Asymmetrical chela growth patterns and correlation of chela shape to inhabited gastropod shells in Pagurus bernhardus (Linnaeus, 1758) (Decapoda, Anomura, Paguridae)

Abstract We investigated the growth patterns and morphological changes in the chelae of the heterochelate hermit crab Pagurus bernhardus using µCT and SPHARM analysis, correlating our findings with three different types of shells offered for protection. In males, the right chela exhibited positive allometric growth compared to the left, leading to significant size differences, whereas females showed isometric growth. This sexually dimorphic growth pattern suggests sexual selection for chela display and agonistic behavior in males. Significant length-specific growth differences were found between the chelae. The left chela exhibited positive allometric growth in the chela finger (pollex) compared to the manus, while the right chela showed isometric growth. If the left chela represents the derived state, genetic factors predominantly influence its growth and shape for biological roles such as food manipulation and grooming. Conversely, if the right chela is the derived state, it may indicate an adaptation for better shell closure in gastropod shells. There was no sexual dimorphism in the length-specific growth pattern and shape changes during ontogenesis. Shape analyses revealed significant changes in chelae morphology during ontogenesis, with correlations to the inhabited shells. These changes were particularly pronounced in the right chela, used for closing the shell aperture. Shape analysis of non-allometric data showed no significant differences between shell groups, indicating that shape changes are primarily due to allometric growth. Phenotypic plasticity may also play a role. Sexual dimorphism was significant in the shape of both chelae and was also evident in non-allometric data, suggesting shape dependence.