Evaluating treatments for shell-boring polychaete (Annelida: Spionidae) infestations of Pacific oysters (Crassostrea gigas) in the US Pacific Northwest

Shell-boring polychaetes are responsible for economic losses on oyster farms around the world, including in Washington State, where these pests were officially reported for the first time in 2017. After burrowing into their host's shell, the polychaetes cause oysters to deposit layers of nacre to isolate the parasite from the oyster's tissues. This creates unsightly blisters that are packed with mud and feces. Burrows and blisters cannot be reversed, but worms within burrows and blisters can be killed, which may reduce the overall burden of parasite infestation on a farm and prevent future burrows and blisters from developing. Treatments for these polychaetes have been tested in other world regions, and we wanted to determine whether these treatments would work for infested Pacific oysters raised in Washington State. We collaborated with local shellfish growers to develop three treatments that could be practically deployed on Washington State oyster farms: drying, a freshwater dip plus drying, and drying with refrigeration. We quantified the extent to which each treatment killed worms, affected oyster physiology, and killed oysters. Experiments were conducted in fall and spring to evaluate seasonal differences. Our results showed that drying and using a freshwater dip plus drying for two days are both highly effective (>95%) at killing worms without negatively impacting oyster survival (4% mortality in the drying treatment in the spring, 0% mortality in all other treatment–season combinations). Use of these treatments on affected farms may help to mitigate the potential effects of shell-boring polychaetes on oyster product value, thus avoiding economic losses. •We co-created 3 interventions with oyster farmers to control shell-boring polychaetes.•We assessed whether treatments killed worms, oysters, or affected oyster physiology.•Drying and freshwater + drying treatments were very effective (>95%) at killing worms.•Treatments caused minimal oyster mortality.•Oxygen consumption and shell growth were not negatively impacted by treatments.