Sexual and geographic differences in morphological traits of Sthenoteuthis oualaniensis and their correlation with environmental factors

The purpleback flying squid (Sthenoteuthis oualaniensis) is a representative pelagic cephalopod species in the South China Sea, playing a crucial role in both ecosystem functioning and fisheries utilization. Based on 929 specimens collected from 22 stations in the southern South China Sea during 2016–2017, nineteen morphological traits were systematically measured. Analysis of variance (ANOVA), principal component analysis (PCA), coefficients of variation (CV), and regression models were applied to evaluate sexual differences, spatial variation, and their relationships with environmental factors. The results showed significant differences between sexes in six traits, including mantle length, arm lengths, fin dimensions, body mass, stomach mass, and gonadal mass (p<0.05). Females exhibited greater energy storage and reproductive investment, while males tended to mature earlier. Growth and reproduction significantly drove variation in body mass, gonadal mass, and feeding-related traits, reflecting dynamic energy allocation across the life cycle. Spatially, female gonadal mass and male stomach mass exhibited the greatest variability, suggesting potential local adaptation. Environmental analyses revealed that salinity had significant negative effects on beak-related parameters, whereas current velocity was positively correlated with certain arm lengths (p<0.05), indicating that key environmental factors play dominant roles in shaping functional traits. Overall, this study elucidates the ecological adaptation mechanisms of S. oualaniensis under complex environmental conditions and provides new insights for resource assessment and fisheries management of cephalopods in the South China Sea.