Invertebrates
Sunflower Sea Star
Pycnopodia helianthoides · also called sunflower star, twenty-armed star
The sunflower sea star is a metre-wide, 24-armed predator that used to prowl every kelp forest from Alaska to Baja, fast enough that urchins fled from it. Then, after 2013, it almost disappeared — and the kelp forests fell with it.
Overview
Pycnopodia helianthoides lived throughout the northeast Pacific, in kelp forests and on rocky and soft bottoms. It hatches with five arms and adds more as it grows, reaching 16–24, and spans about a metre. Running on some 15,000 tube feet, it moves about a metre a minute — fast for a sea star — hunting urchins, snails, clams, and other sea stars.
The collapse
From 2013, a sea star wasting epidemic spread along the Pacific coast, causing stars to lesion, go limp, and disintegrate within days. It struck ~20 species; the sunflower star was hit hardest, losing an estimated 90–95% globally and vanishing from California, Oregon, and Washington. A marine heatwave made it worse.
Kelp forests fell too
The sunflower star was the main check on purple sea urchins. Without it, urchins boomed and grazed down the kelp, turning long stretches of coast into bare urchin barrens — Northern California lost ~95% of its bull kelp.
Status
Critically Endangered since 2020. Captive breeding has succeeded, and trial reintroductions of disease-resistant stars are beginning, but full recovery would take decades.
Sunflower Sea Star: common questions
What is sea star wasting disease?
It is a syndrome, first noticed on a large scale in 2013, in which affected sea stars develop white lesions, lose grip and body tension, twist and pull their own arms off, and then dissolve into a slurry over a few days. It swept the Pacific coast of North America from Mexico to Alaska and affected around 20 species, killing billions of animals in what is considered one of the largest wildlife die-offs ever recorded. The cause has been debated; a densovirus was implicated early on, that link was later questioned, and current thinking points to a combination of a pathogen or pathogens and environmental stress, especially the warm water of the 2014 to 2016 northeast Pacific marine heatwave, which seems to have made the disease far worse.
How did losing one sea star species damage kelp forests?
The sunflower star was the main predator keeping purple sea urchin numbers in check along much of the coast, alongside sea otters in the areas that still have them. When the sunflower stars died, urchin populations, no longer controlled, exploded. Urchins graze kelp, and at high density they mow down whole kelp forests and then persist in a starved, dormant state on the bare rock, an "urchin barren", eating any new kelp before it can establish. Northern California lost an estimated 95 percent of its bull kelp within a few years. Kelp forests are among the most productive habitats on Earth and support hundreds of species, so the knock-on effects have been large.
Can the sunflower sea star recover?
It may, but not quickly on its own. Small numbers survive in the northern part of the range, and there is hope that some individuals carry resistance to the disease. Research institutions in Washington and elsewhere have succeeded in spawning and rearing sunflower stars in captivity, a first for the species, with the aim of eventually releasing disease-resistant animals to seed recovery and help rebuild kelp forests. The species was proposed for listing under the US Endangered Species Act in 2023. Full natural recovery across the range, if it happens, is expected to take decades.
Is it related to the crown-of-thorns starfish?
Both are sea stars, but they are not close relatives and they play opposite roles. The crown-of-thorns is a coral-eating sea star of tropical Indo-Pacific reefs whose population outbreaks damage corals, and it is armoured with venomous spines. The sunflower star is a cold-water northeast Pacific predator of urchins and shellfish, soft-bodied and not venomous, whose collapse has damaged kelp forests. One is a problem when there are too many; the other is a problem because there are now far too few.