The digenean trematode Proctoeces maculatus is an important parasite of the blue mussel Mytilus edulis. The parasite reduces mussel quality and yield, negatively impacting mussel aquaculture efforts. Typically, the trematode is detected by visual observation. To provide a better diagnostic tool able to detect this parasite at any life stage and at low intensities, we designed a species-specific molecular assay to detect P. maculatus in M. edulis tissue. Primers targeting the 18S nuclear ribosomal DNA (rDNA) from P. maculatus were used to develop an endpoint polymerase chain reaction assay and a quantitative polymerase chain reaction (qPCR) assay. Analytical specificity of the assays was demonstrated using DNA from 4 other digenean trematodes. The qPCR assay was linear from 6.79 × 10 2 to 6.79 × 10 7 copies of the cloned target DNA and had a conservative detection limit of 68 copies. The qPCR assay detected single cercariae, and the number of isolated cercariae was linearly correlated with the threshold cycle (C T ). Diagnostic sensitivity of the PCR-based methods was 100%. The assays also detected the parasite in 6 additional samples from the 57 tested through microscopy. We used the assays to verify the presence of encapsulated sporocysts in the mantle and to document infected mussels from Dover, New Hampshire, extending the previously described northern range of the species. Thus, this work has important implications for detection of the parasite in aquaculture and in monitoring its potential spread with climate change.KEY WORDS: Digenean · Aquaculture · Bivalve · Marine · Northwest Atlantic Resale or republication not permitted without written consent of the publisherDis Aquat Org 122: [125][126][127][128][129][130][131][132][133][134][135][136] 2016 and visible abscesses (Sunila et al. 2004). The parasites deplete glycogen and triglyceride levels in the mantle and hepatopancreas of infected mussels (Dennis et al. 1974, Machkevsky & Shchepkina 1985 and cause major organ damage as they spread throughout the mussel (Machkevsky & Gaevskaja 2008). Intense infections cause mussels to gape and detach from the substrate (Machkevsky & Gaevskaja 2008). Thus, infected mussels may be unable to handle the stress of harvesting and processing involved in aquaculture (Cole 1935, Canzonier 1972. Infection slows mussel growth (Machkevsky 1988) and at the highest intensity levels, ultimately leads to mortality (Machkevsky 1982, Machkevsky & Gaevskaja 2008. P. maculatus was implicated in mass mortalities of Mytilus galloprovincialis Lamarck, 1819 in Laguna Vaneta, Italy (Munford et al. 1981), and in the Black Sea (Machkevsky & Parukhin 1981), where the disease is referred to as 'proctecosis' (Machkevsky & Gaevs kaja 2008).P. maculatus has been found along the Gulf Coast (Wardle 1980) and east coast of the USA as far north as Woods Hole, MA. After numerous attempts by previous resear chers to find P. maculatus at sites north of Woods Hole were unsuccessful (Uzmann 1953, Pondick 1983, it was concluded that Woods Hol...