| Purpleback flying squid Sthenoteuthis oualaniensis is an abundant and highly productive species that distributes in the open waters around the Indo-Pacific Ocean covering from the tropical to the subtropical waters.This species is characterized by fast growth,short lifespan,and semelparous reproduction.In the epipelagic to mesopelagic waters,S.oualaniensis plays a critical role in the ecosystem,not only preying on a very wide spectrum of food organisms from mesozooplankton to myctophids,but also supporting diverse marine predators including squids,fishes,sharks,whales and seabirds.Meanwhile,presumably owing to the high abundance,S.oualaniensis increasingly becomes one of the important fishery species.In the central South China Sea,for example,this species is one of the major target species for the small-scale jigging fisheries,as well as for the large-scale light falling-net fisheries.Many studies have carried out about the age growth,reproductive characteristics and resource evaluation of S.oualaniensis in the South China Sea,and a preliminary consensus on the fishery biology of different forms has been reached.However,the research on its reproductive characteristics is still in the primary stage of gonadosomatic index analysis and fecundity analysis of mixed forms,However,the research on its reproductive characteristics is still in the primary stage of gonadosomatic index analysis and fecundity analysis of mixed forms,and the research of reproductive characteristics of different forms and the relationship between reproductive characteristics and environmental factors still needs to be carried out.Therefore,to understand and master the ovipositing pattern,reproductive investment pattern,relationship between reproductive characteristics and environmental factors of different forms of S.oualaniensis is helpful to understand its reproductive adaptability and form differentiation reproductive strategy in the South China Sea ecosystem,and to provide theoretical support for its sustainable development and scientific management of resources.In this dissertation,Sthenoteuthis oualaniensis specimens were collected from the catches of light-falling fisheries in the South China Sea from September to December2012 and from March to May 2013 and from June to July 2017 and 2018.The reproductive characteristics and body condition of purple flying squid Sthenoteuthis oualaniensis were studied,using model performance,morphometric analyses and residual indices analyses.Based on the number of oocytes in the ovaries and oviducts of females and the number of spermatophores in the spermatophoric sac of males,the potential fecundity,batch fecundity and relative fecundity for females and the batch fecundity and relative fecundity for males were quantitatively analyzed to determine the ovulation and ejaculation mode of iris squid.By using the technology of tissue energy density determination combined with residuals analyses,the energy accumulation of somatic and reproductive tissues and the allocation to reproduction in the female and male individuals were analyzed.Based on the measurement of tissue energy and fecundity data,the reproductive investment of female individuals during the whole reproductive period was calculated.The residuals of tissue weight regression on the mantle length and the tissue energy accumulation regression on the mantle length were estimated.Additionally,the carbon and nitrogen isotope ratios??13C and?15N?were determined for gonadal tissues,soma and digestive gland.These analyses could allow for the clarification of reproductive investment pattern.?1?Reproductive biological characters:Both medium and dwarf forms were found allometric growth.The power regression between body weight and mantle length showed that the immature individuals had a smaller regression coefficient a than that for the mature ones.Regarding the regression index b,in contrast,the immature individuals had a larger value than that for the mature ones.The number of female individuals was equal to male for the medium form,whereas for the dwarf form,females significantly outnumbered males.With the progress of maturation,the ratio of female to male in the medium form decreased gradually,while it increased in the dwarf form.Both females and males showed a significant increase in gonadosomatic index as the progress of maturation regardless of forms.Although the number was seasonally changeable,mature individuals were found throughout the sampled months without considering the forms.For the females of the medium form,individuals sampled in December showed the highest gonadosomatic index,the largest sex ratio?femle:male?and the largest number of mature ones,whereas in the dwarf form such observations were found in April.The body condition,represented as the standardized residuals of eviscerated weight regressed on mantle length,showed a better status at maturity for the female and male individuals from both forms.Similar result was abtained with the analyses of gonadal development.Meanwhile,individuals from both forms showed an increase trend in food intake as maturation progressed.?2?Fecundity and spawning pattern of female:In medium form,the potential fecundity of female individuals was estimated from 1076 cells to 220332 cells,in which the ovarian oocytes accounted for 63-100%.The relative fecundity was determined from8.47 cell.g-1to 789.72 cell/g.The index of potential reproductive investment was0.03?0.17.In dwarf form,the potential fecundity of female individuals ranged from1218 to 32843 oocytes.The number of ovarian oocytes ranged from 1178 to 30397 cell,and the ripe eggs in the oviducts ranged from 113 to 2904 cell.In addition,the relative fecundity was?48.93?842.12?cell/g and the overall potential reproductive investment index varied from 0.012 to 0.083.A positive correlation was found in both medium and dwarf forms between the potential fecundity and relative fecundity on mantle length,body weight and nidamental gland length.The batch fecundity varied from 15086 cells to 46711 cells for female medium form individuals,with average spawning events of6.19±1.90.Batch fecundity was estimated from 1895 to 2904 cell based on the ripe eggs in the oviducts for late mature individuals?stage??for female dwarf form individuals,and the number of spawning batch was estimated from 6 to 15,with an average of?9.87±4.36?.Ovarian oocytes were measured from 0.10 mm to 1.06 mm for female medium form individuals and 0.17?0.99 mm for female dwarf form individuals in the major axis length,and the size distribution was unimodel,in which there were significant differences in major axis length of ovarian oocytes between different maturity stages?P<0.05?.Cumulatively,these observations suggested that the female individuals of medium form of S.oualaniensis develop high fecundity that significantly increases with body growth.The ripe egg was relatively small in size and released in batches by the adult females.?3?Fecundity and spawning pattern of male medium:The length and weight of spermatophoric sac showed an increasing trend with sexual maturation,and were measured length of 22?124 mm and weight of 0.03?3.07 g,respectively.The spermatophoric sac's weight showed a power regression on its length.In addition,both the length and weight of spermatophoric sac were linearly correlated with mantle length and body weight.The effective fecundity was estimated from 1 spermatophores at stage?to 144 spermatophores at stage?,and the relative effective fecundity by mantle length was estimated from 0.02 to 1.62 spermatophores per millimeter.The effective fecundity increased with maturity,attaining the highest value at stage?,and the relative effective fecundity by mantle length remained stable at stages?and?.The effective fecundity showed a linear relationship with mantle length and body weight?Fig.4?,while the relative effective fecundity by mantle length had a power regression on mantle length and body weight.The spermatophore length was a power function of its weight,in which they were measured from 4.79 to 36.60 mm and from 0.000 2 to0.020 0 g,respectively.Both the length and weight of spermatophores attained their highest value at stage?,and in addition,both of them were found linear correlation with mantle length and body weight.Therefore,these findings indicated that the spermatophoric sac,spermatophores and effective fecundity in the medium form of male S.oualaniensis from South China Sea showed a consistent increasing trend with sexual maturation,and significantly related to mantle length and body weight.?4?Tissue energy allocation and investment pattern:Significant difference in the energy density was found between different tissues,and medium form showed a significantly higher value than dwarf form.For medium form,the male individuals showed significant difference in the energy density of mantle muscle and spermatophoric complex between sexual maturity stages;the females exhibited significant difference in the energy densty of mantle muscle.For dwarf form,all the tissues analyzed?except the nidamental gland?showed significant difference in the energy density between sexual maturity stages.With respect to female individuals,both somatic and reproductive organs accumulated the largest amount of energy at stage?.There was form-specific in the maximum energy accumulation during the course of maturation for the male individuals,in which both somatic and reproductive organs for the male medium form individuals accumulated the largest amount of energy at stage?,while the arms and testis for the male dwarf form individuals accumulated the largest amount of energy at stage?whereas,other organs including mantle,fins and spermatophoric complex attained the maximum at stage?.For a given maturity stage,the proportion of somatic tissue energy was consistently larger than that of reproductive organs.The proportion of energy accumulated in the reproductive organs of females increased significantly in line with sexual maturation.In male individuals,the proportion increased rapidly from maturity stages?to?for the male dwarf form individuals followed by a non-significant change at stages?and?,while that in medium form increased rapidly in stage?-?,and decreased significantly in stage?-?.The somatic tissues are the largest unit of energy accumulation in the medium form of female S.oualaniensis,and reproductive tissues accumulate energy significantly along with sexual maturation.There are form-and sex-specific in energy density,energy accumulation and relative tissue energy accumulation of somatic and reproductive organs.When all the oocytes in the ovary were ripe,the total energy accumulation in the ripe eggs of the medium form females was estimated370.18±212.67 k J,and that of the dwarf form females was estimated 125.82±46.43 k J.The potential reproductive investment was 400.98±222.65 k J in the medium form and132.56±46.94 k J in the dwarf form when all the oocytes in the ovary matured.And there was a significant difference in the relative potential reproductive investment between the two forms,in which the medium form had a higher relative potential reproductive investment than the dwarf form.This indicates that the relative reproductive investment of dwarf form was higher than that of medium form,although the body size of dwarf form was smaller than that of medium form.?5?Reproductive investment pattern:The growth rate of eviscerated weight,total reproductive weight and digestive gland weight of female was higher than that of male,and that of medium form was higher than that of dwarf form.The condition of the reproductive in male individuals showed a decreasing trend from stages immature to stages maturing followed by an improvement to stage mature,while in female individuals showed an improvement from stages immature to stage spent.By analyzing the positive relationship between the TRW-ML residuals and these two residuals in two forms,EW-ML residuals and DGW-ML residuals,it can be concluded that the reproductive input of all forms was mainly exogenous.The feeding status of medium form and dwarf form was good during gonadal development.The mean values of?13C and?15N in digestive gland,mantle and ovary of medium form were higher than those of dwarf form.The?13C and?15N values of each tissue in the two forms of female individuals showed different trends.The distribution of?13C and?15N values in the ovary of the medium form and dwarf form overlaps with that in the digestive gland tissues,which can be distinguished from that in the mantle,indicating that the distribution of?13C and?15N values in the ovary and digestive gland tissues is more similar.Further ANOSIM test showed that the?13C and?15N values of ovary and digestive gland tissues in the two forms were more similar,especially in the dwarf form.Residuals analyses indicated that a positive correlation was found between the residuals of each somatic tissue energy accumulation regressed on the mantle length and the residuals of reproductive tissues?combined nidamental gland,oviducal complex and ovary in females,while in males combined spermatophoric complex and testis?energy accumulation on the mantle length,suggesting that there was no energy trade-off between somatic and reproductive tissues during the sexual maturation.Meanwhile,according to the reanalysis of residuals from model?regressions of energy accumulation against mantle length,there was slightly negative relationship in the residuals between reproductive and somatic tissues from stages?to?,possibly indicating energy remobilization from soma to develop reproductive tissues,but this process of remobilization did not impact the integrity of somatic tissues.In this paper,the findings improved our understanding of the reproductive characteristics,fecundity,energy accumulation and reproductive investment pattern of medium and drawf forms of S.oualaniensis in the South China Sea.Additionally,the results revealed the form-specific differences in the reproductive strategies of S.oualaniensis and the relationship between reproduction and living environments.The results can not only provide a reference for the study of reproductive strategies of commercial cephalopods,but also provide basic data for the assessment of S.oualaniensis resources in the South China Sea. |
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