Geographic pattern of shell morphology in the endemic freshwater mussel Unio ravoisieri (Bivalvia: Unionidae) from northern Tunisia

Abstract

Unio ravoisieri is an endemic freshwater mussel inhabiting North Africa rivers. Morphological diversity in U. ravoisieri was examined at eight sites covering the three main drainage systems in northern Tunisia. Variation was assessed using 10 shell measurements in 193 specimens. Univariate and multivariate analyses were applied to test the effects of sex and sites on shell traits. Simple and partial Mantel correlations on external and internal shell traits and geographic locations of populations were performed to test for spatial effects. Our results showed significant differences in shell morphometric characters among locations, but no evidence of sexual size dimorphism. Mantel correlations between geographic and all shell morphometric distances were significant, indicating an isolation-by-distance pattern. However, partial Mantel correlation did not detect a significant correlation between internal and external shell morphometric distances after controlling for geographic distance. The nonparametric Spearman correlations between geographical parameters (latitude, longitude and altitude) and external shell traits, as well as growth index were significant, but no significant relationships were demonstrated for internal traits or for condition index. Assessment of the underlying processes involved in this differentiation is not straightforward, because of the likely complex interaction between genetic and environmental factors.