For nearly a century, shellfish in Washington State have experienced ‘summer’ mortality events and these shellfish losses have been difficult to understand. In the literature phytoplankton have been noted but not implicated in these mortality events because spawning stress and bacteria were thought to be the drivers of mortality.
A bloom of Protoceratium reticulatum associated with a massive shellfish mortality event opened the door to earnestly evaluating the role of phytoplankton in summer mortality events in Washington State. The SoundToxins phytoplankton research and monitoring partnership had been documenting increasing numbers of P. reticulatum and Akashiwo sanguinea in summer months, which had been associated with declining shellfish health or mortality at various sites in Puget Sound. However, the door was flung wide open in July 2019, as a dramatic scene of surfacing stressed, gaping Manila clams brought to everyone’s attention that something was wrong. Everything was aligning, pathology reports, observations of the bay, and analytical testing suggesting that yessotoxins (YTX) could be the cause. Subsequent YTX concentrations in clam tissues were measured at a maximum of 0.28 mg/kg and their histology demonstrated damage to digestive glands. A culture of P. reticulatum, isolated from North Bay during this massive bloom and shellfish mortality event, showed YTX reaching 26.6 pg/cell, the highest recorded toxin quota measured in the U.S. to date. This grand mortality event led to the uncovering of previously collected data which further demonstrated YTX and other phytoplankton involvement in shellfish mortality events in Washington State over the last century. Why the needle in the haystack? YTX produced by P. reticulatum were first isolated from scallops in Japan in 1986, so it is a relatively ‘new’ toxin and something not commonly on the radar of researchers.