Shell-boring organisms burrow into the shells of cultivated and wild bivalves, weakening the shell structure. Hence, bivalves spend extra energy to repair the drilled parts and allocate less energy for growth and reproduction. This issue is a severe concern, especially for economically important species. This study investigated temporal and spatial differences in macroborers’ infestation of Portuguese oysters in Taiwan’s farming areas . Macroborers (>100 μm), such as polychaetes and sponges, as well as microborers (<100 μm), have been found in oyster farming areas on the west coast of Taiwan. T he most abundant macroborers were polychaetes. These boring polychaetes had U-shaped drilling channels and 8-shaped external openings.
The infection by boring polychaetes on oysters cultured for eight months could reach 100%. Two boring polychaete species exist in the cultural areas, i.e., Polydora lingshuiensis and Polydora haswelli . T he infection rate was higher in the right valve. Sponge-drilled areas appeared as dense holes on the shell surface, and reticular canals were within the shell. In Qigu, the infestation rate of boring sponges on farming oysters was <33%, and a higher inf estation rate on the left valve . The infection of farming oysters by macroborers was higher in 2022 than in 1990 in Qigu. We found n egative correlations between shell-boring degree ( i.e., 0, 1, 2) to shell density (1.86 – 1.66) and standardized condition index (0. 84 – 0.64) . Boring algae was also observed, but their impact on the oyster industry deserves further evaluation.