Sexual asynchrony in the development of beak pigmentation for the neon flying squid Ommastrephes bartramii in the North Pacific Ocean

The squid beak is a hard structure that displays high variability in both shape and coloring and can contain valuable ecological information. The beak pigmentation process reflects changes in growth and diet. In this study, we proposed species–specific beak pigmentation stages (PS) to supplement a previous classification scheme, which quantifies beak growth based on pigmentation variability. We analyzed sexual differences for Ommastrephes bartramii in the North Pacific Ocean. Distribution of beak PS increased with mantle length (ML). Body and beak measurements and beak weight were significantly different between PSs for both sexes. The relationship between ML and beak measurements differed significantly between the sexes. The observed differences might result from variable growth in different habitats as a result of sex-segregated migration in the North Pacific Ocean. The process of beak pigmentation may reflect variability in growth associated with different feeding regimes. The sexual asynchrony of pigmentation can also give us a supplemental way to discriminate different sexes using beak morphometric.