Detection of a novel species complex of shell‐boring polychaetes in the northeastern United States

Abstract Infestations of commercially cultured eastern oysters ( Crassostrea virginica ) by shell‐boring spionid polychaetes are common in the northeastern United States. Historically, infestations were attributed to Polydora websteri , also known as mud‐blister worm. Among samples of shell‐boring worms sampled from eight oyster farms in northern New England (Maine, New Hampshire, and Massachusetts), we detected P. websteri , as well as worms that were morphologically distinct from P. websteri . Using a combination of light and scanning electron microscopy, along with analysis of molecular variation at the nuclear 18S rRNA and mitochondrial cytochrome c oxidase I (COI) genes, we determined that specimens related to two other species of Polydora , Polydora onagawaensis and Polydora cornuta , were inhabiting burrows within the shells of oysters. Polydora cornuta is not recognized as a shell‐boring species and likely invades existing burrows opportunistically. Our analysis of COI sequences identified three distinct genetic lineages among specimens morphologically identified as P. cornuta ; one of these lineages is much more geographically widespread than previously had been reported. Polydora onagawaensis is considered a shell‐boring species, and our article constitutes the first time this species has been documented in the Northwest Atlantic. We also provide evidence for three divergent COI genetic lineages that occur sympatrically among the specimens provisionally identified as P. onagawaensis in the northeastern United States.