Aims  Our study aimed to characterize the dispersal dynamics and population genetic structure of the introduced golden mussel Limnoperna fortunei throughout its invaded range in South America and to determine how different dispersal methods, that is, human‐mediated dispersal and downstream natural dispersal, contribute to genetic variation among populations. Location  Paraná–Uruguay–Río de la Plata watershed in Argentina, Brazil, Paraguay and Uruguay. Methods  We performed genetic analyses based on a comprehensive sampling strategy encompassing 22 populations (N = 712) throughout the invaded range in South America, using the mitochondrial cytochrome c oxidase subunit I (COI) gene and eight polymorphic nuclear microsatellites. We employed both population genetics and phylogenetic analyses to clarify the dispersal dynamics and population genetic structure. Results  We detected relatively high genetic differentiation between populations (FST = −0.041 to 0.111 for COI, −0.060 to 0.108 for microsatellites) at both fine and large geographical scales. Bayesian clustering and three‐dimensional factorial correspondence analyses consistently revealed two genetically distinct clusters, highlighting genetic discontinuities in the invaded range. Results of all genetic analyses suggest ship‐mediated ‘jump’ dispersal as the dominant mode of spread of golden mussels in South America, while downstream natural dispersal has had limited effects on contemporary genetic patterns. Main conclusions  Our study provides new evidence that post‐establishment dispersal dynamics and genetic patterns vary across geographical scales. While ship‐mediated ‘jump’ dispersal dominates post‐establishment spread of golden mussels in South America, once colonies become established in upstream locations, larvae produced may be advected downstream to infill patchy distributions. Moreover, genetic structuring at fine geographical scales, especially within the same drainages, suggests a further detailed understanding of dynamics of larval dispersal and settlement in different water systems. Knowledge of the mechanisms by which post‐establishment spread occurs can, in some cases, be used to limit dispersal of golden mussels and other introduced species.

Adaptation is an essential step in the establishment and spread of alien species in new environments, with phenotypic plasticity or genetic variability often contributing to this success. The golden mussel Limnoperna fortunei is a biofouling mollusc native to Southeast Asia that was introduced to South America near the Río de la Plata estuary, Argentina, though the species has subsequently spread more than 2000 km upstream. We analyzed morphological and genetic variation in 24 introduced populations of L. fortunei across its South American range. Relative gill area and shell morphology differed significantly, even among geographically proximate populations. Differences in relative gill area were especially marked across the species' range and were negatively correlated with total suspended solids. Whereas mean gill cilia length, filament width, and interfilamental ciliary junction distance did not differ significantly among populations, mean gill cilia density was significantly lower in populations from areas with high suspended solids. Conversely, morphological differences were not related to the number of haplotypes, haplotype diversity, or nucleotide diversity, based upon analyses of the mitochondrial cytochrome c oxidase subunit 1 gene. Our results indicate that introduced populations of golden mussels in South America exhibit pronounced morphological variation in shell and gill metrics that appear to result from developmental plasticity in relation to total suspended sediments, as has been observed in other mussel species. These adaptations may have facilitated spread of this species to a wide range of habitats.