Reproductive Energy Accumulation Of Dosidicus Gigas And Responding To Living Environments

Dosidicus gigas is a typical Ommastrephid species under the genus Dosidicus in the Ommastrephida family and plays an important role in the eastern Pacific ecosystem.Dosidicus gigas is one of the most abundant cephalopod species,supporting the world cephalopod fisheries and being the main targetedspecies in China distant jigging fishery.This species is characterized by fast growth,short life cycle,and semelparous reproduction.In terms of reproductive investment,it has proved that D.gigas adopts mixed income-capital breeding strategy,where the reproductive energy is from concurrernt food intake during the period of sexual maturation,coupled with remobilization of somatic energy reserve when energy is required urgently at physiologically maturing and spawning period.Generally,cephalopods are highly sensitive to environmental fluctuations,and show adaptation to living environments in reproductive investment that is closely related to reproductive strategies.Herein,we observed the growth,reproductive energy accumulation and potential effects from living environment for D.gigas.The findings would put forward our understandings about the squid’s breeding strategy in terms of reproductive investment,and provide scientific reference to support the species’ fishery sustainability and development.The specimens of D.gigas were randomly collected from China distant jigging fishery catches during February to August 2017,January,May to August 2018,May to August 2019 and October to December 2020.The reproductive energy accumulation of D.gigas and responding to living environments were explored based on size growth,tissue energy accumulation and environmental factors,as well as fatty acids analyses of gonad tissue.We applied statistic methodologies of: using model residual analyses and generalized linear mixed effect model(generalized linear mixed-effects models,GLMM)to investigate the relationships between reproductive energy living environment factors.The main results are as follows:(1)Body size growth.Female squids ranged between 163 mm and 468 mm ML and between 123 g and 3034 g BW,and males ranged between 171 mm and 475 mm ML and between145 g and 2681 g BW.Both female and male squids varied significantly in dorsal mantle length and body weight amongsampling months,in which females and males both were found in the largest size in February and relatively smaller size in June to August.Specimens from each sampling month exhibited a unimodal distribution of dorsal mantle lengthconsistently.Both female and male squids sampled during February to April showed likelihood of isometric growth but in poorer body condition,while those sampled from June to August were mostly found allometric growth with better body condition.In addition,to understand the relationships of dorsal mantle length to environmental variables and sampling latitudes,the generalized linear mixed-effect models using sampling month as random effect were performed,and a positive and significant effect of sea surface temperature on the attainment of dorsal mantle length was found for both sexes.Meanwhile,the effect of sampling latitudes on the dorsal mantle length of both sexes was also significant,showing an increasing trend of ML towards the equator.The effect of chlorophyll-a concentration on the dorsal mantle length was only found significant in female squids.Therefore,the monthly change of body size of D.gigas was significant in the equatorial waters of eastern Pacific Ocean,and significantly correlated with the sea surface temperature and chlorophyll a concentration.(2)Relative reproductive energy accumulation.The tissue energy determination technique and generalized linear mixed-effect models(GLMM)were used to explore the potential effects of marine environments on the relative energy accumulation in soma and gonad tissues.The results showed that soma tissue was the largest organ for energy accumulation,accounting for at least 70% of the total energy of D.gigas,while gonad tissues accumulated the rest of the energy from food intake.The relative energy accumulation in soma tissue declined significantly from physiologically maturing stage to functionally mature stage,followed by a significant rebound at the spawning stage.In contrast,the relative energy accumulation in gonad tissues showed an increasing trend after the onset of physiologically maturing,reaching a peak at the stage of functionally mature.The relative energy accumulation in soma was not significantly correlated with sea surface temperature,neither was the relative energy accumulation in gonad tissues.However,there was significant relationship between the relative energy accumulation in both soma and gonad tissues and sea surface chlorophyll-a concentration.There was also significant relationship between the relative energy accumulation in soma and gonad tissues and sampling longitudes in the East Pacific Ocean.These observations indicate that D.gigas increases energy allocation to gonadal development after the onset of physiologically maturing,and maintains somatic condition during the spawning period.The relative energy accumulation in soma and gonad tissues may be more likely related to marine production,due to larger relative energy accumulation associated with higher chlorophyll-a concentration.(3)Reproductiv investment in relation to feeding based on fatty acids analyses.The methodologies of fatty acid biochemical markers and tissue energy density technique were applied to study the possible pathway of reproductive energy in terms of feeding habits.The results showed that the total fatty acids were significantly different seas among sampling regions(R=0.15,P = 0.001),namely Ecudor,Equator and Peru.Additionally,the fatty acid composition showed non-overlap between sexual maturation when multivariate analyses were perfomed on at each sampling regions,confirmed by the analysis of similarities(ANOSIM)(Ecuador: R= 0.458,P = 0.002;Equator:R= 0.514,P = 0.004;Peru:R= 0.773,P = 0.006).There was a significantly positive relationship between ovarian energy density and DHA concentration.In contrast,a negative correlation was detected between ovarian energy density and EPA concentration.The findings indicate that D.gigas may feed on different food items to meet the reproductive energy demand during sexual maturation.