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

초록

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 (cox1, 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.

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