Kelp forests help protect coastlines because water flow slows as it passes through the canopy, baffling wave energy and reducing the impacts of erosion and storm surges. Crucially, they also improve water quality in coastal seas by absorbing nutrient pollution that pours off the land. Agricultural fertilisers, as well as sewage from humans and livestock, contain high concentrations of nitrogen and phosphorus, which are key nutrients for stimulating plants and seaweeds to grow, but too much can be highly problematic. Excess nutrients washing into the sea cause other seaweeds and phytoplankton to grow so fast they swamp ecosystems, choke waterways, and trigger toxic blooms that contaminate fisheries, kill wildlife, and make drinking and bathing water unsafe. When blooms die and decompose, they rob the water of oxygen and create dead zones where little life survives. With its naturally fast growth, kelp removes enough nitrogen from the surrounding water to substantially reduce the chances of problematic blooms, saving tens of thousands of dollars annually in clean-up costs for every acre of intact, healthy forest.

Kelp forests also have many other, incalculable values. They are hot spots of biodiversity, home to unique species that live nowhere else on earth. Kelp forests take their place in healthy, functioning seas and extend their influence far beyond the forest boundaries. Fast-growing kelp are enormously productive and generate huge amounts of energy for food webs around them. Broken chunks of kelp drift off and forge connections to distant ecosystems where animals pick over the forest’s remains. This is similar to terrestrial trees dropping their leaves and earthworms dragging them into their burrows, only instead of feeding animals on the forest floor, much of the food from kelp forests ends up hundreds of miles away.

Nevertheless, compared to many other wild places, kelp forests remain largely overlooked, understudied, and underappreciated. And just like forests on land, they’re vanishing at alarming rates because of habitat destruction and climate change combined—although rather than burning down, kelp forests melt away.

In 2011, when the coast of Western Australia was hit by a record-breaking marine heatwave, kelp forests there began to collapse in a matter of weeks. High temperatures were killing them outright. Prior to the heatwave, roughly three-quarters of the rocky reefs along the coast north of the city of Perth had been covered in thick stands of kelp. By the time temperatures fell back down, long stretches of coast were left deforested. This is the westernmost end of Australia’s Great Southern Reef, a five-thousand-mile-long interconnected fringe of rocky reefs dominated by kelp forests that wrap all the way along the southern coast of the continent, around the island of lutruwita (Tasmania), and north along the coasts of Western Australia and New South Wales. In that one heatwave, this enormous biome became at least seventy-five miles shorter.

Five years later, diving scientists resurveyed the Western Australian coastline and found a major shift had taken place at that shortened end of the Great Southern Reef. Where golden kelp had only recently been flourishing, tropical seaweeds now grew in its place. Instead of complex, lush forests, the seabed was covered in low-lying seaweed turfs, where animals have no place to hide and have far less food to eat. These new habitats contained a small portion of the rich biodiversity in the forests they had replaced.

Aiding this transformation were shoals of tropical fish that moved south with the heatwave and stayed. The pin-spotted spinefoot, a type of rabbitfish, is covered in white spots and has a gold stripe along its back; like all rabbitfish, it is a herbivore. Large shoals of these spinefoots are nibbling away at kelp spores as soon as they settle, thus preventing the forests from regenerating. As the influx of rabbitfish continues to sweep south, beyond the limits of the original heatwave, the seaweed turfs are expanding and pushing the kelp forests even farther back. The spreading of these simplified ecosystems diminishes the ocean environment in species and structural complexity, leaving coastlines more vulnerable to waves and storms, and fisheries stripped of critical habitat.

Turf wars are playing out across many kelp forests around the world. It’s one reason why around half of kelp forests are in decline globally. A mix of troubles is triggering these transitions, including declining water quality and the loss of key species. The changes can be abrupt, or they can unfold more gradually.

Sometimes, when the initial cause of kelp loss is halted and undone, the forests can regrow. That has been happening along the south coast of England, where a successful campaign led to a ban on trawling in 2021, and the kelp ecosystems began recovering.

In many cases, though, as happened in the wake of the 2011 Western Australian heatwave, kelp forests don’t spontaneously return. Too much is changing in these ecosystems, and they’re shifting into alternative states that are stubborn and difficult to reverse. Among many of the kelp forests that have been depleted and rubbed out in recent decades, it’s uncommon to see natural recovery taking place. Leaving lost forests alone to regrow by themselves will not be enough to bring them back.

Rewilding is becoming increasingly popular for terrestrial ecosystems. At the heart of rewilding lies the idea of putting back, and sometimes taking away, certain important species. Their presence or absence can help restore balance and kick-start the recovery of vanished ecosystems. These are keystone species, a concept that originated in Robert Paine’s 1960s studies of starfish on rocky shores. Rewilding efforts tend to focus on bigger animals and most often mammals, species like European beavers, which were hunted to extinction in the British Isles and are now, tentatively, being reintroduced from remnant populations on the continent. The beavers’ role is not merely to reacquaint people with a native species that’s been missing for centuries but to get busy using their huge buckteeth to chew down trees, dam rivers, and construct their lodges. Beavers create wetlands, which become homes for other animals such as water voles and fish. Their constructions slow the flow of water, reducing flooding downstream, which will become ever more important as the climate crisis delivers wilder, more frequent rainstorms.

In North America, wolves have become the most prominent rewilded species, following their reintroduction to Yellowstone National Park in the mid-1990s. The job for these apex predators is to hunt, and also simply to scare, the large populations of grazing elk, which in turn gives aspen and willow trees and the rest of the woodland ecosystem a chance to thrive.

For decades, rewilding has also been happening in the ocean, although it has rarely been called that. Some of the earliest efforts didn’t originally intend to create more resilient, biodiverse ecosystems but were meant to save one particular species from extinction.

Sea otters used to live along all the coasts of the North Pacific, from Hokkaido, Japan, to Baja California, Mexico. Each of the three subspecies—Asian, northern, and southern sea otters—has a distinct range. Their story pivots on the relatively recent return of their ancestors to the ocean, in the past three million years. Other marine mammals have been swimming for much longer—the whales and sea cows for around fifty million years, seals and sea lions for around twenty million years—and they’ve all evolved a thick layer of fat to keep them warm and to serve as an energy source when they dive. Perhaps in time, sea otters too will transition to blubber, but for now they insulate themselves just with fur.

And what incredible fur it is. With up to one million hairs per square inch of skin, otter fur is the thickest and most luxuriant of any animal. An outer layer of waterproof guard hairs keeps the under-fur dry, holding a layer of insulating air next to the skin, akin to emperor penguin feathers. To survive in the cold Pacific, sea otters must keep their fur in good condition, which is why they spend so much time grooming. For hours every day, sea otters float on their backs and rub themselves all over with their paws, fluffing up their fuzzy cheeks and reaching every point on their bodies thanks to their unusually flexible skeleton. With retractable claws they comb their fur and squeeze out water; then with their mouths they blow in air. Sea otter pups have such fluffy fur it acts as a life jacket, keeping them bobbing at the surface while their mothers dive down for food.

Indigenous peoples have long made use of the sea otters’ superdense fur to make clothing. But a commercial trade in their fur did not begin until Russian and European explorers encountered sea otters in the mid-eighteenth century, and hunters began to systematically wipe them out. Sea otters gather in single-sex groups known as rafts, where they groom and rest. Trappers would kill every otter in a raft before moving on to the next, leaving no animals behind to rebuild their numbers. The exploitation was more akin to strip mining than to any form of vaguely sustainable hunting.

By the end of the nineteenth century, the fur trade had collapsed, not through regulation but because otters had become so rare that hunting was no longer profitable. Across the Pacific, the population had crashed by 99 per cent. At most one or two thousand otters were left. Along thousands of miles of previous otter territory, there were none at all. In California, they were all gone except for a single group of around fifty otters, spotted off the coast of Big Sur in the 1930s, that had somehow gone unnoticed and uncaught. They were the last remaining members of the southern sea otter subspecies.

With the mass slaughter finally over, remnant populations of the three otter subspecies began to increase by themselves, slowly. People made attempts to speed up the process by moving sea otters from place to place. The Russians were the first to try this, in 1937, when they sent nine otters by ship and train from the Pacific Ocean to the Barents Sea in the Arctic Ocean, thousands of miles from their native range, planning to establish a new fur trade. Only two otters survived the journey, and commerce in their pelts never emerged.

Americans were more successful in their otter-moving efforts. They were driven by a different ambition, not to carry on killing them but to rescue the species from extinction. Initial attempts had mixed results, and many otters died in transit, especially before handlers worked out what to feed them. Frequently, as soon as otters were released in new environs, they would swim off, never to be seen again. Some swam all the way back to their original territories.

The first translocation that really worked was in 1965. In Prince William Sound, Alaska, forty-one otters were loaded into an amphibious aircraft and flown 450 miles to Shee Kaax (or Chichagof Island) in southeast Alaska. Roughly half of the otters survived the flight, and several were seen alive in the wild the following year. Numerous other translocations continued intermittently until 1990, and some have paid off. Sea otters once again roam parts of the coasts of Washington and British Columbia. In southeast Alaska, an initial population of 412 relocated otters has grown to around 25,000. It hasn’t worked everywhere: there are still no otters along the coasts of Oregon or Mexico. But overall, sea otters have been pushed back from the brink of extinction. Roughly 50,000 otters—a third of the current global population—are alive today thanks to these translocations.

Now, at various spots along the Pacific coast of North America, you can stand on the shore at dusk and watch a raft of sea otters twirl in the floating canopy of a giant kelp forest, anchoring themselves so they don’t drift away while they sleep, holding their four paws aloft to conserve body heat. And otters have been entwining their lives deeper into ocean forest ecosystems. Only when they were brought back did it become obvious how important otters are for keeping balance in the ocean. Their return inspired scientists to construct a new ecological paradigm around them. North Pacific kelp forests, as it turns out, need otters.

The otter-kelp connection comes down, once again, to this mammal’s relatively recent return to an aquatic life and its reliance on its fur. A fur coat is not enough to keep a sea otter warm in the cold Pacific. In addition, it relies on its incredibly high metabolic rate, which runs around three times faster than the metabolism of other warm-blooded mammals of a similar size. Leaky mitochondria, the energy-producing structures inside every cell, churn out heat from an otter’s muscles without any need for them to move or shiver, but this requires a lot of energy. To supply it, sea otters must do a lot of eating. Each day, they consume around a quarter of their body weight, and they’ve evolved to be highly skilled hunters. They’re the only marine mammals that catch prey with their paws and not their teeth, and they use stones to smash open mussels and clams and to whack abalones off rocks. They carry their favourite stone tools with them in pockets of loose skin in their armpits. Depending on what’s available, sea otters eat all sorts of fish and invertebrates, and they’re especially fond of purple sea urchins. They carefully pick them up with their tough paw pads and bite into the underside, where there are fewer spines, then slurp out the insides. Otters can eat so many urchins, their bones and teeth turn purple.^d

Otters eat urchins, and urchins eat kelp—lots of kelp. At first glance, a sea urchin is deceptively simple, little more than a spine-covered sphere gently squeezed and flattened top to bottom. On closer inspection, an elegant, pentaradial symmetry is revealed, showing these animals are obviously close relatives of starfish.^e Turn over an urchin, and there’s a star-shaped mouth with what look like five little bird beaks pinching together. This is the tip of a complex apparatus called Aristotle’s lantern, which is controlled by dozens of muscles that move the teeth in any direction the urchin chooses. This beautiful piece of biological engineering is the secret to an urchin’s tremendous eating abilities, allowing it to rasp at seaweeds lodged in nooks on rocky reefs.

The enormous appetites of otters and urchins tie both animals to the future of the ocean. When the fur trade demolished sea otter populations, urchin numbers exploded. Spiky armadas marched across the seabed, chewing through kelp stipes and swiftly converting former forests into devastated expanses known as urchin barrens. Anywhere from five to seventy-five urchins per square yard of seabed can flip an ecosystem from forest to barren.

When sea otters were gradually restored to their former territories, they helped drive urchin numbers back down, giving kelp spores a better chance of settling, allowing the forests to rise again. In Alaska and British Columbia, when otters came back, so did kelp. This is the kind of ecological cascade that people have been searching for among sharks and largely failing to detect. Sea otters, in contrast, have thoroughly proven their abilities as keystone predators. And their influence reaches even further. They are not only playing their part in bringing back lost ecosystems but also helping to make them more resilient to the changing ocean.

In central California, otters are boosting the resilience of eelgrass. Green blades ten to twenty inches long grow in dense meadows across Elkhorn Slough, a sheltered estuary that flows into Monterey Bay and lies at the heart of a watershed dominated by farmland where many of California’s orange groves and almond orchards grow. Over past decades, sales of artificial fertilisers have exploded, and the estuary has become overloaded with nitrogen and phosphorous. This has been causing algae to proliferate and grow all over the eelgrass, blocking sunlight from reaching the photosynthesising blades and causing the grassy meadows to die off.

When a few young male sea otters made their own way to Elkhorn Slough from the recovering wild population, they were joined by foundling otters that had been rescued and reared at the Monterey Bay Aquarium. Mother otters will abandon their pups if they can’t find enough food, or they sometimes get separated by storms or shark attacks. Young pups that wind up stranded and alone will soon die—unless concerned humans watch out for them and place them in the arms of captive female sea otters. Since 2002, surrogate mothers among the sea otters at the Monterey Bay Aquarium have cared for dozens of stranded pups, teaching them essential life skills, including how to groom their fur coats and use rocks as tools to crack open shellfish. (People watching wear amorphous black capes and welder’s masks to make sure the pups don’t grow accustomed to human presence.) More than thirty rehabilitated otters have been released into Elkhorn Slough where, rather by chance, they’re proving yet again their ability to help ecosystems recover after years of decline.

The otters’ influence on eelgrass ecosystems is more complex than their impact in kelp forests, involving more steps in the food web. Since sea otters have arrived in Elkhorn Slough, they’ve been eating a lot of crabs. A drop in crab numbers favoured the crabs’ prey, in particular eelgrass sea slugs, which crawl around eelgrass blades, slurping and chewing on the smothering algae. With more otters, there are fewer crabs and thus more sea slugs and less algae, and the eelgrass meadows are flourishing, even though the estuary’s waters are still burdened with farmland runoff.

To the north, off British Columbia, sea otters are encouraging eelgrass meadows to burst into bloom. Eelgrass and other types of seagrasses may look like seaweeds, but they are in fact types of flowering plants, as are terrestrial grasses and trees. As otters dig into the seabed searching for clams, they disturb the plants and stimulate them to produce flowers, release pollen, and set seed. The disturbed eelgrass is thus hedging its bets, sending its offspring into the ocean to find somewhere to settle and grow or to lie dormant as seeds in the seabed waiting for favourable conditions to return. In the process, parent eelgrass plants are mixing their genes and boosting the genetic diversity of their offspring.

When they’re left in peace, eelgrass and other seagrass species tend to reproduce by cloning. They split off copies of themselves and create perennial meadows that can spread for miles. These stands of genetically identical plants have in some regions survived for millennia, but their lack of diversity renders them vulnerable. As Charles Darwin figured out long ago, variation is the raw material that enables populations to adapt, evolve, and keep pace with their shifting environment. In parts of British Columbia where sea otters have been back the longest, more flowering has been taking place, and the eelgrass meadows are more genetically diverse—and where there’s more diversity, there’s more resilience to change.

Elsewhere in the world, from Iceland to Ireland, Japan to Australia, many kelp forests that never had any sea otters have also collapsed, and urchin barrens have taken their place. Other urchin predators have been overfished and overhunted, and warming seas are allowing urchins to expand their range and spread their influence. Urchin barrens can be relatively small, a few hundred yards across, and exist in ephemeral mosaics interspersed with patches of kelp forest. They can also be huge and persistent. Entire forests along thousands of miles of coastline have disappeared without a trace. Witnessing this, kelp restorationists are on the rise, and many of them have decided the best way to bring kelp forests back is to get rid of urchins.

A mix of conflicting ideas has come to surround these humble echinoderms. For millennia, humans knew about them from star-crossed rocks buried in earth that could so easily have fallen from the night sky. These were the fossilised remains of ancient urchin species that have existed in the ocean for hundreds of millions of years. Neanderthals collected fossil urchins and carved them into stone tools. Neolithic people buried them with loved ones, sometimes a single urchin clasped in the deceased’s hands, other times hundreds decorating a grave. Across Europe and North Africa, fossil urchins have been pierced to wear as necklaces, used as protective amulets, and lined up on window ledges or buried beneath the thresholds of houses to keep out the devil.

Many people today still think of urchins as animals to treasure. For Māori people in Aotearoa (New Zealand), kina urchins are taongo, a sacred species, as well as kaimoana, a traditional food. A delicacy in Japan, uni is either the testes of male urchins or the ovaries of females. Five in each shell are carefully scooped out and savoured for the creamy texture and the sweet, briny flavour. Orange and yellow uni apparently taste subtly different depending on where in the ocean they came from, similar to the terroirs of French wines.

However, the plight of kelp forests is transforming sea urchins from magical creatures and cherished foods into what many see as little more than spiny pests. There are roughly a thousand living species of urchins, which range from the tideline all the way down into oceanic trenches. Of those thousand, twenty are known to be powerful controllers of kelp growth, including purple urchins in California, black urchins in Chile, green urchins in the North Atlantic, and long-spined urchins in Australia. They can proliferate into swarms and bring down entire forests, and restorationists have tried various ways of wiping them out.

Caustic quicklime spread across the seabed is an effective means of killing urchins. This method was tested in California in the 1970s, but it also obliterates a lot of other marine wildlife. A more selective option is urchin smashing. Often this involves divers swimming down with hammers and bashing urchins to pieces. Stabbing them with knives also works. The fastest reported crushing rate for a diver is more than seventy urchins per minute.

It’s a crude technique, but it does seem to work. In one study off the coast of Hokkaido, Japan, a team of twenty volunteer divers spent two days clearing away urchins from about a half acre of seabed. Eight months later, Saccharhina and Undaria kelp were beginning to grow again. Off Vega Island in northern Norway, after scuba divers meticulously picked off urchins around a small, rocky islet, Laminaria kelp came back. But a diver can collect or smash only so many sea urchins. Most urchin-removal programmes have worked over small, experimental areas. To scale up efforts, a tech start-up company based in Atlanta, Georgia, plans to replace hammer-wielding divers with automated underwater robots equipped with artificial intelligence to detect and dispatch urchins.

Rather than wasting all these urchins and smashing them in the name of reforestation, another possibility is to eat them. If there’s one sure-fire method humans have implemented for effectively removing animals from the ocean, it’s to catch, sell, and eat them. The problem is that sea urchins roaming empty barrens are generally not good to eat. Once all the kelp has gone, these starved animals have shrivelled, unappetising gonads.

One workaround is to gather live urchins from barrens, then fatten them up until their gourmet body parts are ready to eat. An urchin ranch was opened in 2020 in Stavanger, Norway, followed by another in 2022 in Nagato, Japan. The urchins are kept in water tanks out of the ocean and fed on sustainably grown seaweeds; their gonads are destined for high-end sushi restaurants.

Sushi lovers will need to eat a huge amount of uni if they’re going to help keep the kelp forests intact. There are billions of urchins in barrens worldwide. And studies show that kelp forests are likely to return only if almost all the urchins are removed from the area. The urchin population needs to be pushed far below the number that stripped the seabed bare in the first place; the two tipping points, forwards and backwards, are not the same. This is partly because urchins can survive for a long time with nothing to eat. Nicknamed zombie urchins, they hang around ready to nibble any young kelp as soon as it starts to grow. And just like zombies in the movies, urchins in barrens just keep on coming. To maintain a regrowing kelp forest, it’s usually not enough to clear away urchins just once; restorationists need to keep smashing or removing new arrivals over and over.

Killing thousands upon thousands of urchins is undoubtedly a desperate measure and a dismal sign of how drastically ocean ecosystems have shifted out of balance. Urchins have been vilified as an invading scourge, but they’re guilty of nothing more than surviving in the Anthropocene and coping with everything humanity has thrown at them. Wiping out urchins en masse does nothing about the underlying problems of rising temperatures and predator loss, which are worsening and complicating each other.

Humans can kick-start the kelp recovery process by clearing away an urchin swarm—as carefully as possible—to pave the way for wild predators to take over and maintain the longer-term balance. For that to happen, there need to be healthy populations of animals that eat urchins, such as triggerfish, wrasse, crabs, and lobsters, which do much better inside well-protected marine reserves. A study of two of Aotearoa’s no-take zones, including the original one set up at Cape Rodney–Okakari Point by Valentine Chapman and Bill Ballantine in the 1970s, showed that urchins are much more likely to get eaten by predators inside reserves than outside. The marine reserves contain blooming forests of kelp and other seaweeds thanks to the large, abundant silver seabream, blue cod, and spiny lobsters that are keeping urchin numbers in check and discouraging the spread of barrens. Highly protecting more of the ocean from fishing and hunting will give more kelp forests a chance to thrive.

Now that sea otters have proven their abilities to engineer more resilient kelp forests, various conservation groups along the coast of the Pacific Northwest region of North America have started thinking about a new wave of otter reintroductions. In 2021, the US Congress officially recognised sea otters as critical for ocean ecosystems and instructed the Fish and Wildlife Service to investigate the feasibility of bringing back otters to more parts of the coast where they’re still missing more than a century after fur hunting ended.

One proposal is to move otters into the biggest remaining gap in their historical range, reaching north from San Francisco Bay into Oregon. California otters are not moving north into these areas by themselves, maybe because they would have to run the gauntlet of great white sharks that are shifting northwards as coastal waters warm. Great whites don’t normally prey on sea otters. They’re not nearly fatty enough to be worth eating. But one investigative chomp is easily lethal for a small marine mammal. Dead otters with shark bites have been washing up on California beaches in increasing numbers. What’s more, otters are more likely to be picked off by sharks when they leave the shadowy canopy of kelp forests. In areas where kelp is dwindling, shark-bitten otters are far more common than they are in places where forests are still intact. As the ocean changes around them, otters may well need kelp forests just as much as the forests need otters.

If otters were returned to more of their former North American range, they would play a part in the fight against the climate crisis by helping underwater seagrass meadows and kelp forests flourish, and in the process harness more carbon dioxide from the atmosphere. These otter-renewed habitats would have the added benefit of nurturing local fisheries by welcoming in new generations of young fish.

People living near coastlines where otters could be reintroduced are generally in favour and want to see the species back in their neighbourhoods. Among the indigenous peoples who have shared their views on the matter, many are enthusiastic and hope the otter reintroductions will help restore balance to the ocean. The Confederated Tribes of the Coos, Lower Umpqua, and Siuslaw Indians in Oregon describe feeling a kinship with otters, which were once an important part of the ocean environment.

By contrast, voices of concern are coming from some communities involved in fishing for species that sea otters like to eat, including crabs, sea urchins, and abalone. Conflict has raged on and off for decades between environmentalists and fishers over whether sea otters should be encouraged or even allowed at all in California waters. Though fishers today generally appreciate the critical role otters play in ecosystems, in the past some claimed otters have an unfair advantage because they don’t have to abide by seasonal restrictions or catch limits and can help themselves to as much precious shellfish as they want.

The future of kelp forests is not all about otters and urchins. Up until a couple hundred years ago, enormous Steller’s sea cows lived in the kelp forests of the North Pacific and may well have boosted the ecosystem’s resilience. These thirty-foot-long giants had so much blubber they couldn’t dive down, and so they spent their time floating near the surface browsing on kelp. This would have opened up the canopy and allowed more light to pass through to the understory, likely allowing the seabed to flourish with a garden of colourful seaweeds. Today, in darker, denser forests, where there are no more floating megaherbivores thinning out the canopy, seaweeds struggle to grow on the seabed.

A speculative idea, based on mathematical models, hints that these sea cow–enhanced forests may have coped better with heatwaves compared to modern kelp forests; the understory seaweeds could have helped feed urchins and dampened their tendency to fell whole forests. Nobody is seriously considering genetically engineering massive manatees or dugongs to replace the absent Steller’s sea cows. But one group of scientists is suggesting it might be worth trying to mimic the sea-cow effect in some other way, perhaps by selectively cutting away the upper fronds in kelp forests.

More realistic is the return of a lost species that has been missing from kelp forests for only a few years. Sunflower sea stars are the size of bicycle wheels, among the biggest of all the sea stars.^f They have velvety-looking skin and dozens of golden, red, or purple arms. Draped across the seabed, they don’t look like formidable hunters, but when they sense food, they can show a turn of speed, using thousands of tube feet to glide along by a body length per minute—fast for an echinoderm.

Fellow native species of the North American Pacific coast, sea otters and sunflower sea stars used to work together to keep sea urchins in check. The furry mammals generally bother catching only the most nutritious urchins, those larger than a baseball. These are too big for sunflower sea stars, which instead go for urchins a finger’s width across or smaller. The sea stars are such effective predators, little urchins learned to hide under the spines of bigger urchins, presumably where they won’t be noticed. By devouring urchins of different sizes, the otters and sea stars effectively prevented kelp forests from being overgrazed.

But in 2013, sunflower sea stars, along with dozens of other sea star species, were afflicted by a horrifying plague. Sea star wasting disease, as it was later named, caused a stricken sea star to break out in skin lesions, its internal organs spilling out of its body and its arms falling off, with no chance of regrowing new ones. Millions of sea stars wasted away, leaving behind smears of sickly slime and piles of white bony ossicles, the structures that form their spiny exoskeletons.