Evaluation of stock variation and sexual dimorphism of beak shape of neon flying squid, Ommastrephes bartramii, based on geometric morphometrics

Variability in the shape of hard structures such as mandibles (beaks) due to stock variation or sexual dimorphism is common in cephalopods, and could prove useful in the discrimination of stocks or sexes. In this study, geometric morphometrics of upper and lower beak's shape are used to analyze the variation between sexes and stocks in Ommastrephes bartramii of the northwestern Pacific Ocean. This species is targeted by the Chinese jigging fleet during the fishing period May-October 2013. Variation in the beak's pigmentation stage was also investigated. According to our analyses, upper and lower beak shapes of O. bartramii differed significantly between eastern and western stocks of O. bartramii (MANCOVA, P < 0.01), and different allometric growths were also detected. The degree of pigmentation by pigmentation stage, changed in upper beaks between the two stocks and in lower beaks with beak size. In the western stock, only lower beak shape and pigmentation stages of upper beak shape showed significant differences between sexes, while the degree of pigmentation stage changed with different beak sizes in the lower beak. Squid stocks could be effectively identified based on the results of a principal component analysis of beak shape, but sexes could not be identified with this technique. We hypothesize that reason for these differences may be caused by the variation of feeding habits between stocks but the sharing of a similar habitat by both sexes.