From Core to Front: Microplastic Loads Along a Signal Crayfish ( <i>Pacifastacus leniusculus</i> ) Invasion Gradient

Plastic pollution is one of the most persistent threats to aquatic ecosystems, yet studies in low-disturbed mountain aquatic environments remain scarce. This study examined the presence of macro- and microplastics in three rivers in the Montesinho Natural Park and adjacent areas, Portugal. It investigated how intrapopulational ecological traits of the signal crayfish (Pacifastacus leniusculus), a non-native species widely distributed across Europe, may influence contaminant accumulation. Sampling was conducted at five sites along an invasion gradient (front-core), where macroplastics along the margins and signal crayfish were collected, followed by laboratory analyses for the morphological and polymeric characterization of the plastics. Chemical identification was performed using Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) and Raman spectroscopy. Behavioural assays of boldness and aggressiveness were carried out on 100 captured signal crayfish individuals. Macroplastics were detected in all sampling sites. Microplastic abundance was higher at the invasion front, and the digestive tract contained significantly more particles than the gills, suggesting that the digestive tract is the primary route of exposure. No differences were detected between sex or individual size. Fibers were the dominant form, mainly in blue and black colors, and polymers such as polyethylene terephthalate (PET), polyethylene (PE), and polypropylene (PP) prevailed. Although personality traits differed between the invasion front and the core, with generally bolder and more aggressive individuals at the front, behavioural responses also showed sex-related patterns, with females tending to leave the shelter faster and males showing higher aggressiveness. However, no statistically significant correlations were found between animal personality and microplastic accumulation. Overall, our results indicate that plastic contamination depends on multiple factors arising from interactions between local environmental factors, including hydromorphological processes, and intrapopulational ecological traits (i.e., diet). This study highlights the importance of integrating behavioural analyses to better understand differential exposure to contaminants and their potential environmental implications for non-native species and possible legacy for native communities.