a variety of shellfish susceptible to yessotoxins
Teri King
Not only are oysters susceptible to yessotoxins, but so are other shellfish like clams and mussels.

Around 50 years ago, Pacific oysters in the Puget Sound started dying at noticeably increasing rates during the summer, causing residents and scientists to wonder why. Researchers in what is now the School of Aquatic and Fishery Sciences investigated many factors that may cause mortalities such as bacteria, reproductive stress related to spawning and changes in other environmental conditions.

The evidence collected pointed towards stress on the animals when they spawn, as the Pacific oyster spends a lot of their energy on reproduction. To help alleviate that stress, the team—led by Stan Allen and Sandra Downing, then graduate students of UW fisheries professor Kenneth Chew, in collaboration with the West Coast shellfish industry—created the triploid oyster, a genetic modification that prevents oysters from reproducing. An added bonus was it produced an “all-season” oyster, meaning the oysters would not be mushy with egg or sperm, a distasteful attribute to those who like oysters on the half-shell in the summer during spawning season.

Yet despite this successful work, oysters kept dying during the warm summer months on farms in Puget Sound. The hunt for the reason continued with more research, again looking closely at bacteria and other environmental factors with no conclusive findings.

clams killed by yessotoxins
Teri King
Countless clams and oysters killed by yessotoxins in Puget Sound.

Fast forward to the summer of 2019. Teri King, who was once a student assistant in the Chew Laboratory and is now an aquaculture and marine water quality specialist at Washington Sea Grant, found herself back at the same site she frequented when collecting samples on the triploid oyster project in the 1980s. She was contacted by residents and shellfish farmers to help figure out strange behavior exhibited by the clams in the bay; they were pushing themselves out of the sediment onto the surface where they died within a tide or two. King observed that no birds or other animals that normally scavenge dead or dying marine life were consuming this easy prey. Shoreline residents had observed a similar pattern.

King gathered samples from the dying clams and oysters, and nearby water samples too. From there, she had the information to start piecing the mystery together. Her team identified high concentrations of yessotoxins produced by the phytoplankton Protoceratium reticulatum which grows naturally in the bay but with a sinister ability to kill shellfish.

“This bay took my dignity as a student in the 80s when I literally sunk waist-deep into the bay with my lab book floating out to sea for the oyster triploid project, but gave my dignity back to me as a researcher when I found a yessotoxin link to summer mortality events,” said King.

An overlooked killer

Yessotoxins were not a new concept to King, who heard about them previously in a food science class. Discovered in 1986, yessotoxins were the new kids on the block in the phytoplankton and toxin world, and often overlooked even though they could kill massive amounts of shellfish particularly during the warmer summer months when they bloomed. As other causes of mortalities began to be ruled out, investigators finally began considering biotoxins such as yessotoxins.

cysts that release yessotoxins
Teri King
Tiny cysts from the phytoplankton that reslease yessotoxins.

Yessotoxins in Puget Sound are released from a species of microscopic plankton known as dinoflagellates, which are able to swim in the water using two tails. When conditions are not good for the phytoplankton’s survival, they are able to drop out of the water column into a cyst formation and spring back to life when conditions are right. Many species of dinoflagellates produce cysts, which set them apart from other algae by making them highly resistant to stressful environmental conditions.

How yessotoxins affect oysters

Shellfish, such as clams and oysters, consume phytoplankton and other particles floating freely in the water. In the process, they filter massive amounts of water which makes them a key player in maintaining good water quality. King refers to shellfish as the canaries in the coal mine for water quality issues, the grazers of the sea.

Knowing the animals are essentially sampling the surrounding water continuously through feeding, King examined the clam’s digestive system to see what they had been eating in hopes of determining what was causing death. She was able to match plankton cells and cysts in the clam’s digestive system to those in her water samples. This, it seemed, was a ‘smoking gun’ especially since none of the other environmental conditions could be identified as an obvious culprit.

She sent the water samples to colleague to Vera Trainer at the NOAA Northwest Fisheries Science Center to culture the phytoplankton species, and she sent shellfish tissue samples to Misty Peacock from the Salish Sea Institute at the Northwest Indian College hoping they could analyze the samples for yessotoxins. A shellfish farmer also sent samples to their pathologist for clinical diagnosis. The pathology report noted unexplained damage to the digestive tract. King and Trainer were also able to find reports of yessotoxin poisoning of shellfish worldwide that had similar damage to digestive tissues.

Staying ahead of the blooms

Knowing this, King reached out to the partners of the SoundToxins phytoplankton monitoring program, a group she manages through her work at Washington Sea Grant. The program trains tribes, shellfish growers, environmental learning centers, university staff and private residents to monitor and record phytoplankton at sites around the Puget Sound. Data collected by the group since 2010 helped King and Trainer further link yessotoxins to many of the die off events. Looking back through the literature Trainer found that this species of phytoplankton had been mentioned numerous times as being present when summer shellfish mortality events had been recorded.

Because research surrounding yessotoxins is relatively new, close collaboration between Washington Sea Grant, SoundToxins, the Northwest Indian College, Washington Department of Fish and Wildlife, tribes and shellfish growers is required to understand them more fully. Intensive surveys funded by NOAA MERHAB are planned for upcoming summers at different shellfish farms to study yessotoxins over time to see how they react to a changing climate, determine the level of toxicity that is lethal to an animal and when during an oyster’s growth is it the most susceptible to the toxin. Filling in these gaps will help researchers better predict the timing of phytoplankton blooms, allowing shellfish farmers to deploy mitigation procedures like harvesting earlier, moving the stock to cleaner waters or enhancing water filtration. There’s no way to kill these toxins, which forces growers to figure out ways to live with it and minimize its effects on shellfish.

“This work really is a team effort,” said King. “At Washington Sea Grant, it’s our job to partner with agencies, tribes, shellfish producers and researchers to bring everyone together to understand the problem and figure out solutions. It’s really important that we have a diverse group of people bringing in different perspectives and areas of expertise at the table.”

Researchers have just scratched the surface in understanding these issues and yessotoxins have been largely overlooked globally, but now researchers have another likely culprit explaining mass shellfish mortality events and can develop research approaches to understand them even further. That alone brings much needed relief to scientists and oyster farmers alike.

King and her collaborators published their research in The Journal of Harmful Algae.

Story by Joy Chu