Worm Alert! Three Alien Species of Polychaete Genus Polydora detected in Pacific Oysters along the Swedish West Coast

Abstract

Shell-boring polychaetes like the mud-blister worm Polydora websteri cause infections in commercially important bivalves, leading to economic loss within the shellfish industry worldwide. Recently, P. websteri was detected in invasive Pacific oysters (Magallana gigas) established along the Swedish west coast. This study assessed the species diversity, morphology and genetic structure of polydorid worms inhabiting Pacific oysters collected from 11 infected sites between Askim and Strömstad. A combination of morphological analysis and molecular identification using mitochondrial cytochrome c oxidase I (COI) genes was used to identify polydorid species. Three species were detected in oyster mud-blisters: P. websteri, P. cornuta and P. onagawaensis. Polydora cornuta is not formally recognised as a shell-boring species but thought to inhabit blisters created by other boring polydorids. During extraction, multiple species were found to inhabit the same oyster. An Assemble Species by Automatic Partitioning (ASAP) analysis suggested clusters based on genetic distance between COI sequences of each species. The genetic variation within and between these clusters was further analysed using the Kimura 2 Parameter model (K2P). Phylogenetic maximum-likelihood and haplotype network analyses identified a single COI clade of P. websteri, with Swedish specimens grouped within the dominant global COI haplotype. Three previously known COI haplotype clusters of P. cornuta were detected, and Swedish COI haplotypes were placed in a geographically diverse cluster. For P. onagawaensis, three previously known COI haplotype clusters were detected together with preliminary evidence of a fourth, previously undescribed European cluster consisting of Swedish and French COI haplotypes. Overall, this study provides updated knowledge on the diversity, morphology, and genetic structure of polydorids in Swedish Pacific oysters which can aid in further sampling and species identification of Polydora spp. in Sweden. This knowledge can be contribute to reduce economic cost and improve the efficiency of harvesting efforts of Pacific oysters in Sweden.