Decadal assessment of the non-native bloodworm Glycera dibranchiata Ehlers, 1868 in a northern Spanish estuary

We report the first record of a potentially self-sustaining population of the American bloodworm Glycera dibranchiata outside its native NW Atlantic range, established for over a decade in the Villaviciosa estuary (Cantabrian Sea, Bay of Biscay). Its presence is likely linked to imports for recreational fishing bait. Morphological and genetical analyses, including mitochondrial COI sequencing, confirmed species identity and traced origins. Two haplotypes were identified, one matching Canadian populations, suggesting either long-term persistence since initial introduction (2010) or repeated introductions from similar sources. Substantial divergence between Cantabrian Sea and NW Atlantic specimens suggests a possible a cryptic species complex, highlighting limitations of traditional taxonomy and the need for integrative approaches to accurately delineate species boundaries in widely traded, ecologically impactful taxa. Additionally, ectosymbiotic ciliate protozoans and endoparasitic apicomplexan gregarines at different developmental stages were found in all examined specimens, raising concerns about ecosystem impacts, including potential public health risks, threats to endangered taxa, and mass mortalities of native hosts.