Morphological and genetic variability in an alien invasive mussel across an environmental gradient in South America

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 Rio 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.