Infestation dynamics and microanatomy of shell-boring Polydora haswelli in single-seed Pacific oyster (Crassostrea gigas) aquaculture in South Korea

The economic viability of Pacific oyster ( Crassostrea gigas ) aquaculture is increasingly threatened by shell-boring spionids ( Polydora spp. ), which degrade shell integrity and market value. As the South Korean industry shifts from traditional cultch-on-longline systems toward single-seed (cultchless) culture, understanding how this management transition influences Polydora infestation dynamics is critical. We integrated molecular barcoding ( cox 1, 18S rRNA gene), field epidemiology, and histological analysis to determine if single-seed systems offer a protective advantage against Polydora infestation compared to traditional and wild environments. Polydora haswelli was identified as the primary agent across four major farming regions. Single-seed oysters exhibited moderate prevalence (55.83–75.42%) but consistently low infestation intensity (3.10–4.57%). In contrast, cultch-based oysters, wild oysters, and scallops showed higher prevalence (81.67–83.75%) and markedly greater shell-damage intensity (14.90–19.46%). However, a distinct temporal vulnerability was observed in juvenile single-seed oysters, where prevalence reached 100% during autumn outbreaks. Beyond epidemiology, our histological observations provide the first detailed account of internal organ architecture and reproductive organization in P. haswelli , revealing specific reproductive cycles that coincide with these peak infestation periods. Collectively, the results indicate widespread but host- and region-specific impacts of Polydora in Korean shellfish aquaculture and suggest that the ongoing transition from traditional cultch-based production to single-seed culture may help reduce shell damage under Polydora pressure, while highlighting the need for early detection and seasonally targeted monitoring, particularly for juvenile oysters. ● Polydora haswelli is the dominant shell-boring species in Korean single-seed oyster farms. ● Single-seed oysters show lower infestation intensity than cultch-based and wild oysters. ● Juvenile single-seed oysters can experience rapid, high-intensity outbreaks in late summer–autumn. ● This study provides the first detailed histological description of P. haswelli internal and gonadal structures.