Sexual dimorphism and ontogenetic variation in the carapace ofA. pallipes(Lereboullet, 1858)

In this study we aimed to describe the morphological variability of the carapace of Austropotamobius pallipes (the only native river‐dwelling crayfish in central Italy) by geometric morphometrics. In particular, we investigated aspects of morphological variation related to sexual dimorphism and ontogenesis. In the white‐clawed crayfish, differences in carapace form due to sexual maturity and dimorphism between adults were significant. In particular, our study showed that the carapace form undergoes most changes during ontogeny and that sexual dimorphism occurs only between adult specimens. In fact, most ontogenetic changes in the carapace shape are allometric and almost inevitably it implies divergence in the ontogenetic allometric trajectories. Major changes take place mainly after reaching maturity. Morphological differences between males and females, expressed in both size and shape, can be explained by allometry and interpreted as a direct effect of growth, different ecology between sexes and amongst age classes, and sexual factors, the latter mainly regarding fertility and clutch size. This suggests that, in A. pallipes, allometry is an important aspect in ontogeny and most shape differences among age classes (mainly regarding the relative rostrum shortening and the lateral stretching of the entire carapace region) are likely to be size‐related adjustments.