Factors influencing beak morphology of Sthenoteuthis oualaniensis in the northwest Indian Ocean.

With 1009 Sthenoteuthis oualaniensis samples collected by Chinese light falling-net fishery during February to May in the northwest Indian Ocean, we analyzed the influencing factors of the morphology change of the beak. There were significant differences in the morphologic growth between genders, different gonad maturity stages, and different mantle length. The increment of characteristic parameters growth peaked in the mantle length between 201 and 250 mm, and in the Ⅱ and Ⅲ stages of gonad maturity for females and males, respectively. There was no significant difference in the ratio of morphometric parameters to crest length (CL) between the genders, diffe-rent gonad maturity stages, and different mantle length, except for the ratio of upper hood length to upper crest length (UWL/UCL) and lower hood length to lower crest length (LHL/LCL). The ratio of each characteristic parameter to CL was nearly stable with the change of sex, gonad maturity and mantle length, suggesting consistent growth of each dome of beak. Our results suggested the mantle length of 201 mm to 250 mm seemed to be the threshold for the morphologic growth of beak on the body size, while the gonad maturity of Ⅱ and Ⅲ stages seemed to be the threshold for the morphologic growth of beak on the gonad maturity.