Growth Of Gladius And Feeding Migration Characteristics Of Sthenoteuthis Oualaniensis In The Northwest Indian Ocean

Sthenoteuthis oualaniensis belongs to Sthenoteuthis,Ommastrephidae,and was a wide distribution in the Indian Ocean,the equatorial Pacific and subtropical sea areas.It has great exploitation potential and is an important cephalopod resource in China.The gladius is a chitin structure that grows from the caudal fin to the head and has support function in the dorsal midline of the cephalopod mantle.Because of the irreversibility of growth,it is a good carrier to record the complete life history of cephalopod.According to the samples of S.oualaniensis collected in the northwest Indian Ocean in 2019 by the Chinese light falling-net fishery fleets.Combined with basic biological information,external morphological parameters and and stable isotope information of gladius,the growth and feeding characteristics of S.oualaniensis were studied.This will provide basis and reference for further research on the conditions affecting the growth of cephalopods.The main research results are as follows:（1）Gladius morphology characteristics of Sthenoteuthis oualaniensis in the northwest Indian Ocean.To explore the growth characteristics of S.oualaniensis,the morphological characteristics of gladius structure were studied based on the 1388samples collected in the northwest Indian Ocean in the autumn of 2019（September to November）.Principal component analysis showed that gladius length（GL）,cones length（CL）,vanes length（VL）,greatest width of cones（GWC）and greatest width of proostracum（GWP）could be used as characteristic parameters to describe the change in the shape of the gladius.Results of the analysis of covariance（ANCOVA）showed that there were significant differences between sexes in the growth relationship between all characteristic parameters and mantel length（ML）except VL,and there were significant differences between sexes in the growth relationship between all characteristic parameters and body weight（BW）.According to equation fitting and Akaike Information Criterion（AIC）analysis,The parameters of the gladius were significantly correlated with mantel length and body weight,and could be expressed as linear functions,power functions and logarithmic functions.（2）Factors influencing gladius morphology of S.oualaniensis in the northwest Indian Ocean.To explore the factors affecting the external morphology of gladius of S.oualaniensis,the gladius length（GL）,cones length（CL）,vanes length（VL）,greatest width of cones（GWC）and greatest width of proostracum（GWP）were used as characteristic parameters to describe the change in the shape of the gladius.Analysis of variance（ANOVA）and least significant difference method（LSD）were used to study the effects of sex,individual size,and gonadal maturity on changes in morphology and relative size.The results showed that the inner shell shape characteristics were significantly different（P<0.05）.There was no significant difference in VL/GL values between different mantle length groups（P>0.05）,but there was significant difference among different gonad maturity（P<0.05）.However,GWC/GWP values were significantly different between different mantle length groups and gonad maturity（P<0.05）.The results showed that gonadal maturity stagesⅡandⅢmay be the inflection points of the changes of the morphology of gladius of S.oualaniensis.In the process of individual growth,the growth rate of each structure of gladius length was basically the same,while the growth rate of cones was slower than that of proostracum.（3）Feeding ecology of Sthenoteuthis oualaniensis based on carbon and nitrogen stable isotope analysis in the Northwest Indian Ocean.S.oualaniensis samples were collected by Chinese light falling-net fishery in March to May 2019 in the northwest Indian Ocean,from which the trophic niche and feeding ecology of S.oualaniensis were studied using stable isotope technology.Results show no significant difference inδ¹³C between female and male individuals with gladius length（P>0.05）,butδ¹⁵N of male individuals had significant differences with gladius length（P<0.05）.The trophic niche of the males（TA（the total area of convex hull）=0.6152‰,and SEA_C（corrected standard ellipse area）=2.35²‰）was greater than the females’（TA=0.362‰,SEA_C=1.58²‰）.The gladius growth of both males and females had no significant relation withδ¹³C content（P>0.05）,but a significant relation withδ¹⁵N（P<0.05）.The changes inδ¹³C（1.28‰）andδ¹⁵N（3.46‰）indicate a migratory phenomenon that the migration and trophic level of S.oualaniensis were improved as it grew.Furthermore,the females were in a higher trophic level than males.