PREVALENCE, DISTRIBUTION, AND CONTROL OF SHELL-BORING POLYCHAETES ON OYSTER FARMS FROM CALIFORNIA TO ALASKA

Marine polychaetes in the genus Polydora (and other related genera) burrow into the shells of commercially important bivalves, creating unsightly blisters and burrows. Because they are unappealing to consumers, these blisters are an economic burden on producers whose stock goes to the half-shell market. The US west coast is the leading producer of aquaculture oysters in the country, making it important to understand the prevalence of these pests in Pacific oysters. We obtained over 3,000 oysters from 35 farms in four states (CA, OR, WA and AK) to determine infestation prevalence. Prevalence per state ranged from 13 to 37%. Polychaetes were extracted from infested shells for molecular analyses. We used mitochondrial (CO1) and nuclear (18S) genes for species-level identification, and these genetic markers confirmed the presence of P. websteri and P. hoplura, as well as other polychaete species. Given the difficulties faced by growers in controlling this pest, we consulted shellfish farmers from Washington State to create treatments that are both effective and easy to apply on farms. We tested three treatments in experiments carried out with Pacific oysters infested with shell-boring polychaetes collected from Washington State. Our experimental design consisted of three stages: (1) the main experiment to test treatments, (2) a growth assessment, (3) and respirometry trials. We subjected 675 oysters to three treatments: 'Dry', 'Fresh-Dry', 'Refrigeration' and a Control. The 'Dry' treatment was 100% effective at killing worms and the other two treatments were nearly as effective. The growth assessment indicated that none of the treatments negatively affected oyster growth, and the respirometry trials showed no differences in oxygen consumption between treatments. Findings from this study will allow us to advise the US west coast shellfish industry on the progress of the spread of shell-boring polychaetes and strategies for reducing the economic impacts of these globally-distributed pests.