The hermit crab Clibanarius signatus Heller, 1861 inhabits varied intertidal habitats of the Red Sea coast, such as rocky shores and mangrove littoral salt marshes. Shield-shape variation among three populations of C. signatus was analyzed with geometric morphometric methods. Shape variation was studied through multivariate analyses using configurations aligned by the generalized Procrustes analysis. Shape variation was explored through principal component analysis. The ordination of the populations and the sexes was investigated using discriminant analysis of canonical variables. Centroid size, as a measure of overall size, was used to estimate size variation among the three populations and the sexes. The results revealed the presence of shield-size variation among the three populations and confirmed the size sexual dimorphism in two populations. Moreover, the analysis revealed the occurrence of two morphotypes based on a covariation between shield shape and shape of occupied shells. The geographic distance was not a good predictor of shield shape. Cross-validation analyses correctly reclassified more than 70% of individuals and 66% of sexes to their correct group. It was suggested that association in shield-shell shape could be the result of the phenotypic plasticity of this species.

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Background Isopods shape features are sensitive and respond to several selective pressures which may result in variations of these features. These pressures might reflect the heterogeneity of the environment where an animal lives. Land isopods Porcellionides pruinosus were collected from an agricultural field. Landmarks geometric morphometrics was applied to evaluate its shape changes during two different seasons. The present work aims to (i) assess and characterize morphological changes in body shape of P. pruinosus as a response to seasonal variations, (ii) determine differences in the body shape during ontogeny, (iii) examine the effect of intraspecific allometry to interpret the observed variations in the species, and (iv) clarify whether the body shape of P. pruinosus can be used as a sexual differentiating trait. Results Juveniles showed no seasonal variations in the body shape, which were detected among adults, females and males as shown by PCA, DFA, and MANOVA. The adult winter forms have large bodies, small heads, broad pereons, and short but wide telsons. The adult summer forms have small slender bodies, slightly stretched heads, and relatively long telsons. Juveniles’ growth to adulthood showed body shape changes in the head and pereon, that include shrank of the head in the anteroposterior direction and its level became slightly lower than the body. The pereon becomes broader and the two anterolateral projections of the first pereonite extend anteriorly, reaching a little beyond the posterior margin of the eyes. Present species showed a shape sexual dimorphism which includes the broader body and more convex pereon in females and a small waist between the second and third pleonites in males. Shape sexual dimorphism was attributed to reproductive activity. Both allometric trajectories of juveniles and adults (ontogenetic allometry) and of sexes (static allometry) were parallel. Conclusions The landmark geometric morphometric technique was able to reveal the seasonal shape variations in terrestrial isopod P. pruinosus. Also, this method provides information about shape variations between juveniles and adults, as well as about shape sexual dimorphism.