A Study On Morphology And Reproductive Biology Of Purple Flying Squid Sthenoteuthis Oualaniensis In The Central And Southern South China Sea

Sthenoteuthis oualaniensis is a warm water specie of oceanic Ommastrephidae,head of copepods,and thought to distribute in tropical and subtropical waters of India and Pacific oceans.S.oualaniensis plays an important role in the ecological system,which is the prey of some marine species and also the predator of the marine system,and also one of the main target species of fishing.With the decline of inshore fisheries resources,abundant S.oualaniensis resources in the in the central and southern South China Sea has become a new type specie of fishing.Domestic and foreign scholars have studied the morphology,age,growth and population composition of S.oualaniensis,it has complex population structure,and most area of the studies focus in the India ocean.In order to know the morphology and reproductive biology in the purple flying squid S.oualaniensis,the indicators including mantle length(ML),Arm Length-I(AL1),Arm Length-Ⅱ(AL2),Arm Length-Ⅲ(AL3),Arm Length-Ⅳ(AL4),Tentacle Length(TL),Fin Length(FL),Fin Width(FW),Body weight,Net body weight,gonad weight and maturity of individuals of dwarf and medium forms of S.oualaniensis were measured in this study.We analyzed the relationship between mantle length and the other morphometric indicators in the purple flying squid S.oualaniensis,obtained the best models for female and male individuals reflecting the relationship between each morphometric indicator and ML through model fitting screening,respectively,analized the morphological variations among the squid species using principal component analysis,and studied the reproductive biology.The results showed:(1)We established the regression equation with morphometric indicators as variables and ML as the dependent variable.The results showed that the best model fitted for most male and female individuals of two forms was power function,the linear function model and logarithmic function model take the second and third place.Through comparing two forms power function between mantle length and the other morphometric indicators,the result showed that: Significant differences of coefficient in AL1-ML,AL2-ML,AL3-ML,TL-ML,FL-ML and power exponent in AL1-ML,AL2-ML,AL3-ML,AL4-ML,TL-ML,FL-ML,FW-ML were identified between the female of medium and dwarf forms.Significant differences of coefficient in AL1-ML,AL2-ML,AL3-ML,AL4-ML,TL-ML,FL-ML,FW-ML were identified and power exponent in AL1-ML,AL2-ML,AL3-ML,AL4-ML,TL-ML,FL-ML,FW-ML were not identified between the male of medium and dwarf forms.It showed that the length of all morphometric indicators increased obviously with the elevation of gonadal maturity,the average rate of female was bigger than male,and the average rate of medium form was bigger than dwarf form.The analysis of fin length and fin width showed that the ratio of fin length and fin width was different with the increasing of fin length between medium and dwarf forms,and the difference is more obvious in male individuals.(2)Medium and dwarf forms have different mantle length distribution.The non-overlapping ranges of the mantle length are 59-78 mm and 126-225 mm for medium and dwarf forms in female individuals,which contains 45% of medium and 10% of dwarf forms.The non-overlapping ranges of the mantle length are 56-80 mm and 95-151 mm for medium and dwarf forms in male individuals,which contains 98% of medium and 58% of dwarf forms.Significant differences in all the ratios of the 7 morphological indicators to mantle length between the medium and dwarf forms of males and females were found by tests of significance(P<0.05).Significant differences between males and females in AL1(Arm Length-I)/ML,FL(Fin Length)/ML,and FW(Fin Width)/ML were identified in the dwarf form(P<0.05),while significant differences between males and females in AL1/ML,AL2/ML,AL3/ML,TL(Tentacle Length)/ML,FL/ML,and FW/ML were found in the medium form(P<0.05).The analysis of coefficients of difference indicated that the morphological variations between the medium and dwarf forms generally were within species(C.D were less than 1.28 except FW/ML of males),but the FW/ML variations between the medium and dwarf forms of males attained a level of difference between subspecies(C.D=1.45).Principal component analysis indicated that the second principal component represented the growing differences among the wrists and fins,and the third principal component represented the growing differences among the wrists.The discriminant functions based on the morphological ratios had the identification accuracy of 85.9% and 92.9% respectively for two forms of females and males,which could provide a reference for classification of the squid species.(3)The medium form could be distinguished into cohort I and cohort II which accounted for 69% and 31% of the total variation,respectively.The percentage of females of the two cohorts decreased first with increasing ML,then increased and reached the ratio of 100% finally.Using Logistic regressing,the MLs at first maturity for females and males of cohort I were estimated to be 164 mm and 112 mm,respectively,and those for females and males of cohort II were 158 mm and 109 mm,respectively.The cohort I concentrated in the waters of Xisha Islands and Nansha Islands,whereas the cohort II was located in the waters of Nansha Islands.Significant differences in all the ratios of the 8 morphological indicators between males and females of cohort I and cohort II were found(P<0.05).Significant differences between cohort I and cohort II in ML,AL1,AL2,AL4,TL and FW were identified in females(P<0.05),while significant differences between cohort I and cohort II in ML,AL1,AL2,AL3,AL4 and FL were found in males(P<0.01).The analysis of coefficients of difference indicated that the morphological variations between cohort I and cohort II indicated a clear division within the species.(4)There existed two forms,a medium and a dwarf form with significant difference in the sex maturity(coefficient of maturity,gonad somatic index and gonad weight)between them,the ratio of female to male of the medium form was about 1: 1,and most of immaturity of that form was female,and most of maturity of that form was male.Females were larger than males of dwarf form.The medium form spent longer to become maturity than dwarf form.The male spent longer to become maturity than female.There was a Positive correlation between sex maturity and latitude in dwarf form.The mature period and reproduction had seasonal characteristic.The difference of sex maturity were identified to be important key features to differentiate dwarf population from the medium form.