Shell-burrowing worms (Annelida: Polychaeta: Spionida) Boccardiella quintinenis sp. Nov. (Gracida-Olivera, Cruz-Flores, Cáceres-Martínez, 2023) and Polydora websteri (Hartman, 1943) infesting cultured Magallana gigas from Bahía San Quintin, Baja Cali

The Pacific oyster (Magallana gigas) is one of the most important species in global bivalve aquaculture. In Mexico, a significant portion of its production originates from Bahía de San Quintín, Baja California (BSQ). Like other bivalves, it is susceptible to infestations by burrowing worms (Polychaeta: Spionidae), these infestations result in a negative effect for growth, survival and commercialization of the host. Polydorid infestations have been implicated in the collapse of oyster aquaculture farms in several countries. In BSQ, these worms have been recorded since 1997, where recurrent infestations are associated with at least two phenotypes. Their precise taxonomic identity has not yet been conclusively determined. To analyze these aspects, worms were obtained from oysters of nine farms and identified by conventional taxonomy complemented with scanning electron microscopy and by analysis of nuclear genes and the complete mitochondrial genome. The results indicated that one phenotype corresponds to Polydora websteri. The second phenotype showed that, morphologically and molecularly it belongs to the genus Boccardiella without coinciding with any of the species described. Mitochondrial analyses led to the consideration of this worm as a new species, which was designated Boccardiella quintinensis and was more abundant than P. websteri (72% versus 28%). This study contributes to a better understanding of the diversity of polychaete infestations and the associated risks of transfaunation in aquaculture systems. Furthermore, it proposes the use of specific molecular markers for the detection and monitoring of B. quintinensis, providing valuable tools for early diagnosis and management of infestations in oyster farming.