Pollution versus biopollution? Failure of Xenostrobus securis invasion in a heavily polluted estuary of the Bay of Biscay

Invasive species tolerant to pollution may gain a competitive advantage in contaminated environments. This study aimed to assess the accumulation of microplastics (MPs) and heavy metals (HMs) in native ( Mytilus galloprovincialis ) and invasive ( Xenostrobus securis ) mussels in three estuaries - Avilés, Nalón, and Ribadesella – in the Southwest Bay of Biscay, Spain. X. securis invaded the Avilés estuary a decade ago, but current sampling indicates that its population is now declining (< 4%), with no expansion to other Asturian estuaries. X. securis exhibited significantly higher concentrations and bioconcentration of MPs. Some individuals of both species in Avilés had Cd and Hg levels (μg kg −1 ), that exceeded the permitted limits for human consumption, whereas most Mytilus from Nalón and Ribadesella remained within safe limits. Multiple regression analyses revealed that, while the pollutants analysed did not affect the condition of M. galloprovincialis , Cd had a potential negative effect on X. securis . The spatial arrangement of X. securis , often growing on M. galloprovincialis in multispecies assemblages, suggests complex species interactions. X. securis may indirectly benefit native mussels by retaining MPs, since its filtration rate is higher. These findings improve understanding of pollutant accumulation in native and invasive species and highlight the urgent need for stringent control of HMs pollution in the region, due to the presence of unsafe mussels.