Effects of difference of individual size on beak morphology of Sthenoteuthis oualaniensis in the Xisha Islands of South China Sea

Beak is an important hard tissue of Cephalopoda which is usually used in the study of fisheries biology such as population identification, growth and stock assessment and so on. Based on the 860 samples of Sthenoteuthis oualaniensis collected by light falling-net fishery during May to August in the year of 2017 in the Xisha Islands waters of the South China Sea, the morphologic growth of beak impacted by gonad maturity and individual size was analyzed. The result of principal component analysis of twelve morphologic indices indicated that the upper hood length (UHL), upper crest length (UCL), upper rostrum width (URW), upper lateral wall length (ULWL), lower hood length (LHL), lower crest length (LCL),lower rostrum length (LRL) and the lower lateral wall length (LLWL) could be used to describe the length growth features of beak, and the HL/CL, URW/UCL, LRL/LCL, LWL/CL and WL/CL could be used as the indicators of entire growth of beak. The analysis of variance (ANOVA) and the least-significant difference (LSD) indicated that there were significant differences in the morphologic growth between sexes, gonad maturity and different mantle length, however, the values of HL/CL, URW/UCL, LRL/LCL, LWL/CL and WL/CL were nearly constant with the change of sex, gonad maturity and mantle length. This study suggested that there are significant effects of gonad maturity and individual size on beak morphology and the mantle length of 121mm to 150 mm, and the gonad maturity of Ⅲ stage seemed to be the inflection points of the morphologic growth of beak of S. oualaniensis.