Abstract
Morphological variability plays a key role in the establishment and spread of invasive fish species, influencing essential life functions and adaptability to new environments. In this study, we examined the morphological diversity of the topmouth gudgeon, Pseudorasbora parva (Temminck & Schlegel, 1846), across both its native and invasive ranges to assess patterns of evolutionary adaptation during its invasive process. Our findings reveal remarkable morphological plasticity across all studied populations, regardless of geographic origin or climatic conditions. Notably, populations from Taiwan and Japan exhibit significantly larger mouth gapes, potentially reflecting differences in feeding ecology. A distinct north–south gradient in fin morphology was also observed, with northern populations displaying shorter dorsal and pectoral fins. Meanwhile, European populations exhibit greater morphological fragmentation, shaped by a complex interplay of evolutionary history, genetic divergence and environmental pressures. These results underscore the role of morphological adaptation in invasion dynamics and highlight how phenotypic plasticity contributes to the ecological success of P. parva in novel environments.