A mysterious worm threatens the future of Washington oysters
A dozen twists knife were all it took to tarnish the spotless reputation of Washington oysters. It was 2017, and Teri King, an aquaculture specialist for Washington Sea Grant, a marine research institute, had been asked to shell shellfish at a seafood event in Shelton, Washington. She was there to teach people about the local oyster industry, which is prized for producing delicious half-shells with perfect pearly white interiors. But his lesson soon took a dark turn. As she stuck her knife under the lip of an oyster, she split a hidden blister inside the shell.
King watched in disbelief as the black slime bled into the raw meat. “I don’t know what’s going on here,” she recalled telling her audience. “But let’s get you some better oysters.” Much to his embarrassment, it continued. It took 13 or 14 oysters before she finally produced a presentable half shell. King had noticed these blisters occasionally during her 30-year career, but she had never seen so many at once.
From Massachusetts to the Gulf of Mexico, it is common to find oyster shells in the United States marred by dark blisters and burrows – the scars of shell-boring mudworms. The most commonly recorded is Polydora websteri. Measuring approximately three quarters of an inch, P.websteri makes its home by digging into oyster shells and sticking its palps into the ocean for food. In response, the oyster secretes a fragile layer of shell between itself and the invader, like an older brother dividing a common room. Behind this barrier, the worm continues to burrow more space, creating a bubble that gradually fills with its own waste and sludge from the seabed. Mudworms generally do not kill oysters or poison meat for humans. But they can stunt the growth of oysters and the marketability of tanks.
nobody knows where P.websteri origin, but over the centuries it has circled the globe with the global oyster trade, triggering major infestations that have collapsed half-shell businesses in South Africa, Australia, New Zealand and in Hawaii. More recently, the parasite has been reported in Oregon, California and British Columbia. Washington, however, seemed unaffected, a half-shell haven. Today, the majority of oysters in the state are sold raw, and King knew an invasion of mudworms could upend the market. In the absence of previous scientific reports of the parasites in Washington, however, she could not say if this was a recent invasion, or if a small number had been here for centuries and were only proliferating now due some changes in the environment. King decided to call Chelsea Wood, a parasite ecologist from the University of Washington.
In 2018, King, Wood, and Julieta Martinelli, a postdoctoral researcher in Wood’s lab, started a project to assess P.websteri’s spent in Washington, using the structure and chemistry of oyster shells as a record of marine conditions over time. They started by mapping the distribution of modern mudworms along the west coast of the United States. From California to Alaska, more than 30 oyster farmers and tribes donated shells. With careful tweezers, the researchers extracted mudworms from 25% of the samples. Washington oysters were no exception: in parts of southern Puget Sound, the infestation rate was as high as 53%.
But there was a difference. Almost all West Coast worms had DNA that matched P.websteri, suggesting species invasion. In Puget Sound, Hood Canal, Samish Bay and Willapa Bay, however, about three-quarters of the blisters came from a mysterious parasite, which matched no other mudworm species on record. At least not the records kept by humans.
In the fall, Martinelli invited me to Wood’s lab to see the blisters for myself. Opening the top drawer of a closet, she flipped through bags of seashells like files in a filing cabinet, squinting through the clear plastic. “Hey, I don’t know if you can see them,” she said, opening a bag and shaking a seashell. “It’s crazy how your eye is trained.” On the inside rim, I could just make out a black pinprick that Martinelli identified as a mudworm burrow. Picking up another shell, she showed me the next, more obvious stage of infection, which looked like a small charred blister on a pizza crust. Martinelli could not tell by sight if it was caused by P.websteri or the mysterious parasite. But, in some ways, it didn’t matter: either one can cause significant damage.
To track the infestation, Martinelli and his colleagues made “wanted” signs showing bloodworms and their markings, so oyster farmers could contact them if they see any. The posters will be hung in peeling halls along the West Coast. Chris Burns, a natural resources technician for the Jamestown S’Klallam Tribe, told me he had no knowledge of the pests until Martinelli showed him the blemishes. “They’re there, but they’re not really a problem at the moment,” he said. “It’s not that they couldn’t become one.” While Tribe Beach in Sequim Bay has relatively low infestation rates, in other areas blisters are a bigger problem.
Martinelli suspects that these hotspots present environmental conditions that best suit the parasite. She is now comparing shorelines to see if factors such as farm density, ocean temperature or tidal heights can explain the recent rise. But it’s hard to draw conclusions without a historical point of reference. Martinelli needed to compare modern conditions to those of the past.
Fortunately, ancient oyster shells are an unrivaled archive. Not only can seashells retain traces of shell-boring parasites for more than half a billion years, but they also faithfully record changes in the marine environment. As an oyster grows, it extracts dissolved calcium and bicarbonate from the water to create a calcite shell. This process forms periodic growth lines from the inside out that keep chronological records of water temperature, acidity, salinity and turbidity, similar to tree rings. trees record weather history. By crushing and chemically analyzing oyster shells from different eras, it is possible to follow the changing conditions of an ecosystem, revealing increasing pollution, warming waters, increasing acidification or the invasion of species, which might help explain when mudworms arrived in Washington and why they’re suddenly flourishing.
But in a tsunami-prone region like the Washington coast, oyster shells are easily washed away. For months in 2019, Martinelli scoured the shore for ancient samples to no avail. After hearing about his struggles, the Jamestown S’Klallam tribe presented him with a collection of thousand-year-old seashells from a cooking pit near their shore.
Today, most Washington beach oysters are invasive, introduced from Japan just over a century ago. Populations of the only native oyster, the Olympia, have fallen by more than 95%, due to overexploitation and pollution. Now, however, several tribes are trying to revive Olympia oysters to restore the foundations of their shores. Burns, who leads the Jamestown S’Klallam Tribe Restoration Project, hopes Martinelli’s research can provide restoration goals for the tribe. Ancient seashells, for example, could reveal Sequim Bay’s water quality before a 19th-century mill smothered its native oysters with floating logs and sinking bark.
Martinelli found only one other collection of ancient oysters. One day, a local geologist informed her of a raised stretch of shoreline near Hood Canal. Guided by a map and GPS coordinates, Martinelli carried 10 buckets to the Theler Wetlands Belfair nature reserve. After meandering along a series of floating boardwalks, she jumped into a small stream. Weaving through the shallows, she noticed something shiny in the muddy banks. With the tip of her boot, she discovered a layer of oyster shells polished by a thousand years of erosion. Back at the lab, Martinelli began preparing the shells for chemical analysis to determine what Olympia’s habitat once looked like. It was then that she noticed dark marks around the edges of the hull. To the naked eye, they looked exactly like modern oyster burrows.
Historically, parasites have been considered a drain on ecosystems. But recent research suggests that they are in fact important managers of natural communities, keeping food webs, biodiversity and key species, including oysters, in balance. Parasitism is arguably the most widespread way of life on Earth. Yet scientists estimate that up to 95% of parasitic worms remain undescribed.
Calling the new mudworm in Washington a mystery is therefore a little misleading: all mudworms are mysteries. “We have almost no information about what happened with wildlife diseases in the past,” Wood told me. “Almost none.” Martinelli and Wood don’t yet know if the mysterious worm is responsible for the burrows of ancient Olympias. To figure this out, they plan to scan ancient and modern oyster burrows and compare their characteristics. If the mysterious worm has really been around for millennia, then the next question is: Why did we just notice it?
It’s unclear how Washington oysters will fare if mudworms continue to proliferate. Even though the mysterious worm turns out to be native, the habitat it evolved in has been altered by centuries of human influence. Groups of invasive oysters are now concentrated on the Washington coasts, where ocean acidification could weaken their shells. The records they create in the decades to come could tell a whole new chapter for Washington’s shores and its oysters.
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