| Sthenoteuthis oualaniensis belongs to Sthenoteuthis,Ommastrephidae,Cephalopoda,and was a warm-water pelagic species with a wide distribution in tropical and subtropical waters of the Pacific and Indian Oceans(40°N-40°S).With its short life cycle and rapid growth,it was a commercially valuable economic cephalopod and plays an important ecological role in the ecosystems.As a feeding organ,beak were as resistant to corrosion,structurally stable and rich in information storage as hard tissue structures such as statolith and inner shell.Due to its large size,ease of extraction and ease of determination,was also widely used in studies of cephalopod fisheries ecology and resource assessment.According to the 1896 Sthenoteuthis oualaniensis samples collected by Chinese light falling-net fishery during the same months(February-May)in the northwest Indian Ocean,this study presents the differences in cephalopod population structure,morphology and growth characteristics of beak between 2019(Ei-Ni(?)o)and 2020(normal),providing a scientific basis for subsequent studies on the ecology of the Indian Ocean cephalopod fishery and resource assessment using the beak.(I)Basic biology of Sthenoteuthis oualaniensis in the Northwest Indian Ocean between different years.The result indicated that the mantle length(ML)range and dominant ML group of 2019 were larger than 2020,especially for females.The ML range of females in both years were 129～347mm and 284～582mm,and the dominant ML group were 171～220mm and 481～530mm,respectively.The ML range of males were 138～273mm and 94～235mm,and both of the dominant ML group were171～220mm.The analysis of covariance(ANCOVA)showed that the relationships between body weight(BW)and ML of Sthenoteuthis oualaniensis in different years was significantly different between the sexes.By fitting and optimizing the equations and comparing the AIC,the relationship between ML and BW of males were most suitable to be expressed by exponential functions,except for the males of 2020 when the relationship between ML and BW of males was most suitable to be expressed by a exponential function.The relationships between ML and BW for both the 2019 and2020 were best expressed as power functions,except for the relationship between ML and BW for the 2020 males,which were best expressed by a exponential function.The gonad maturity composition varied between different years,the females were mostly at immature stages(I and II)in 2019 but mature stages(stages III and IV)in2020,while the males were mostly at stages II and III in 2019 but mature stages(III and IV)in 2020.The age composition and hatching characteristics differed between years,the females ranged from 97～263d and 148～267d,and dominant age groups were 171～200d and 231～260d with hatching dates from June to December and from May to October,and peaked in August-September and July-August,respectively.The males age ranged from 131～253d and 133～221d,with a dominant age 171～200d for both years,and the hatching dates were from August to December and from September to December and the peak periods were September-November and October-November,respectively.(II)Beak morphological growth characteristic of Sthenoteuthis oualaniensis in the northwest Indian Ocean in different years.The principal component analysis showed that the upper hood length(UHL),the upper crest length(UCL),the upper lateral wall length(ULWL),the lower crest length(LCL),the lower wing length(LWL)and the lower lateral wall length(LLWL)could be used as the morphological characteristic parameters to study the morphological changes of the beak.The analysis of covariance(ANCOVA)showed that there were significant differences in the relationships between all of the morphological characteristic parameters and mantel length(ML)except for UHL in the two-year female sample,and there were not significant differences in the relationships between all of the morphological characteristic parameters and mantel length in the two-year female sample.ANCOVA showed that there were significant differences in the relationships between all of the morphological characteristic parameters and mantel length(ML)for males and females in 2019.The relationships between all of the morphological characteristic parameters for females and UHL,UCL,ULWL for males and ML were optimally described by the linear functionss while the relationships between LLWL,LWL and ML were optimally described by logarithm functionss.There were not significant differences in the relationships between all of the morphological characteristic parameters and mantel length(ML)for males and females in 2020.The relationships between all of the morphological characteristic parameters and ML were optimally described by the linear functionss except for ULWL.The relationship between beak and body weight was significantly different between the sexes in both samples.The relationships between all of the morphological characteristic parameters and body weight were optimally described by power functions for females,and logarithm functions for males except for LCL,which was optimally described by linear functions in 2019.The relationships between all of the morphological characteristic parameters and body weight were optimally described by logarithm functionss for females,and linear functions for males except for LCL,which was optimally described by logarithm functions in 2019.(III)The effects of individual differences on the morphology of the beak of Sthenoteuthis oualaniensis in the Northwest Indian Ocean between different years.Analysis of variance(ANOVA)showed that there were significant differences between sexes,different gonad maturity and different mantle length.The least significant difference(LSD)showed that the variation of each characteristic parameter varied between any two mantle length groups and gonadal maturity in different years.The ratio of each characteristic parameter to mantle length was only significantly different between sexes in 2019.The ratio of each characteristic parameter to mantle length was not significantly different between different gonadal maturity and mantle length groups in 2020.Moreover,the ratio of each characteristic parameter to CL were stable.In summary,the morphologyl changes of beak in the Northwest Indian Ocean were significantly different among individuals in different years,and basically increased with the increase of the mantle length of Sthenoteuthis oualaniensis.(IV)Beak microstructure and growth characteristic of Sthenoteuthis oualaniensis in the north-west Indian Ocean in different years.Each growth stripe was smaller in spacing and width,more lightly pigmented,with rounded,unnotched and less worn rostral tips,and more abnormal stripe changes for beak microstructure in 2019.ANOVA showed that the relationship between the characteristic parameters of beak and age at day differed significantly between the sexes in different years.The relationship between all profile parameters and age were optimally described by exponential functions for females in 2019 and logarithm functions for females in 2020.The relationship between all profile parameters and age were optimally described by power functions,except for LCL for 2019 males,and linear functions for 2020 males.The AGR and IGRG of beak showed a clear opposite inverted "U"-shaped trend with increasing age in all individuals except for males in 2020,which showed a decreasing and then increasing "U"-shaped trend.The AGR of each characteristic parameter was generally higher in 2020,while the IGRG of each characteristic parameter was higher in 2019. |
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