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Body condition and reproductive investment of Dosidicus gigas in the equatorial waters of eastern Pacific Ocean

  • Corresponding author: Dongming LIN, dmlin@shou.edu.cn
  • Received Date: 2018-06-12
    Accepted Date: 2018-11-20
  • Extant cephalopod species are characterized by the lag of sexual maturation, when compared to their body growth. And they have evolved a range of flexible reproductive strategies, which are associated with different characteristics of body growth and reproductive investment, and the latter is vital important to understand their reproduction. In this study, the specimens of Dosidicus gigas, collected from equatorial waters of eastern Pacific Ocean during a jigging fishery period from February to April 2017, were used to explore their body condition and reproductive investment, by applying methodologies of morphometric measurement and residual index analyses. Both female and male individuals were found isometric growth, according to weight-length relationship analysis. Female D. gigas attained maturation from mantle length (ML) of 270 mm and body weight (BW) of 766 g; whereas, males were found maturity from 270 mm ML and 479 g BW. Both female and male individuals had the lowest rate of zero stomach fullness degree (FD), with the majority being found at FD of 1-2. Along with the sampled months, D. gigas showed a better body condition, but which was the worst during the sexual maturation period. The gonadosomatic index (GSI) was measured a mean value of 2.26%±1.33% for sexual mature females, and nidamental index was estimated 34.34%±18.62%. Mature males had a mean GSI of 1.31%±0.35%. The body condition indices, measured as the standardized residuals of body weight and mantle length relationship, and reproductive investment indices, measured as the standardized residuals of reproductive system weight and mantle length relationship, showed a decreasing trend with sexual maturation. When corresponding to stomach fullness degree, the body condition index showed a decreasing trend, whereas the reproductive investment index increased with higher stomach fullness degree. However, there was a significantly positive relationship between indices of body condition and reproductive investment. Therefore, these evidences indicated that D. gigas continue feeding during sexual maturation, and the energy sourcing for reproduction is mostly from food intake. Meanwhile, energy reserve in the muscle tissues might be mobilized to meet the high energy demand during gonadal development.
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Body condition and reproductive investment of Dosidicus gigas in the equatorial waters of eastern Pacific Ocean

    Corresponding author: Dongming LIN, dmlin@shou.edu.cn
  • 1. College of Marine Sciences, Shanghai Ocean University, Shanghai    201306, China
  • 2. Key Laboratory of Sustainable Exploitation of Oceanic Fisheries Resources, Ministry of Education, Shanghai Ocean University, Shanghai    201306, China
  • 3. National Engineering Research Center for Oceanic Fisheries, Shanghai Ocean University, Shanghai    201306, China
  • 4. Collaborative Innovation Center for Distant-water Fisheries, Shanghai Ocean University, Shanghai    201306, China

Abstract: Extant cephalopod species are characterized by the lag of sexual maturation, when compared to their body growth. And they have evolved a range of flexible reproductive strategies, which are associated with different characteristics of body growth and reproductive investment, and the latter is vital important to understand their reproduction. In this study, the specimens of Dosidicus gigas, collected from equatorial waters of eastern Pacific Ocean during a jigging fishery period from February to April 2017, were used to explore their body condition and reproductive investment, by applying methodologies of morphometric measurement and residual index analyses. Both female and male individuals were found isometric growth, according to weight-length relationship analysis. Female D. gigas attained maturation from mantle length (ML) of 270 mm and body weight (BW) of 766 g; whereas, males were found maturity from 270 mm ML and 479 g BW. Both female and male individuals had the lowest rate of zero stomach fullness degree (FD), with the majority being found at FD of 1-2. Along with the sampled months, D. gigas showed a better body condition, but which was the worst during the sexual maturation period. The gonadosomatic index (GSI) was measured a mean value of 2.26%±1.33% for sexual mature females, and nidamental index was estimated 34.34%±18.62%. Mature males had a mean GSI of 1.31%±0.35%. The body condition indices, measured as the standardized residuals of body weight and mantle length relationship, and reproductive investment indices, measured as the standardized residuals of reproductive system weight and mantle length relationship, showed a decreasing trend with sexual maturation. When corresponding to stomach fullness degree, the body condition index showed a decreasing trend, whereas the reproductive investment index increased with higher stomach fullness degree. However, there was a significantly positive relationship between indices of body condition and reproductive investment. Therefore, these evidences indicated that D. gigas continue feeding during sexual maturation, and the energy sourcing for reproduction is mostly from food intake. Meanwhile, energy reserve in the muscle tissues might be mobilized to meet the high energy demand during gonadal development.

  • 茎柔鱼(Dosidicus gigas),隶属于头足纲(Ceohalopada),枪形目(Teuthoidea),柔鱼科(Ommastrephidae),茎柔鱼属(Dosidicus),广泛分布于太平洋中部以东海域,主要集中于0°~18° S的南美大陆架以西200~250 nmile的外海,栖息于表层至1 200 m水层中 [1-3]。茎柔鱼资源丰富,产量大,最高年产量超过 85万t[4],是智利、秘鲁和美国加利福尼亚重要的头足类资源,也是中国鱿钓渔业重要的捕捞对象,具有较高的经济价值[1]

    生殖投入是自然生物繁衍后代、延续生命的重要生活史策略之一,反映生物个体生长发育过程中摄食盈余能量对生殖的投入分配水平[5]。头足类属种生长速率快、生命周期较短,除鹦鹉螺属(Nautilus)外均为终生一次繁殖[6]。一般的,这些种类性腺发育滞后于个体生长,性腺开始发育后,所积累或所摄食的能量均大部分投入生殖行为及配子发生成熟等[7-8]。但是,这些种类对环境波动敏感性高,其生殖投入方式表现出一定的环境适变性,并且与繁殖产卵策略密切相关[9]。因此,研究这些种类的生殖投入对于掌握它们的生长繁殖过程有着重要的意义[5,10]。目前,各国学者对茎柔鱼年龄生长[11]、种群结构[12]、摄食生态[13]等生物学特性方面的研究较为深入,而在其体征生长和生殖投入方面的研究较少。因此,本研究将分析东太平洋赤道海域茎柔鱼的体征生长和生殖投入,探究茎柔鱼在生长发育过程中的体征生长特性和生殖投入分配及其变化过程,以期进一步认知其生物学特性,为可持续开发利用该种类资源提供理论基础。

1.   材料与方法
  • 样本来自我国远洋鱿钓渔船的渔获物,采集时间为2017年2—4月,是茎柔鱼产卵高峰期的第1个峰期(第2峰期在9—11月)[14];采集调查海域为东太平洋赤道海域1°21′N~1°9′S和109°23′~118°36′W(图1)。根据远洋鱿钓渔船作业流动性的特点,每个站点随机采集10尾,共采集茎柔鱼样本537尾,雌性399尾,雄性138尾(表1)。经冷冻保藏后运回实验室进行分析。

    Figure 1.  Survey area and sampling station of D. gigas in the open equatorial Pacific Ocean

    月份
    month
    数量
    number
    雌性 female 雄性 male
    胴长/mm mantle length体质量/g body weight 胴长/mm mantle length体质量/g body weight
    2月 February 95 326~452 972~3 622 345~475 1 191~2 681
    3月 March 262 256~455 428~3 284 268~457 479~2 457
    4月 April 180 228~498 570~3 691 292~438 633~2 486

    Table 1.  Biological information of samples by months

  • 样本于实验室解冻后,进行常规的生物学测定,测定内容包括胴长(mantel length,ML),体质量(body weight,BW),雌性的卵巢重(ovary weight, OvaW)、缠卵腺重(nidamental gland weight, NGW)、缠卵腺长(length of nidamental gland, NGL)、输卵管复合体重(输卵管+卵管腺)(oviducal complex weight, OCW),雄性的精巢重(testis weight, TesW)、精荚复合体重(spermatophoric complex weight, SCW),性腺成熟度(maturity stage, MAT),胃饱满度(stomach fullness degree, FD)。胴长测定精确至1 mm,缠卵腺长测定精确至0.1 mm;体质量测定精确至1 g,其他重量测定精确至0.1 mg。

    生殖系统发育划分,以Guerra等[15]和Hatfield等[16]的性腺成熟等级划分标准为基础,结合实验观察,将茎柔鱼性腺成熟度划分为Ⅰ~Ⅴ期5个等级。其中Ⅰ期是未发育期,雌雄个体的性腺组织较小,呈半透明状;Ⅱ期为生理发育期,性腺组织开始增大变厚,雌性个体可以清晰地看到卵巢中的颗粒状的结构,雄性个体的精荚中有白色的精团;Ⅲ期为生理成熟期,雌性的卵巢呈草青色、能看到大量的卵母细胞,雄性的精巢呈现亮白色,在尼氏囊中可见部分不成形的精荚;Ⅳ期是成熟排卵期,雌性个体的卵巢发育成熟,在输卵管中有成熟卵子,雄性的尼氏囊内有分层次排列的精荚,精荚腺中有白色的精团;Ⅴ期是繁殖后期,雌雄性腺组织萎缩松软。本实验样本鉴别为Ⅰ~Ⅳ期,未发现Ⅴ期样本。

    摄食等级划分,以Zuev等[17]胃饱满度划分等级为标准,划分为0~5级共6个等级。其中,0级为空胃,1级为可见零星食物,2级为胃含物体积不超过胃腔1/2,3级为胃含物体积超过胃腔1/2,4级为满胃但不膨胀,5级为饱满且胃壁膨胀凸出。

  • 按雌雄分类统计,采用频率分布方法分析胴长组成和体质量组成。同时,计算渔获样本的平均胴长、平均体质量,以及优势胴长分布和优势体质量分布及其所占的比例。

  • 采用非线性回归方法,分别进行雌雄个体体质量与胴长的幂函数曲线拟合,求取体质量与胴长(BW-ML)之间的关系:

    式中,BW为体质量(g),ML为胴长(mm),a、b为待估算参数。

    性腺指数(gonadosomatic index, GSI)[18]

    式中,OvaW为雌性卵巢重(g),TesW为雄性精巢重(g),BW为体质量(g)。

    缠卵腺指数(nidamental gland index, NGI)[16]

    式中,NGL为缠卵腺长(mm),ML为胴长(mm)。

    精荚复合体指数(spermatophoric complex index, SCI)[19]

    式中,SCW为精荚复合体重(g),BW为体质量(g)。

  • 根据体质量和胴长(BW-ML)的幂函数回归关系,求取残差并标准化。运用最小二乘法,分别进行雌、雄个体的生殖系统重量与胴长(RsW-ML)的线性回归,求取残差并标准化;其中,雌性个体的生殖系统重为卵巢、缠卵腺和输卵管复合体的合计重量,雄性个体的生殖系统重为精巢、精荚复合体的合计重量。残差值是基于个体水平又独立于个体大小,能够反映出个体或者组织的质量特征[7-8,20-21]:残差值为负值,表示质量特征不佳;残差值为正值,表示质量特征良好。因此,体质量和胴长关系残差值(BW-DML residuals)作为个体的体征指标(body condition index, CI)[22],指标值为正时,表征个体质量特征良好;指标值为负时,则表征个体质量特征不佳。生殖系统重量和胴长关系残差值(RsW-DML residuals)作为生殖投入指标(reproductive investment index, RI)[23],指标值为正值时,代表性腺的质量特征良好;指标值为负值时,代表性腺的质量特征欠佳。

    利用t检验分析雌、雄个体体质量—胴长幂函数关系b值与匀速生长b值=3的差异性,协方差分析(ANCOVA)检验雌雄个体之间体质量—胴长关系的差异性;非参数Kruskal-Wallis方差分析(K-W ANOVA)检验不同月份、不同性腺成熟度等级个体的摄食差异性;单因素方差分析(ANOVA)检验体征指标、生殖投入指标以及性腺指数等随月份推移、性腺发育、摄食强度等之间的差异性,并利用Tukey HSD进行随后差异性检验。利用Spearman-rank相关系数(Sr)分析,检验茎柔鱼的生殖投入指标与体征指标之间的关系,确定性腺组织生长的能量投入是否来自肌肉组织存储的能量[23]

2.   结果
  • 茎柔鱼的胴长为228~498 mm,体质量为428~3 961 g(表1)。其中,雌性个体平均胴长为(355.61±56.78) mm,平均体质量为(1 380.20±654.78) g,最小性腺成熟个体的胴长和体质量分别为270 mm和766 g,最大的未成熟个体胴长和体质量分别为454 mm和3 143 g。有2个优势胴长组(图2-a),分别为270~350和390~450 mm,分别占总数的34.62%和29.81%;2个优势体质量组(图2-b),分别为500~1 300和1 800~2 500 g,分别占53.52%和27.89%。雄性个体平均胴长为(340.63±40.39) mm,优势胴长为270~370 mm,占总数的83.90%(图2-c);平均体质量为(1 092.68±440.00) g,优势体质量为600~1 400 g,占总数的80.30%(图2-d);最小性腺成熟个体的胴长和体质量分别为270 mm和479 g,最大的未成熟个体胴长和体质量分别为475 mm和2 681 g。

    Figure 2.  Distribution of mantle length and body weight for female and male D. gigas

    茎柔鱼的胴长与体质量关系式,雌性个体为BW=0.000 012×DML3.01,拟合回归系数R2=0.91;雄性个体为BW=0.000 020×DML2.99,拟合回归系数R2=0.92(图3)。t检验显示,雌雄个体的体质量—胴长关系b值均与匀速生长假设b=3之间不存在显著性差异(雌性:t=0.23, P=0.82;雄性:t= −0.16, P=0.87)。但是,协方差分析(ANCOVA)结果显示,雌雄个体的胴长—体质量关系存在显著性差异(F=7.53,P=0.006)。

    Figure 3.  Relationships between body weight and mantle length for female and male D. gigas

  • 分析显示,不同月份之间,东太平洋赤道海域茎柔鱼雌雄个体的摄食等级不同(图4)。其中,雌性个体在2月份的摄食等级以1~2级为主(K-W ANOVA, χ2=42.86, P<0.001),约占该月份样本总数的65.70%;3、4月的摄食等级以1级为主(K-W ANOVA, 3月,χ2=139.65, P<0.001;4月, χ2=93.81, P<0.001),分别占每个月份样本总数的56.14%和48.72%。雄性个体2月份的摄食情况比较稳定,摄食等级以1~3级为主(K-W ANOVA, χ2=7.15, P=0.007 5),合计占样本总数的80.01%;3月份摄食等级以1级为主(K-W ANOVA, χ2=48.34, P<0.001),占该月份样本总数的49.30%;4月的摄食等级均以1~2级为主(K-W ANOVA, χ2=16.47, P<0.001),占该月份样本总数的76.73%。

    Figure 4.  Distribution of stomach fullness degree by months in female and male D. gigas

    此外,根据不同性腺成熟度的摄食情况分析发现,雌性个体性腺成熟度Ⅰ~Ⅳ期的摄食等级均以1级为主(K-W ANOVA, Ⅰ期,χ2=140.53, P<0.001;Ⅱ期,χ2=162.36, P<0.001;Ⅲ期,χ2=21.02, P<0.001;Ⅳ期,χ2=6.67, P=0.009 8),分别占每个成熟期样本总数的59.12%、45.54%、66.67%和62.5%;Ⅲ期、Ⅳ期的空胃率占比均为0。雄性个体性腺成熟度Ⅰ期和Ⅱ期时均以摄食等级1级为主(K-W ANOVA, Ⅰ期,χ2=17.04, P<0.001;Ⅱ期,χ2=10.49, P=0.001 2),分别占每期样本总数的63.16%和61.91%;Ⅲ期和Ⅳ期则以摄食等级1~2级为主(K-W ANOVA, Ⅲ期,χ2=31.51, P<0.001;Ⅳ期,χ2=8.57, P=0.003 4),分别占每期样本总数的49.27%和55.55%,空胃率则分别为1.45%和3.70%(图5)。

    Figure 5.  Distribution of stomach fullness degree by sexual maturation in female and male D. gigas

  • 体征指标与胴长之间关系的散点图分布显示,雌雄个体的质量体征呈凹形分布(图6-a, 6-b)。其中,雌性个体的质量体征在胴长300~425 mm时表现为最差(ANOVA, F=4.10,P=0.019);雄性个体的质量体征则在300~375 mm时为最差,但是与其他胴长组的质量体征没有统计学差异(ANOVA, F=1.25,P=0.304)。

    Figure 6.  Distribution of body condition index (CI) in female and male D. gigas

    同时,不同月份之间,雌性个体的质量体征没有显著性差异(ANOVA, F=1.13,P=0.32);但是,个体的质量体征随着月份推移逐渐趋好,在4月份时为最佳(图6-c)。雄性个体的质量体征指标在各月份之间差异显著(ANOVA, F=4.19,P=0.017),在3月份时质量特征较佳(图6-d)。

  • 茎柔鱼雌性个体的性腺指数为0.01%~3.96%,缠卵腺指数为1.57%~52.08%(图7-a)。其中,性腺指数在生理性成熟期(Ⅲ期)增长显著(ANOVA,F=124.21, P<0.001);在成熟排卵期(Ⅳ期)达到最大值,平均值为2.26%±1.33%。缠卵腺指数自生理发育期(Ⅱ期)后迅速增长(ANOVA,F=103.99, P<0.001);在Ⅳ期达到最大值,平均值为34.34%±18.62%。

    Figure 7.  Gonadosomatic index(GSI), nidamental gland index(NDI) and spermatophoric complex index(SPI) in female and male D. gigas

    雄性个体的性腺指数为0.85%~1.22%,精荚复合体指数为3.09%~4.22%(图7-b)。其中,性腺指数随着性腺发育不断增长(ANOVA,F=38.77, P<0.001);在Ⅳ期达到最大值,平均值为1.31%±0.35%。精荚复合体指数在生理成熟期(Ⅲ期)最大,平均值为0.40%±0.34%;但不同成熟度之间没有统计学差异(ANOVA,F=0.80, P=0.56)。

  • 残差指标分析显示,茎柔鱼雌性个体的体征指标(体质量—胴长关系残差值)和生殖投入指标(生殖系统重量—胴长残差值)均随着性腺发育成熟而下降,并在生理性发育期(Ⅱ期)之后下降显著(ANOVA, 体征指标,F=12.09, P<0.001;生殖投入指标,F=108.97, P<0.001)(图8-a)。但是,随着摄食强度增加,体征指标呈下降趋势(ANOVA, F=1.92, P=0.90),生殖投入指标则略上升(ANOVA, F=0.85, P=0.517)(图8-b)。此外,Spearman-rank相关系数(Sr)分析显示,雌性个体的体征指标和生殖投入指标存在显著的统计学相关性(Sr=0.159, P=0.006)。

    Figure 8.  Body condition index (CI) and reproductive investment index (RI) in female and male D. gigas

    随着性腺发育,雄性个体的体征指标在Ⅳ期时较差,但是不同成熟度之间没有统计学差异(ANOVA, F=2.12, P=0.100);生殖投入指标则随着性腺发育显著下降,Ⅳ期时指标值最低(ANOVA, F=13.56, P<0.001)(图8-c)。同时,二者随着摄食强度均没有统计学差异(ANOVA, 体征指标,F=1.10, P=0.363;生殖投入指标,F=1.29, P=0.278);但是在胃饱满度3级及以上,生殖投入指标均为正值,而体征指标呈下降趋势(图8-d)。类似于雌性个体,雄性个体的体征指标和生殖投入指标也存在显著的统计学相关性(Sr=0.215, P=0.033)。

3.   讨论
  • 茎柔鱼生长速率快,通常为1年生,部分海域群体的寿命可达2年[24],而且不同海域群体的胴长范围及其优势胴长组各不相同[25]。其中,秘鲁外海的茎柔鱼群体胴长范围为200~880 mm,优势胴长为240~480 mm[26];智利外海群体的胴长范围为287~702 mm,优势胴长为380~430 mm[27];哥斯达黎加外海群体的胴长范围为205~429 mm,优势胴长为260~360 mm[28]。本实验东太平洋赤道海域茎柔鱼群体的胴长范围为228~498 mm,与哥斯达黎加外海群体的相类似。

    一般的,茎柔鱼因其寿命短、全年生长、全年繁殖等生活史特点而形成了相对复杂的群体结构,并依据性腺成熟个体大小划分为多个体型群[3]:小型群体,雌雄成熟个体的胴长范围分别为140~340 mm和130~260 mm;中型群体,雌雄成熟个体的胴长范围分别为280~600 mm和240~420 mm;以及大型群体,雌雄成熟个体的胴长范围分别为550~1 000 mm和400~500 mm。本实验茎柔鱼雌雄成熟个体的最小胴长均为270 mm,以此初步推测这些实验样本可能来自中型群体。同时,雌性个体存在2个优势胴长组及其体质量组,说明它们可能来自于不同的产卵群体,因为茎柔鱼终生一次发育繁殖、繁殖时间短,产卵后便死去,不可能存在繁殖后继续生长和(或)世代重叠的个体[3,6]。然而,具体的群体划分及其产卵群体确定仍需要借助耳石微结构等技术进行产卵孵化时间的推算[11]

    此外,头足类存在着雌雄二态型及其性腺发育的雌雄差异性,并反映在个体的体质量—胴长幂函数关系的b值上[25]。一般的,当b=3时,个体为匀速生长;当b>3时,为正异速生长;当b<3时,为负异速生长,反映了生物个体的生长率及其身体形态发育等特征[29]。本实验茎柔鱼雌雄个体的体质量—胴长幂函数关系的b值分别为3.01和2.99。t检验显示雌雄个体拟合函数的b值均与匀速生长假设的b=3不存在显著性差异(P>0.05),说明该实验茎柔鱼雌雄个体均为等速生长。但是,雌雄个体之间的胴长—体质量关系存在显著的差异性(ANCOVA, P=0.006),这可能是茎柔鱼雌雄个体性腺发育差异性的一种表现,前者性腺发育较晚但成体较大,后者性腺发育较早但个体相对偏小[3]。此外,本实验的b值均略小于陆化杰等[30]研究所得的同地理海域雌雄个体胴长—体质量幂函数关系的b值(3.09和3.02),可能与该海域不同年份的食物丰度差异及其栖息环境条件的变化等密切相关[28]

  • 已有研究表明,茎柔鱼在其较短的生命周期内进行着索饵场—产卵场之间的洄游活动,并且在产卵洄游过程中持续摄食以积累组织能量[9]。同时,类似于其他头足类属种,茎柔鱼为机会主义捕食者,在食物资源不充足时甚至会发生同类残食以满足自身的生长发育需要[31]。对摄食等级的分析显示,本实验茎柔鱼雌雄个体的空胃率低,也说明这些个体保持着持续摄食行为。然而,这些实验个体的摄食等级比较集中在1~2级,这可能与其较高的食物消化率相关。因为茎柔鱼群体的摄食活动一般发生在日出前后,而捕捞作业却是在夜间进行[32]。Jereb等[3]曾报道柔鱼类胃含物的完全消化吸收时间周期为3~8 h,平均每小时消耗食物重量为其体质量的0.6%左右。同时,海域生产力在一定程度上影响着头足类的摄食行为,进而影响这些种类的摄食强度[6]。东太平洋赤道海域生态系统具有较低的浮游植物群落,并且与上升流强度密切相关[33]。本实验茎柔鱼个体相对较低的摄食等级也可能是由于该海域相对较低的浮游生物量所致。

    然而,本实验发现性腺发育成熟度Ⅲ~Ⅳ期的茎柔鱼个体,雌性个体的空胃率为零,雄性个体的空胃率仅为1.45%~3.70%,说明这些个体自生理性成熟之后并没有停止或逐步停止摄食行为。这种摄食行为不同于同科属下的阿根廷滑柔鱼(Illex argentinus)自生理性成熟后逐渐降低摄食强度或停止摄食的情况[34-35],可能是它们之间不同的产卵繁殖策略所致。茎柔鱼的产卵繁殖策略表现为多次产卵,每次产卵间歇保持摄食及体细胞生长[9,34];而阿根廷滑柔鱼产卵繁殖策略为终端间歇性产卵,产卵期间停止摄食和体细胞生长[9, 36]

    此外,基于残差分析的体征生长显示,茎柔鱼雌雄个体的体征质量均随着月份推移逐渐变好。这与胃饱满度分析的空胃率低的结果是相一致的,说明这些个体持续摄食以保持个体的生长发育。同时,本实验研究发现雌性茎柔鱼个体在胴长300~425 mm、雄性个体在胴长300~375 mm时的体征质量最差,表明该胴长范围的个体生长状况不佳。根据上述胴长组成分析,茎柔鱼样本应该来自于中型群[3],而生长状况不佳的个体也正好处于中型群的性腺成熟个体的胴长范围之内。这在一定程度上说明了该胴长范围内体征质量较差的原因可能与其性腺生长发育密切相关。因为柔鱼类的性腺发育相对滞后于个体生长[7-8],而生理性发育开始之后性腺发育所需的能量需求很高,有时候甚至以牺牲个体的肌肉生长为代价[37-38]

  • 现生头足类,除鹦鹉螺属的种类外,均表现为终生一次发育繁殖,交配繁殖后便死去[3]。但是,这些种类的生殖投入类型往往表现为属种特殊性,或倾向于内源性,即生殖投入来源于储存肌肉等组织的能量;或倾向于外源性,即生殖投入来源于繁殖产卵期间捕食食物的消化吸收[38-40]。McGrath等[8]认为,成熟个体性腺指数低于15%的种类,其生殖投入类型倾向于外源性。本实验赤道外海茎柔鱼个体的性腺指数及其附属腺指数(雌性的缠卵腺指数、雄性的精荚复合体指数)均增长显著,但是成熟个体的性腺指数较低,结果与秘鲁外海茎柔鱼群体的相一致[12]。因此,可以初步推断茎柔鱼的生殖投入倾向于外源性,而这些个体在性腺成熟后的空胃率低则可为本推断提供一定的佐证。

    同时,本实验结果显示,赤道海域茎柔鱼在胃饱满度3级及以上的个体生殖投入指标均为正值,并随着胃饱满度等级增加呈上升趋势,而体征指标则呈下降趋势。这进一步说明了该种类的生殖投入倾向于外源性,即以食物摄食为主。然而,本实验也发现茎柔鱼雌雄个体的体征指标和生殖投入指标之间均表现为正相关关系,并存在显著的统计学相关性(P<0.05)。此外,自生理发育开始之后,生殖投入指标和体征指标均呈下降趋势,并多表现为负值,表明这些个体的体征质量和生殖投入均不理想,则有可能说明当时的外界环境(如饵料生物)有所改变,并影响着个体生长和生殖投入的变化[5]。而且这种变化将会产生2种结果:①停止生殖发育,以待下一个繁殖周期的到来,如大部分鱼类[5];②牺牲个体生长,将肌肉等组织存储的能量转化供给生殖发育所需,如阿根廷滑柔鱼[38,40]。茎柔鱼为终生一次发育繁殖的属种[3],不存在多个繁殖周期,第2种情况的发生更符合这些种类的繁殖发育。但是,具体的投入及其来源情况仍需要后续的稳定同位素和脂肪酸等生物标记测定的判别,以及能量积累测定的量化分析。

    综上所述,本实验通过传统的性腺指数、胃饱满度等分析手段,结合残差指标分析方法,初步阐明了赤道海域茎柔鱼的体征生长过程及其生殖投入类型。赤道海域茎柔鱼雌雄个体均为匀速生长;随着月份推移,体征质量逐渐变好,而在个体性腺发育阶段,体征质量最差;生殖投入倾向于外源性,发育过程中肌肉组织存储的能量可能部分转化用于生殖发育。这些结果将为进一步认识茎柔鱼的生物学特性提供基础,也为深入开展大洋性柔鱼类生殖投入策略及其选择适应性研究提供科学支撑,以更科学合理地、可持续地开发利用这些种类资源。

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