Abstract: Statolith is one of the most important hard tissues in Cephalopods. It is an important material for studying age, growth, population structure, and life history of Cephalopods. The morphological growth characteristics of statolith in Sthenoteuthis oualaniensis were examined based on samples collected in the Northwest Indian Ocean from February to May of 2019 (the period of El Nino occurrence) by the Chinese light falling-net fishery fleets. The results showed that wing dome of the statolith was large, while dorsal dome and rostrum were relatively small. The results of principal component analysis showed that total statolith length (TSL), wing length (WL), lateral dome length (LDL) and maximum width (MW) could be used as morphological indicators for the morphological growth of statolith. The analysis of covariance (ANCOVA) showed significant sex differences in the relationships between these morphological characteristic parameters versus mantle length (ML) and body weight (BW). Based on the results of the Akaike’s information criterion (AIC), logarithm function best described the relationships between ML and TSL or WL, while linear function for the relationships between ML and LDL or MW for males. The relationships between ML and TSL, LDL or MW were best described by logarithm functions for females. The relationships between BW and TSL or WL were best described by logarithm functions but power function for LDL, MW for males. The relationships between BW and TSL, WL, LDL, MW were best described by logarithm functions for females. The absolute growth rate of statolith increased while the relative growth rate decreased with individual growth. Our results provide scientific basis for understanding population structure and life cycle of Sthenoteuthis oualaniensis and the effects of extreme weather on growth characteristics of hard tissues of Cephalopods.

Key words: Sthenoteuthis oualaniensis, morphological characteristics of statolith, growth, the Northwest Indian Ocean,  , El Nino.