Integrating morphological and genetic data at different climate and geographic scales in an endangered freshwater mussel Unio durieui (Bivalvia: Unionidae) endemic to northern Tunisia

The freshwater mussel Unio durieui is a rare bivalve endemic to North African watersheds. Although this is an important species from an ecological and conservation perspective, information on its genetic diversity and population structure is lacking. Here, we combine geometric morphometric and molecular genetic analyses to characterize and evaluate the status of populations of this endangered species from four Tunisian watershed basins (Cap Bon, Ichkeul, northwest and Mejerda River basins). By using a multiparameter approach, we sought to test the hypothesis that populations from these geographic regions could be considered as distinct management units. Shell outline shapes were assessed using elliptic Fourier analysis followed by linear discriminant analysis of principal components. Multivariate analysis applied on normalized Fourier coefficients showed highly significant differences in shell shape among the populations. Population genetic analysis based on inter-simple sequence repeats revealed low levels of diversity and high differentiation between the populations studied. Two groups of populations are clearly evident, thus separating samples from the Northwest from those from the other watersheds. Although there is no evidence for isolation by distance patterns, genetic and phenotypic diversity appeared significantly correlated with climatic variables. These findings suggest isolation by environment and the nominal species U. durieui may constitute two biological species. Conservation programme actions should consider this substructure in establishing effective recovery strategies for this endangered species.

Automated summary

This study used geometric morphometrics and molecular genetics to analyze the population structure and genetic diversity of the endangered species Unio durieui in four Tunisian watershed basins. The results showed significant differences in shell shape among populations from different geographic regions, and genetic and phenotypic diversity were found to be correlated with climatic variables. These findings suggest that the nominal species U. durieui may consist of two biological species due to isolation by environment.