Effects of sexual maturity and body size on statolith shape of Sthenoteuthis oualaniensis in the Northwest Indian Ocean

In this paper, the effects of sexual maturity and body size on the statolith shape of Sthenoteuthis oualaniensis in the northwest Indian Ocean were studied, the growth inflection points of statolith shape were evaluated, which would provide a basis for the subsequent studies on the population identification and life history of S. oualaniensis. Statolith, which is mainly composed of dorsal dome, lateral dome, rostrum dome and wing region, is one of the most important hard tissues of Cephalopods to study the age, growth, population structure and life history. According to the samples of S. oualaniensis totaled 1 009 in number collected in the northwest Indian Ocean from February to May of 2019 by the Chinese light falling-net fishery fleets, the total statolith length (TSL), lateral dome length (LDL), wing length (WL) and maximum width (MW) are considered as the indicators of external form growth, and the ratio of MW to TSL is considered as the indicator of changes in statolith shape. The external form growth and statolith shape changes affected by sex, gonad maturity and individual size are analyzed by the analysis of variance (ANOVA) and Least-significant difference (LSD). The results indicated that the changes of TSL, LDL, WL and MW between different sexes were statistically significant. For both female and male samples, the changes of TSL, LDL, WL and MW between different gonad maturity and different mantle length ranges were also statistically significant. However, changes of ratio of MW to TSL in different sexes showed no significant differences but changes in different gonad maturity and different mantle length ranges showed very significant differences. The study showed that sexual maturation and size of S. oualaniensis had an obvious effect on its statolith growth in both males and females. Stage III gonadal maturity and 300-500 mm mantle length are the growth inflection points of statolith shape of female individuals, while there is no distinct inflection point in male individuals. The characteristic parameters of statolith increased rapidly with the growth of gonad and mantle length before the inflection point, while increased slowly after the inflection point.