Ontogenetic Variation and Sexual Dimorphism of Beaks among Four Cephalopod Species Based on Geometric Morphometrics

Simple Summary Octopus minor, Uroteuthis edulis, Sepia esculenta and Sthenoteuthis oualaniensis are important economic cephalopod species in coastal waters of China. They are very important role in marine ecosystems as relevant prey for large marine fish, and marine mammals are a critical component of the food chain. As the main feeding organ of cephalopods, the beak has a stable structure and is resistant to corrosion. However, the sexual dimorphism on the beak of the O. minor, U. edulis and S. esculenta and Sthenoteuthis oualaniensis at different ontogenetic stages is unknown, neither has it been determined whether variation of beak shape relates to changes in habitat environment and feeding preference at different ontogenetic stages. Using a geometric morphometrics approach, we found that the beaks of O. minor, U. edulis, S. esculenta, and Sthenoteuthis oualaniensis showed a pattern of variation, displaying sexual dimorphism and allometry between ontogenetic stages. Habitat may drive variation in beak shape. This study has furthered our understanding of the beak shape ontogenetic variation among the four species. We discuss the potential factors underlying the beak shape variation and provide a basis for the understanding of cephalopod phenotypic plasticity and its ecological significance. Investigating the ontogenetic variation of biological individuals helps us to fully understand the characteristics of evolution. In order to explore the ontogenetic variation and sexual dimorphism of the beak shape in Octopus minor, Uroteuthis edulis, Sepia esculenta and Sthenoteuthis oualaniensis of the China's coastal waters, the differences between immature and mature stages and the sex-linked differences in the beak shape and size were analyzed with geometric morphometrics methods in this study. The results of Procrustes analysis of variance, principal component analysis and multivariate regression showed that the shapes of the upper beaks of O. minor, U. edulis and S. esculenta differed significantly among various ontogenetic stages (p < 0.05). The shapes of the lower beaks of U. edulis, S. esculenta and Sthenoteuthis oualaniensis were also significantly different among various ontogenetic stages (p < 0.05). The results of thin-plate spline deformation grids showed that the beaks of the four cephalopod species presented different variation patterns. This study gives us basic beak geometry morphology information for Octopus minor, Uroteuthis edulis, Sepia esculenta and Sthenoteuthis oualaniensis present in China's coastal waters. The ontogenetic differences in beak shape might be related to extrinsic factors (diet difference and intra and interspecific competition) in habitat.