The 24-armed sea star coming back to save California's kelp forests

Off the coast of Monterey, California, divers are lowering a many-armed predator onto the seabed that has been missing from these waters for the better part of a decade. The scientists watching from the surface believe its return could decide whether the region's kelp forests survive.

The animal is the sunflower sea star, a starfish with up to 24 arms that was once counted in the billions along North America's Pacific shore. Disease erased more than 90 per cent of them, and without their chief predator the purple sea urchin population exploded, stripping the kelp forests bare. Now laboratory-raised stars are being released onto the ocean floor off California in the first controlled trials of their kind.

The 24-armed plan

The effort is being led by the Sunflower Star Laboratory, a nonprofit organisation based in Moss Landing, California, whose entire mission is breeding the critically endangered stars for restoration. Co-founders Ashley Kidd and Reuven Bank told the Guardian that the stars — which they note can carry up to 24 arms — simply need to be brought back.

The trial animals began life far from Monterey. They were raised from eggs spawned at San Diego's Birch Aquarium on Valentine's Day in 2024 — a batch nicknamed the Cupid Cohort — and grown in laboratory tanks until they were robust juveniles. This summer the laboratory took 16 of them out to Tankers Reef in Monterey Bay.

In the first phase, the 16 stars spent a month inside enclosed pods set down on an urchin barren. The stars ate voraciously and put on more than half their body mass on average, while 99 per cent of adult urchins within three feet of the cages moved away — an effect that persisted for weeks after the stars were removed.

The second phase tested the stars in the open. Divers supervised 18 uncaged "walkabouts" in which the 2.5-year-old stars roamed freely for up to two hours at a time. These were tests of release and monitoring methods rather than a genuine attempt to seed a wild population, but they are the first known free-roaming sunflower sea stars in Monterey Bay since 2018. One particularly energetic individual travelled more than 50 metres in a single two-hour session, a promising sign for how far released stars might range.

The urgency is written in the numbers. As Bank told the Guardian, "The known number of sunflower stars in Monterey Bay is zero."

Why it matters

Kelp forests are often called the redwoods of the sea. The giant algae grow fast — up to two feet a day — and their towering canopies shelter thousands of marine species, from the small fish that hide among the fronds to the larger animals that hunt them. The forests are nurseries for commercially important fish, which means their collapse is a fisheries problem as well as an ecological one.

They are a climate asset too. Kelp forests worldwide draw down roughly 200 million tonnes of carbon dioxide a year, pulling planet-warming emissions from the atmosphere to fuel their growth. Losing them does not just strip the coast of its habitat — it erases a working carbon sink.

There is also a plain economic case. Harvested kelp is a juggernaut of an industry worth billions of dollars, its extracts ending up in products as varied as toothpaste, shampoos, puddings and cakes. Fishers who work the coast have backed the sea star recovery effort, recognising that healthy kelp forests support their livelihoods.

Researchers estimate that about 95 per cent of the region's kelp forests have been damaged or lost in the past decade, according to kelp-watch monitoring data — a collapse driven largely by urchin overgrazing.

How the kelp forests collapsed

The disaster began around 2013, when an unprecedented marine heatwave warmed the waters of the west coast. Into that stressed environment came sea star wasting disease: lesions on the skin, twisted arms, limbs detaching and the body dissolving into a shapeless mass. It became the largest documented disease event in a non-commercial marine species.

The toll was almost incomprehensible. More than five billion sea stars died along the Pacific coast of North America across more than 20 species. The sunflower star was hit hardest — an estimated 90 per cent of its population vanished in the first five years, and in the southern part of its range the losses reached 99 per cent. The coastline from Alaska to Baja California had once supported around six billion of them.

With the stars gone, their main prey was suddenly free to multiply. The purple sea urchin can live for 70 years and produce upwards of 500 million eggs in a lifetime. Freed from predation, numbers detonated: one count found 350 million purple urchins on a single Oregon reef, an increase of more than 10,000 per cent since 2014. The grazing armies converted lush forests into "urchin barrens" — bare rock dotted with starving urchins that can persist for years in an emaciated state, locking the system in its altered condition.

Satellite analysis of US Geological Survey Landsat imagery going back decades confirmed the scale: more than 95 per cent of northern California's bull kelp canopy vanished. The forests had survived earlier heatwaves and El Niño events, but warming water plus the removal of the sunflower star — the kelp's only real urchin predator — proved fatal.

For ten years the cause of the wasting itself eluded researchers — an early theory blaming a densovirus was discarded when the virus kept appearing in healthy animals. The breakthrough came in the summer of 2025, when a four-year investigation led by the Hakai Institute with the University of British Columbia and the University of Washington identified a bacterium, Vibrio pectenicida, as the cause. Published in Nature Ecology & Evolution in August 2025, the finding finally gave the recovery effort a known enemy — and evidence that warming oceans help the pathogen thrive.

The science of urchin control

The sunflower sea star earns its reputation as the wolves of the sea. Using an overwhelming battery of tube feet, it reels in a purple urchin and consumes it whole, leaving behind only spines and an empty skeleton. Research from Oregon State University added a telling detail: the stars are so eager for urchins that they will even eat the nutritionally worthless, starving "zombie" urchins found in barrens — exactly the animals that otherwise lock the ecosystem in its ruined state.

But the greater power of the star may be psychological. Urchins can detect the predator's approach through chemical cues in the water, and they react by hiding and sharply reducing their feeding. Researchers describe a "halo of fear" around each star, within which urchins cut back grazing for nearly five metres in every direction. In the Monterey trials the effect was dramatic: virtually all adult urchins near the cages fled, and their absence persisted for weeks.

The implication is that recovery does not require the stars to eat every urchin. Their mere presence may buy young kelp the breathing room it needs to take root and grow — which is why a modest number of reintroduced predators could, in theory, tip a whole reef system back toward forest. Long-term field studies in California suggest that once sunflower stars reach sufficient densities, their dampening effect on urchin numbers can even offset the warming-driven boost that heat gives urchin population growth.

What happens next

The Monterey work is deliberately a stepping stone. The laboratory describes the caged and walkabout trials as experiments in behaviour and method — how lab-raised stars move, how they grow, how urchins respond — rather than the start of a wild population. If the trials succeed, the team hopes to reintroduce the species to the bay permanently within the next few years; Bank has said the transition to permanent outplanting could come within one to three years.

The state is beginning to put money behind that ambition. At its meeting on 30 September 2025, the California Ocean Protection Council approved more than half a million dollars for reintroduction work alongside The Nature Conservancy, the California Academy of Sciences and the state Department of Fish and Wildlife. Readers comparing the scale of conservation grants across countries can translate the figures into their own currency with the currency converter.

The California effort is part of a wider arc. Researchers at the University of Washington's Friday Harbor Laboratories are raising hundreds of stars in seawater tanks, learning how to breed the little-studied animals and even cure them when wasting disease gets into the lab. In British Columbia, the Snuneymuxw First Nation is leading an Indigenous-run pilot on eastern Vancouver Island — backed by the 2026 cohort of the Colossal Species Reintroduction Fund — building the broodstock and genetic baselines that future releases will need.

Obstacles remain. Wasting disease is still active in the wild — a planned October 2025 release in Washington's San Juan Islands had to be called off when the disease surged through the area. The stars are listed as critically endangered by the International Union for Conservation of Nature but hold no federal endangered-species status in the United States, and any reintroduction will need disease-resilient, genetically diverse animals if it is to last.

Still, Bank argues the logic is simple: "Restoring the sunflower star," he told the Guardian, "is the single most important step you can take to saving our beleaguered kelp forests here in California." In Monterey Bay, where the known wild count is zero, every star returned to the seafloor is the start of something the coast has not seen in years.

FAQs

What exactly happened in the Monterey Bay trials?

The Sunflower Star Laboratory placed 16 laboratory-raised sunflower sea stars in enclosed pods on an urchin barren at Tankers Reef for a month, then carried out 18 supervised free-roaming sessions in which the 2.5-year-old stars explored the seafloor for up to two hours. One star travelled more than 50 metres in a single session. These were the first free-roaming sunflower sea stars recorded in Monterey Bay since 2018, in a bay whose known wild population is zero.

How can sea stars control urchins without eating all of them?

Through what researchers call a "halo of fear": urchins detect the predator's chemical scent and respond by hiding and cutting back on feeding within a radius of nearly five metres. In the Monterey cages, 99 per cent of adult urchins within three feet moved away, and the deterrent effect lasted for weeks after the stars were removed. This means even a modest number of stars could give young kelp enough undisturbed growing time to re-establish.

What caused the mass sea star die-off, and has the threat passed?

Sea star wasting disease killed more than five billion sea stars across 20-plus species starting in 2013, and only in 2025 did a Hakai Institute-led team identify the culprit: a bacterium, Vibrio pectenicida, with findings published in Nature Ecology & Evolution. The threat has not passed — a planned October 2025 release in Washington's San Juan Islands was called off when wasting disease surged through the area. Future reintroductions will depend on screening for disease-resilient animals.

Why are kelp forests worth this much effort?

Beyond shelter for thousands of marine species, kelp forests serve as nurseries for commercially valuable fish and draw down about 200 million tonnes of carbon dioxide globally each year. Harvested kelp underpins a multi-billion-dollar industry spanning food, cosmetics and pharmaceuticals. Northern California alone has lost more than 95 per cent of its bull kelp canopy, so the stakes span ecology, climate and the regional fishing economy.

When will sunflower sea stars be permanently returned to the wild?

The Sunflower Star Laboratory says permanent outplanting in California could begin within one to three years if the current trials validate their methods. A December 2025 cage trial in Monterey Bay saw 47 of 48 juveniles survive, and the California Ocean Protection Council approved more than $500,000 in reintroduction funding in late 2025. The main outstanding risks are the still-active wasting disease and securing federal protection for a species listed as critically endangered by the IUCN but not by the US government.