellites have recorded a huge clump of Sargassum, up to five thousand miles long and three hundred miles wide, stretching all the way across the Atlantic, from the coast of West Africa to the Caribbean, in an area to the south of the Sargasso Sea that used to be open blue ocean. Scientists named it the Great Atlantic Sargassum Belt.

The most obvious culprit to blame for the explosion was Sargassum escaping from the Sargasso Sea, and yet studies showed that in fact this is a distinct southerly population that had previously never proliferated. Increasing sea temperatures are likely playing their part in this dramatic expansion for the simple reason that warmth speeds up seaweed growth. A critical connection between ocean and land is also largely responsible. At the centre of this disaster is the Amazon Basin, where accelerating deforestation and burgeoning soya plantations and cattle ranches are releasing excess nutrients into the Orinoco and Amazon Rivers and out to the Atlantic. Likewise, land-based runoff from farmland and sewage systems in North America’s Mississippi Basin is also adding to the nutrient load offshore. Specifically, increasing amounts of nitrogen have been linked to the sudden boom in Sargassum. And with no circulating currents to contain the seaweed, as happens in the Sargasso Sea, the Atlantic bloom is being swept westwards towards land.

An ideal solution has yet to be found for getting rid of Sargassum after it has washed up on the shore. Composting and using it as fertiliser are not straightforward options, because Sargassum contains significant amounts of arsenic, which would contaminate soils and groundwater. Other ideas include pressing the weeds into building blocks or processing the seaweed mass, along with the plastic debris that gets caught up in it, to make biofuels. A company called Seaweed Generation is developing robotic collectors that would gather the floating Sargassum mats, bundle them up, and sink them into the deep ocean several miles down, in the process removing carbon from the atmosphere. The ecological impacts this would have are still to be fully explored, from the floating ecosystems at the surface that could also get scooped up, to the deep-sea animals that may, or may not, appreciate the extra food.

Even if effective ways are invented to clear beaches and help safeguard coastal ecosystems from Sargassum, this problematic seaweed will keep on arriving unless something can be done to stop the rampant destruction of terrestrial forests and the surge of farmland taking their place. The message from the Great Atlantic Sargassum Belt is that the future of earth’s land and ocean are bound tightly together.

While efforts are ongoing to clear up the Sargassum deluge from the Caribbean to Florida, conservationists and scientists farther north have been working hard for the past two decades to bring back a long-lost green ecosystem. Like vast underwater prairies carpeting tens of thousands of acres of seabed, meadows of eelgrass used to thrive in coastal lagoons of the US state of Virginia. In the 1930s, a deadly pandemic broke out of seagrass wasting disease, caused by a slime mould called Labyrinthula. Combined with a devastating hurricane, this completely eradicated all of Virginia’s underwater meadows.

For the next seventy years, no eelgrass grew in the inshore lagoons, although it was gradually returning to nearby Chesapeake Bay, which had also been stripped bare by the disease. Then, early in the twenty-first century, the situation turned around. Locals had noticed small patches of eelgrass in the lagoons, and scientists from Virginia Institute of Marine Science found the conditions were right for meadows to grow—if there were enough eelgrass plants and their seeds to kick-start the populations.

Thus began a programme to actively replant Virginia’s missing eelgrass. Teams of scientists and volunteers worked together to gather up seedpods from existing meadows in the Chesapeake and scatter them in dense patches in empty areas of Hog Island Bay, Spider Crab Bay, Cobb Bay, and South Bay. Over the course of twenty years, more than seventy million seeds were sown, which quickly germinated, grew, and began producing their own seeds, and so the eelgrass meadows have spread. From around five hundred replanted acres, close to nine thousand acres are now covered in meadows.

The regreening of Virginia’s lagoons has been the world’s most successful seagrass restoration project. It’s inspiring similar attempts elsewhere to bring back the ocean’s underwater meadows, which are highly threatened by pollution and sediments pouring off land and destruction by boat anchors, trawlers, and dredgers. Seagrass meadows grow along the coasts of every continent except Antarctica, and already a third have disappeared globally, some at a horrifying rate: Casco Bay, an inlet in the Gulf of Maine, lost more than half its eelgrass meadows, from 5,012 to 2,286 acres, in just four years, between 2018 and 2022.

The seventy or so species of seagrasses, including eelgrass, play a similarly critical role in the ocean as kelp forests: they buffer waves, protect coastlines from erosion, and provide habitat for a rich mix of other species. As the recovering Virginia lagoons are showing, when meadows return, so do a host of animals. Here, crab and fish populations have rebounded, including large shoals of silver perch and pinfish. Tundra swans and redhead ducks are among the birds that are calling in at the aquatic grasslands in greater numbers than a few decades ago. Scientists have also reintroduced a now-thriving population of bay scallops, which rely on the eelgrass, resting on the blades at an early stage in their life cycle before settling down on the seabed.

Virginia’s thriving eelgrass ecosystems are also improving water quality in the lagoons, trapping sediments in their roots and creating sparkling-clear waters. And the underwater grasses are storing more carbon and nitrogen in the seabed. Globally, seagrass meadows likely store close to twenty billion tons of carbon, equal to more than half of the global annual emissions in the mid-2020s. Keeping existing seagrass meadows intact will prevent the release of a great deal more carbon into the atmosphere. And that carbon capture could increase further if more seagrasses were helped to recover. A 2022 study projecting the future of seagrass meadows highlighted the importance of not only protecting seagrasses but helping to restore them too. Combining conservation efforts with restoration programmes, like the one in Virginia, could see underwater meadows increase by more than a third by 2070, effectively dialling back the damage done over past decades.

Likewise, mangrove forests will likely do much better in the years ahead with a blend of conservation and effective restoration. In 105 countries, mangroves grow between the tides on sheltered shorelines, where their looping, twisted roots and trunks flood daily, then get exposed to air and sun when the tide falls. In the later decades of the twentieth century, 1 per cent of these semiaquatic tropical forests were being lost every year, most of them cut down to make way for agriculture and fish farms, a rate of deforestation that has since been reduced; a further 2 per cent were lost between 2000 and 2016.

Historically dismissed as disease-ridden swamps, mangrove forests are another of the sea’s green habitats that provide critical benefits for coastal communities, creating storm buffers, nurturing countless young fish and other animals, and supporting the livelihoods of millions of small-scale fishers.

Globally, mangrove forests lock up close to ten billion tons of carbon in their roots, trunks, and soils. Many restoration programmes have chased the dream of increasing carbon sequestration but not always with good results. Scandalous amounts of money and goodwill have been wasted as projects have carelessly jumped on a green-washed bandwagon, poking millions of seedpods, the shape and size of cigars, into the seabed, only for all the seedlings to die.

The Philippines, for instance, which lost nearly three-quarters of its mangrove forests in the last hundred years, saw one of the most intensive and pointless efforts to restore mangrove forests. Over the course of twenty years, conservationists planted millions of mangrove seeds over an area of more than one hundred thousand acres. Some seeds germinated and began to grow, but most were doomed. A later survey found the new mangrove trees were almost entirely dead, dying, or dismally stunted. Restoration efforts failed because planters didn’t take into account the mangroves’ biology and the conditions they need to survive. Across the Philippines, and in many other restoration projects, seeds have been planted in places that would never have supported mangrove trees; they were stuck into shorelines of the wrong elevation and tidal regime; some were even planted on top of seagrass meadows, destroying one threatened ecosystem in failed attempts to grow another. Far more important than planting seeds is to ensure the hydrological conditions are just right for mangroves to grow—for instance, by digging away sediments or breaking down dams so there’s just enough but not too much seawater flooding in and out every day. Mangrove seeds are buoyant and float for days or months on ocean currents, some drifting thousands of miles before sprouting roots to anchor themselves in place. If the shoreline conditions are right, seeds will drift in, and damaged or lost forests will naturally rejuvenate.

Compared to drifting mangrove seeds, kelp spores don’t tend to travel far. Depending on the species, the next generation of kelp can disperse a few miles or almost no distance at all. Many kelp spores simply scatter among the holdfasts of their parents. The shady, sheltered canopy of adult kelp is commonly where sporelings grow best. Too much light can cause tender young kelp to lose its pigments and bleach white, while chunks of its tissue rot away and die. And the fronds of adult kelp undulating in the swell can sweep away sea urchins and help protect sporelings from being eaten. This means that efforts to restore kelp forests are most likely to be successful when there are large surviving forests nearby. The ocean’s underwater forests need existing forests to grow, which raises a problem when there are none for miles around. Then it’s not enough for kelp restorationists to manipulate grazing and predatory species. They also need to try to regrow the forests.

Japan has the longest history of kelp restoration. Isoyake^j is the term for declining kelp forests, and there are records going back centuries of efforts to help them recover, commonly by constructing artificial reefs for kelp spores to settle on. Fishers in the early part of the eighteenth century threw stones onto barren areas of seabed to encourage kelp regrowth, and essentially the same practice remains popular today. Over the past few decades, off the Pacific coast of Shizuoka Prefecture, not far from the foothills of Mount Fuji, concrete blocks have been installed inside healthy kelp forests, providing substrate for spores to fix on. Moving the sporeling-covered blocks to areas of isoyake has restored more than three square miles of Ecklonia forest.

It takes decades, centuries even, for a terrestrial tropical rain forest or an oak woodland to regrow. Meanwhile, a mature kelp forest can establish within a few years. Kelp restorationists are still in the experimental stages of figuring out how to make that happen in the ever-shifting conditions of the Anthropocene ocean.

In the 1980s, beaches in the Australian city of Sydney were frequently closed to swimmers because masses of poorly treated sewage were allowed to pour straight into the sea. Below the waterline, the pollution killed off dense stands of a jagged seaweed known as crayweed,^k although the disappearance went unnoticed until much later. In 2007, a team of diving scientists surveyed the coastline and found dense crayweed canopies were common on the rocky reefs of New South Wales. But along more than forty miles of Sydney’s metropolitan coastline, they didn’t find a single seaweed blade. By then, the city’s sewage was being diverted along pipes offshore, and water quality inshore had substantially improved. Still, though, the shallow, rocky reefs remained empty of crayweed.

In 2011, a team of scientists at the Sydney Institute of Marine Science decided to test whether crayweed could grow again in the empty areas. They took mature adults from existing seaweed forests north and south of Sydney and transplanted them into the forty-mile gap, fixing them to bare rocks with plastic mesh and cable ties.^l Six months later, some of the transplants had survived, and they were busy scattering the next generation of young sporelings across the seabed.

From that initial success, Operation Crayweed was born. Restoration efforts were scaled up, initially paid for by a crowdfunding campaign in December 2015 that prompted people to “plant an underwater tree for Christmas.” Support from members of the public was critical in kick-starting the project, paving the way for state investment. Operation Crayweed is also reaching beyond the science of reforestation and rolling out art installations and exhibitions along the coast to make new connections between local people and the lost forests. Schoolchildren dressed up as sea dragons and octopuses have been parading along the Sydney seafront, imagining what it’s like to be these animals living in the underwater forest that’s regrowing just offshore. Previously, many Sydneysiders didn’t realise the city’s water quality had greatly improved, and they had no idea crayweed forests used to grow on their doorstep. Now, at more than a dozen sites across the city, people can look out to sea at the marker buoys and know that’s where the forests are coming back, gradually filling in the gap in these easterly reaches of Australia’s Great Southern Reef.

In another part of this immense network of underwater forests, ancient human connections that have remained unbroken for millennia are now at risk of coming to an end. In lutruwita (Tasmania), the shark tooth–shaped island south of the Australian mainland, Aboriginal people traditionally hunt for food in the giant kelp forests of their sea country and use the kelp as an important material to make objects such as water-carrying containers. Aboriginal women visit kelp forests at low tide to collect seashells, including maireener, or rainbow kelp shells, which they rub with sand to reveal the shining mother-of-pearl underneath. These and other tiny shells are pierced, threaded into necklaces, and used as gifts and tokens of honour. Maireener shells are now much harder to find, and shell necklaces could become a lost art because lutruwita’s giant kelp forests are disappearing.

In recent years, propelled by climate change, the warm ocean current that flows polewards along the east coast of mainland Australia has been pushing farther south. This is the prime reason why the seas around lutruwita are heating four times faster than the global average, and why the lutruwita giant kelp forests are dying. There are bays where, just a few decades ago, fishermen cut channels through the giant kelp to get their boats in and out, and now there’s no kelp at all. Only 5 per cent of lutruwita’s giant kelp forests still stand, and that 5 per cent could hold the secret for the future of the forests.

People from the weetapoona Aboriginal Corporation have been collaborating with marine biologists from the University of Tasmania to trial a new form of restoration they hope will see the return of kelp to their sea country. They selected Trumpeter Bay, on the southeast coast of lutruwita, as a site where kelp have been outplanted in the hope they will kick-start the return of lost forests.

These outplantings originate in the surviving naturally occurring stands of kelp dotted around lutruwita. Cayne Layton and his team at the university visited some of the remaining patches of kelp forests and collected spore-producing fronds, leaving the intact adults behind. In the lab, the fronds released spores, which developed into microscopic kelp females and males, known as gametophytes, which look like tiny twigs. These form an important stage in kelp’s unusual, two-phase life cycle, a part that’s normally hidden from view. Minute females and males produce eggs and sperm, which fuse and grow into the large kelp that form the structure of a forest.

Conveniently, this miniature, gametophyte stage can be cooled and stored under red light, which arrests its development; and more gametophytes can easily be made by snapping the tiny twigs in two. It’s a practice perfected in Japan and Korea, where kelp is important food, much of it reared by the aquaculture industry. Cultures of kelp gametophytes have been kept alive in cool storage for decades.

In lutruwita, Layton and colleagues store their kelp gametophytes in a refrigerator the size of a hotel minibar. This is a compact, living repository, the kelp equivalent of a seed bank. It provides material for Layton’s studies in rearing lab-grown sporelings in aquarium tanks at different temperatures, to identify those that can naturally cope with heat. Some young kelp even grow well at seventy-four degrees Fahrenheit, way above temperatures the kelp parents would have experienced in lutruwita’s warming sea.

Layton’s team reared more of these super-kelp strains and transplanted them into the wild, including in Trumpeter Bay. As expected in the turbulent seas of lutruwita, not all the trial outplantings have taken hold. But enough are surviving to show the technique works. Crucially, some of the super-kelp have now reached maturity in the wild and are producing their own spores, like the crayweeds in Sydney. This is the main aim of the lutruwita initiative, to put enough kelp back in the ocean to kick-start the natural cycles, so that ultimately forests regenerate by themselves—and hopefully keep growing as the ocean continues to warm.

There is no silver bullet for creating future-proof, self-sustaining underwater forests or, for that matter, any other important green habitats in the ocean. In lutruwita and elsewhere, restorationists are busy testing out ways to replant and restore kelp forests, seagrass meadows, and mangrove forests. They’re showing that restoration is doable, but it’s complicated and nuanced. Each species and habitat in each region will likely need its own methods and solutions. And that’s going to require much more funding, in many more countries, to support the kinds of detailed research and scientific trials that are exploring what works best. So far, successes in restoring lost kelp forests have mostly been modest in size, but they are showing what’s possible. The obvious next big steps will be finding ways to effectively scale up, while avoiding the pitfalls that other restoration efforts have crashed into.

The future of kelp forests will depend on finding practical ways of keeping these ecosystems alive in the changing ocean, ideally without having to sell out to carbon markets. And perhaps more than anything, it will depend on weaving underwater forests more deeply into people’s lives and minds. The greater the number of people who know and care, the less likely it is that the ocean’s forests will be allowed to disappear.

a Brown seaweeds are included within the proposed kingdom Chromista, and they are a taxonomic class, Phaeophyceae, within which is the kelp order, Laminariales.

b I think it looks like the lollipop-shaped Truffula tree from Dr. Seuss’s book The Lorax.

c Red seaweeds, or rhodophytes, belong to a separate kingdom and are more closely related to land-based plants than kelp are.

d This is described as echinochromicity, after the echinochrome pigments found in sea urchins, which happen to have antiviral and antibacterial properties and have been investigated as potential therapeutic drugs against Covid-19.

e Sea urchins, starfish, sand dollars, brittle stars, feather stars, and sea cucumbers are all echinoderms, a phylum of animals named for their spiny skin (from the Greek word echino, meaning “spiny,” and the Latin word derm, meaning “skin”). Within the echinoderms, sea urchins belong to the echinoid class.

f Or starfish, as they are interchangeably called by people (like me) who aren’t too bothered by the ichthyological overtones.

g At the time of writing.

h I doubt it will ever be possible to rear such animals as great white sharks or blue whales in captivity, and I hope it never comes to that.