Sandy Molnar
In the shadowy ocean floor off the coast of California, killer sunflower sea stars race across the sea bed at a staggering seven meters per minute, ferocious and hungry. The beasts flay their prey, their many arms moving with unsettling grace, ripping apart purple sea urchins, spines scattering like whispers of dread through the water.
Unfortunately, these killing stars have had their lights dimmed; their population was decimated by sea star wasting disease, a mysterious pathogen that appeared after the unprecedented 2013/2014 Pacific Ocean heat wave known forebodingly as “the Blob.” The disease, which gruesomely eats away at sea star flesh, progresses rapidly and can lead to death in just a few days. It has devastated at least 20 sea star species since it appeared in 2013, including the sunflower sea star which has lost over 90% of its overall population and is the first sea star species to be listed as endangered. Now, they are thought to be locally extinct south of Northern California, and even in their lightest-hit region in Alaska, the population has dropped 60%.
These killer sea stars are no average echinoderm. They are the key species that were keeping kelp forest ecosystems in check before their decline. Sunflower sea stars prey on purple sea urchins, a species whose main prey is bull kelp: the organism that provides the habitat of kelp forests across the western coast of North America. Without the sea stars to keep them in check, purple sea urchin numbers have exploded, and they’ve mowed down kelp forests in devastating numbers, leaving behind a barren ocean wasteland.
The devastation of these kelp forests has had wide-reaching implications. Kelp forests sequester up to 20 times as much carbon per acre as terrestrial forests, helping to mitigate some of the effects of climate change by removing greenhouse gases from the atmosphere.
They similarly remove nitrogen and phosphorus pollutants from the water column, a venture that can potentially save millions of dollars in water treatment. In Europe, kelp has been used to fertilize, increase crop yields, and treat illnesses. Most of all, however, kelp forests provide critical three-dimensional habitats for thousands of species, including snails, sea otters, sea lions, harbor seals, elephant seals, sea stars, sea cucumbers, crustaceans, hundreds of fish, and even gray whales. Many of these species rely on and cannot survive without kelp forests. Without sunflower sea stars, kelp forest ecosystems have crumbled across the western coast of North America. In Northern California, only 4% of kelp forests remain as compared to the area 10 years ago.
Some initiatives have sprung up to save these essential kelp forests, but it has not been easy to replant the bull kelp. Unfortunately, purple sea urchins often stay behind in an emaciated, "zombie" state, and new frond growths are often immediately munched. Even other predators, such as sea otters, which are common in Monterey Bay, don't prefer the starved sea urchins, and leave dead kelp forests behind to prey elsewhere.
Conservation teams in Mendocino County have stepped in for the missing sea stars to remove the urchin fiends. Currently, the accepted method is hand harvesting with vacuums - an approach that can be dangerous due to cold, remote, and treacherous conditions in Northern California waters. Some researchers are experimenting with traps, which are safer. In Humbolt Bay, another team of researchers is testing whether kelp forests can be farm-grown. So far, the results are promising.
But this farm-grown kelp cannot be transplanted to the wild without the proper protection from our killer sea stars. At the University of Washington, marine biologist Jason Hodin is championing a lab to spawn and grow sunflower sea stars in the wet labs at Friday Harbor, Washington. The project has been ongoing since 2019, with now 16 fully grown adults having produced more than a hundred living offspring. They live in plastic tubs filled with filtered seawater until they are big enough to move to the outdoor tanks.
Uniquely, these sea stars have gonads in each of their arms. After trying less invasive methods to induce the development of sperm and eggs, researchers have turned to amputating an arm to release the cells. The sea stars don’t mind too much - they grow them right back. In the wild, they can drop their arms in the face of a threat the same way lizards drop their tails. If a part of the central disk is attached to the arm, even a whole new sea star may grow.
The sea star offspring start as microscopic plankton, floating in the water column, before settling on the ocean (or tub) floor as a tiny five-pointed star about the size of the tip of a ballpoint pen. They grow extraordinarily quickly, adding limbs clockwise as they go until they reach their adult size - up to a meter arm span - with between 15 and 24 arms.
Hodin predicts a unique mingling behavior of intertwining around each other could be part of why the disease transmits so readily in this species, even compared to other affected species of sea stars. Unfortunately, even ten years later, little is known about the disease that wiped them out. Originally, the cause was thought to be a densovirus, but this was later debunked, and other theories are now being explored. Warm water, which we saw a large rise in after the Blob, seems to exacerbate symptoms. Morphology of sea stars may also play a part and could help explain why some species were more heavily impacted than others. Researchers are understandably hesitant to release the lab-raised stars into the sea before the disease is better understood, so their hard work isn’t wiped out. Epidemiology labs continue to study the disease.
Nonetheless, there are more and more glimmers of hope in this story. Many populations of sea stars have had significant juvenile input since the original crash ten years ago. Some species even saw a baby boom after the initial dieout. Observations of sunflower sea stars in particular have been trending upwards in both California and Oregon over the last two years. While typically only 1-3 stars are observed in one area, a recent dive in Oregon found as many as 60-70 juveniles in one area.
If you are near the West Coast and want to contribute, you can send observations of both healthy and sick sea stars to UC Santa Cruz using their observation log. Researchers can’t be everywhere - sunflower stars have not been seen in marine monitoring plots since 2015, but citizen observations tell us where they still are. Your contributions could make a difference in bringing back these killer sea stars from the brink of extinction and saving our kelp forests.
Citations
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