Infestation of cultivated Pacific oysters by shell-boring polychaetes along the US West Coast: Prevalence is associated with season, culture method, and pH

Shell-boring polychaetes have contributed to the collapse of several mariculture operations around the world. These pests burrow into the shells of bivalves, creating mud blisters that are unappealing to consumers and which make oysters less valuable on the half-shell market. The US Pacific region produces 38% of the farmed shellfish in the US, making it important to understand the prevalence and drivers of parasite infestation in this region. We sampled Pacific oysters (Crassostrea gigas; n = 4158) from 35 shellfish farms over four seasons (two winters and two summers) in four states (northern California (CA), Oregon (OR), Washington (WA), and Alaska (AK)) to document the prevalence of shell-boring polychaetes. We extracted worms from infested oysters and used mitochondrial (CO1, n = 139) and nuclear (18S rRNA, n = 224) markers to determine species identities. To identify the environmental correlates that were associated with infestation, we pooled environmental data from seven monitoring stations in Washington. We assessed whether seawater surface temperature (SST), salinity, and pH were associated with shell-boring polychaete infestation. Our sampling confirmed the presence of Polydora websteri in the study region, in addition to four other species of shell-boring polychaetes and seven unidentified haplotypes. The mean prevalences across all shell-boring polychaete species ranged from 23 to 45% across seasons between states. In general, prevalence was higher in the winter and among oysters cultured on the bottom versus in tumbled bags, but these results varied across states. We also found greater infestation by shell -boring polychaetes at less acidified sites (pH = 8-8.2). This work is the most comprehensive dataset to char-acterize shell-boring polychaetes along the US West Coast, providing an important baseline of prevalence, species distribution, and environmental associations.