Spionid worms infesting cultured oysters in Bahia San Quintin, Mexico

The Pacific oyster (Magallana gigas) is a key aquaculture species worldwide, with Bahia de San Quintin, Baja California, representing a major production area in Mexico. However, oyster farming is affected by infestations of burrowing polychaete worms (Spionidae), which negatively impact growth, survival, and market value, and have caused industry losses globally. In San Quintin, recurrent infestations display two phenotypes whose taxonomy remained unresolved. Using morphological, ultrastructural, and genomic analyses of worms collected from nine farms, we identified one phenotype as Polydora websteri, while the second corresponded to an undescribed species of the genus Boccardiella. Mitochondrial genome analyses support its designation as a new species, Boccardiella quintinensis, which was more prevalent than P. websteri (72% vs. 28%). This work clarifies the diversity of polychaete infestations in the region and provides molecular markers for improved detection and management in oyster aquaculture..