上海海洋大学学报  2020, Vol. 29 Issue (2): 234-241    PDF    
长江野生中华绒螯蟹降海洄游过程中组织系数和脂类组成的变化
封强梅1,2,3, 龙晓文1,2,3, 姜晓东1,2,3, 成永旭1,3,4, 吴旭干1,3,4     
1. 上海海洋大学 农业农村部鱼类营养和环境生态研究中心, 上海 201306;
2. 上海海洋大学 农业农村部淡水水产种质资源重点实验室, 上海 201306;
3. 上海海洋大学 上海水产养殖工程技术研究中心, 上海 201306;
4. 上海海洋大学 水产科学国家级实验教学示范中心, 上海 201306
摘要:采用活体解剖和生化分析方法,首先比较了长江野生中华绒螯蟹(简称河蟹)降海洄游过程中,在淡水区域与长江口低盐度海水中的肝胰腺指数(hepatosomatic index,HSI)和性腺指数(gonadosomatic index,GSI)的变化,进一步比较了两种群河蟹肝胰腺和肌肉中总脂含量及脂肪酸含量差异。结果表明:(1)淡水组雄蟹的HSI和雌蟹的GSI均显著高于对应的半咸水组(P < 0.05);无论雌性个体还是雄性个体,淡水组肝胰腺中的总脂含量均显著高于半咸水组(P < 0.05),淡水组和半咸水组肌肉中的总脂含量无显著差异(P>0.05);(2)就肝胰腺中的脂肪酸组成而言,淡水组雄蟹和雌蟹肝胰腺中的C20:1n9、C18:2n6和C18:3n3显著高于半咸水组,而前者的C18:1n7、C20:5n3和C22:5n3均低于半咸水组,因此淡水组肝胰腺中总n-3多不饱和脂肪酸(∑n-3 PUFA)以及∑n-3 PUFA /总n-6多不饱和脂肪酸(∑n-6 PUFA)的值显著低于半咸水组,而∑n-6PUFA含量较高(P < 0.05);(3)就雄体肌肉的脂肪酸组成而言,淡水组雄蟹肌肉中的C17:0、C18:0、C20:1n9、C18:2n6、C18:3n3和C20:2n6百分含量显著高于半咸水组,而半咸水组的C16:1n7、C20:5n3和C22:5n3含量显著高于淡水组,故其∑n-3 PUFA和∑n-3 PUFA/∑n-6 PUFA的值较高(P < 0.05);淡水组雌蟹肌肉中除C17:1n7和C20:4n6显著高于半咸水组(P < 0.05)外,两组雌体其余脂肪酸变化规律基本与雄体类似。综上,长江中野生河蟹成体由淡水区洄游至咸水区时,肝胰腺指数及其总脂含量均显著降低,咸水区野生亲本已经完成交配和产卵,其肝胰腺和肌肉中的总n-3长链多不饱和脂肪酸(∑n-3-LC-PUFA,≥C20:2n)百分含量显著升高。
关键词中华绒螯蟹    长江野生成蟹    降海洄游    脂类组成    肝胰腺指数    性腺指数    

中华绒螯蟹(Eriocheir sinensis,以下简称河蟹)是一种洄游性的甲壳类动物,广泛分布于我国东部沿海地区[1],是重要的经济蟹类。众所周知,自然条件下野生河蟹的生活史较为复杂,表现为:河蟹生殖蜕壳之前的大部分时间都在淡水河流和湖泊中生活,而生殖蜕壳后的河蟹则成群结队地向下游河口区或浅海处进行洄游[2-4],在降海洄游过程中河蟹性腺逐步发育,最终在半咸水中发育成熟并进行交配、产卵和孵化[5-6],随后幼体孵化后在河口半咸水中发育为大眼幼体后开始溯河洄游到淡水中进行生长发育[6]。研究表明,长江水系野生河蟹种群具有优异的养殖性能和经济价值[7-8],但迄今为止,对其成蟹降海洄游所知甚少,故研究其洄游规律具有一定的理论意义和现实意义。

脂类是重要的营养成分之一,对于河蟹的生长发育十分重要[9-10]。肝胰腺是甲壳动物最主要的营养物质消化吸收和储存器官,其营养状况可以间接反映甲壳动物饵料的营养价值[11-12]。脂类是肝胰腺中储存的主要能源物质和性腺发育的必需营养物质,对河蟹降海洄游和性腺发育起着重要作用[13-14]。此外,河蟹作为广盐性蟹类中高渗调节的代表种类[15],盐度是河蟹洄游过程中的主要环境因子,对甲壳动物脂质代谢存在一定的影响[16-17],且甲壳动物细胞膜上的脂肪酸组成对其渗透压调节起着重要作用[18-21]。先前的研究表明,长江野生河蟹每年11月洄游到江苏镇江江段性腺发育已经基本成熟,翌年3—4月在长江口崇明岛东侧的产卵场中发现的河蟹雌体已经报卵[22],雄体已经有过交配行为[5-6]。因此,在野生河蟹降海洄游过程中,采集性腺发育基本成熟(没有交配和产卵)的河蟹群体和产卵场中已经交配和抱卵的群体,比较这两种群体的肝胰腺指数、性腺指数、总脂含量和脂肪酸组成,对于理解河蟹的降海洄游、营养状况、渗透压调节与脂类营养的关系具有一定指导价值。迄今为止,尚未见长江野生河蟹洄游过程中肝胰腺指数、性腺指数及其脂类营养组成的报道。鉴于此,本文首先比较了长江水系野生淡水与半咸水成蟹的肝胰腺指数和性腺指数,并进一步测定和比较了肝胰腺和肌肉的总脂含量和脂肪酸组成,旨在为进一步理解长江野生河蟹的降海洄游规律和生态生理适应机制提供基础资料。

1 材料与方法 1.1 实验用蟹和解剖

2015年11月在江苏镇江附近的长江干流(32.11°N, 119.27°E)捕捞降海洄游的淡水中的野生河蟹,长江河口区的半咸水中的河蟹(雌蟹为抱卵蟹)于2016年4月初捕捞自崇明岛东侧的河蟹产卵场(31.62°N, 121.40°E),所有实验用蟹均由当地渔民采用蟹笼或小型底拖网捕捞而得,淡水的盐度为0,河口区半咸水的盐度为15。随机挑选体质量接近的淡水河蟹与半咸水河蟹雌雄各10只用于后续实验,淡水河蟹雌雄个体的平均规格分别为(109.02 ± 1.45)g和(135.06 ± 2.43)g,半咸水河蟹雌雄个体的平均规格分别为(112.49±2.83)g和(133.55±3.11)g。所有实验用蟹活体运输至上海海洋大学甲壳动物营养繁殖实验室用于后续实验。用干毛巾擦干河蟹体表面水分后,用电子天平(JY1002,上海浦春计量仪器有限公司,精确度=0.01 g)测量体质量,并采用游标卡尺(M150,上海精美量具厂,准确度=0.02 mm)测量其甲壳长和甲壳宽。活体解剖所有个体,取出全部肝胰腺和性腺并准确称量,用于计算肝胰腺指数(hepatosomatic index, HSI)和性腺指数(gonadosomatic index, GSI)。河蟹躯体经冷冻处理后,精刮出大部分肌肉,随后将肝胰腺、性腺和肌肉样品保存于-40 ℃冰箱,用于后续的营养成分测定。HSI和GSI的计算公式如下:

    (1)
    (2)

式中:IHS为肝胰腺指数;IGS为性腺指数;WH为肝胰腺质量,g;WG为性腺质量,g;W为体质量,g。

1.2 总脂和脂肪酸测定

参考FOLCH等[23]的方法,采用V(氯仿):V(甲醇)=2:1溶液提取样品中总脂并测定其含量。根据MORRISON等[24]的方法,采用14%的三氟化硼-甲醇溶液对以上总脂样品进行甲脂化处理,然后采用Agilent 7890B-5977A气相色谱-质谱联用仪(GC-MS)进行脂肪酸分析。色谱柱为Omegawax 320毛细管柱(30 m×0.25 mm×0.25 μm; Supelco, Billefonte, PA, USA),进样口温度为240 ℃,辅助加热器温度为245 ℃。升温程序:由40 ℃以10 ℃/min速率升至170 ℃,再以2 ℃/min速率升至220 ℃,保留1 min,最后以2 ℃/min速率升至230 ℃,保留5 min。根据脂肪酸混标出峰时间对样品的脂肪酸进行定性,然后采用峰面积归一化法对脂肪酸进行相对定量。

1.3 数据分析

采用SPSS 20.0软件对数据进行统计分析,数据以平均值±标准误(Mean±SE)表示。采用Levene法进行方差齐性检验,当数据不满足齐性方差时对百分比数据进行反正弦或者平方根处理,用独立样本t检验(independent samples t-test)进行显著性分析,P < 0.05为差异显著,P < 0.01为差异极显著。

2 结果 2.1 野生长江淡水与半咸水河蟹组织系数和总脂含量的比较

野生长江淡水与半咸水河蟹的组织系数和总脂含量见表 1。对雄体而言,淡水组河蟹的肝胰腺指数和肝胰腺总脂含量均显著高于半咸水组(P < 0.05),但两组间雄蟹的性腺指数和肌肉总脂含量均无显著性差异(P> 0.05)。对雌体而言,性腺指数和肝胰腺总脂含量均在淡水组较高(P < 0.05),而两者间的肝胰腺指数和肌肉总脂含量无显著性差异(P> 0.05)。

表 1 野生长江淡水与半咸水河蟹组织系数和总脂含量的比较(%湿质量) Tab.1 Tissue indices and total lipid content of wild E. sinensis between freshwater and brackish water in the Yangtze River(% wet mass)
2.2 野生长江淡水与半咸水河蟹脂肪酸含量的比较

野生长江淡水与半咸水河蟹肝胰腺的脂肪酸含量见表 2。半咸水组雄蟹肝胰腺中的C16:0、C18:0和总饱和脂肪酸(total saturated fatty acids, ∑SFA)含量显著高于淡水组(P < 0.05);半咸水组雄蟹肝胰腺中的C17:1n7和C18:1n7含量较高(P < 0.05),但两组间总单不饱和脂肪酸(total monounsaturated fatty acids, ∑MUFA)含量无显著性差异(P>0.05);淡水组肝胰腺中的C18:2n6、C18:3n3和总n-6多不饱和脂肪酸(total n-6 polyunsaturated fatty acids, ∑n-6 PUFA)含量显著高于半咸水组(P < 0.05),而C20:5n3、C22:5n3、总n-3多不饱和脂肪酸(total n-3 polyunsaturated fatty acids, ∑n-3 PUFA)和长链多不饱和脂肪酸(total long chain polyunsaturated fatty acids, ∑LC-PUFA)含量及∑n-3 PUFA/∑n-6PUFA比值均在半咸水组较高(P < 0.05)。对雌蟹而言,淡水组和半咸水组肝胰腺中的∑SFA含量无显著性差异(P>0.05);淡水组肝胰腺中的C20:1n9含量显著高于半咸水组,而半咸水组中C18:1n7的含量较高(P < 0.05),但是两组间的∑MUFA含量无显著性差异(P>0.05);C18:2n6、C18:3n3和∑n-6 PUFA含量均在淡水组较高(P < 0.05),半咸水组肝胰腺中的C20:5n3、C22:5n3、C22:6n3、∑n-3 PUFA和∑LC-PUFA含量及∑n-3 PUFA/∑n-6 PUFA的值显著高于淡水组(P < 0.05)。

表 2 野生长江淡水与半咸水河蟹肝胰腺脂肪酸含量的比较(%总脂肪酸) Tab.2 Comparison of the fatty acid composition in the hepatopancreas of wild E. sinensis between freshwater and brackish water in the Yangtze River (% total fatty acids)

对雄蟹肌肉中的脂肪酸组成而言,淡水组肌肉中的C17:0、C18:0和∑SFA含量显著高于半咸水组(P < 0.05),但两组间的∑MUFA含量无显著性差异(P>0.05);淡水组肌肉中的C18:2n6、C18:3n3、C20:2n6和∑n-6 PUFA含量显著高于半咸水组(P < 0.05),而C20:5n3、C22:5n3、∑n-3 PUFA和∑LC-PUFA含量及∑n-3 PUFA/∑n-6 PUFA的值均在半咸水组较高(P < 0.05)。在雌蟹中,淡水组肌肉中的C17:0、C18:0和∑SFA含量显著高于半咸水组(P < 0.05);半咸水组肌肉中的C16:1n7、C18:1n9、C18:1n7和∑MUFA的含量显著高于淡水组(P < 0.05);淡水组雌蟹肌肉中的C18:3n3、C20:2n6、C20:4n6和∑n-6 PUFA含量较高(P < 0.05),而C20:5n3、∑n-3PUFA和∑LC-PUFA含量及∑n-3 PUFA/∑n-6 PUFA值均在半咸水组较高(P < 0.05,表 3)。

表 3 野生长江淡水与半咸水河蟹肌肉脂肪酸含量的比较(%总脂肪酸) Tab.3 Comparison of fatty acid composition in the muscle of wild E. sinensis between freshwater and brackish water in the Yangtze River (% total fatty acids)
3 讨论 3.1 河蟹洄游过程中组织系数和总脂的变化

河蟹是一种广盐性的甲壳动物,具有很强的渗透压调节能力[25-26],生殖蜕壳后通常向河口区或浅海进行降海洄游,在河口区完成交配、产卵、孵化等[27],盐度可以促进河蟹的性腺发育和交配[28-29]。本研究结果表明淡水组雄体的肝胰腺指数及其总脂含量均高于半咸水组,这可能是因为雄体洄游过程中肝胰腺中大量脂肪被消耗,故肝胰腺指数下降较多。淡水组和半咸水组雌体的肝胰腺指数并没有显著差异,推测是因为本次采样的镇江江段的河蟹雌体卵巢发育已经接近成熟(卵巢指数为9.14%),雌体在长江中降海洄游和卵巢发育过程中已经消耗了肝胰腺中的大量脂类来提供能量,或者将肝胰腺中的脂类转运到卵巢中满足卵巢对脂类的需要[9, 14],从而导致肝胰腺指数已经下降到6%左右。河蟹肝胰腺中的脂类以甘油三酯为主,甘油三酯是甲壳动物的主要能源物质[9],因此,在河蟹雌体降河洄游过程中肝胰腺中的总脂含量有所下降,可能是肝胰腺中的甘油三酯被用于氧化供能所导致的。

咸水中河蟹雄体的性腺指数低于淡水组,这可能是因为雄体在咸水中交配后,雄体交配过程中将精荚输送到雌体的纳精囊内[6],导致性腺指数下降。半咸水组雌体性腺指数低于淡水组,是因为雌体已经完成第一次排卵,第一次排卵后的卵巢指数通常在1%~2%[30],然而雌体在抱卵过程中,卵巢开始第二次发育,故本研究中半咸水组的卵巢指数可达6.75%[31]。无论雌体还是雄体,各组河蟹肌肉中总脂含量均无显著差异,这是因为肌肉中的常规生化组成通常比较稳定,才能维持肌肉正常的生理功能[32]

3.2 河蟹洄游过程中脂肪酸含量的变化

先前研究表明,水体盐度的变化会影响甲壳动物组织中的脂肪酸组成,如锯缘青蟹(Scylla serrata)肌肉和肝胰腺中的长链多不饱和脂肪酸(LC-PUFA)和总n-3多不饱和脂肪酸的(∑n-3 PUFA)含量随水体盐度的升高呈下降趋势。而较高盐度亦可显著降低河蟹肝胰腺中EPA(C20:5n3)和DHA(C22:6n3)的含量[33]。研究表明DHA和EPA不仅是机体维持细胞结构和功能的重要脂肪酸,且是河蟹性腺发育中卵巢中储存的主要多不饱和脂肪酸,在其产卵、受精和孵化等过程中起着重要作用[34-35]。本研究中半咸水组河蟹肝胰腺中的EPA、DHA和∑n-3 PUFA的含量均显著高于淡水组,其可能原因有两点:(1)河蟹洄游过程中,肝胰腺中的饱和及单不饱和脂肪酸被作为能源物质而利用,因此肝胰腺中的EPA、DHA和∑n-3 PUFA相对总脂肪酸而言,其相对百分比例呈现增加趋势;(2)水体盐度对河蟹肝胰腺中的脂质代谢存在一定的影响,随着水体盐度提高,肝胰腺中的EPA、DHA和∑n-3 PUFA被相对保留下来。此外,本研究中淡水组河蟹肌肉的EPA和DHA均显著低于半咸水组,而C18:2n6含量相对较高,这可能是肌肉中的脂肪酸PUFA组成存在一定的补偿效应,组织中DHA和EPA百分含量上升,可能会导致∑n-6PUFA百分含量下降。先前研究表明,提高了组织中的n-3 LC-PUFA百分含量有利于提高甲壳动物的渗透压调节能力[19, 29],这可能是半咸水组肝胰腺和肌肉中n-3 LC-PUFA百分含量较高的重要原因之一。

本研究比较野生河蟹降海洄游过程中淡水组和半咸水组河蟹的性腺指数、肝胰腺指数和脂类组成,可以为进一步研究长江野生河蟹的降海洄游规律和生态生理适应机制提供参考资料,但是尚不清楚造成这些变化的具体原因,下一步研究需要室内单因素和多因素调控实验,进一步探讨降海洄游、水体盐度和交配产卵等因素对河蟹脂质代谢的影响,从而解析河蟹降海洄游过程中脂类组成变化的具体原因,探讨河蟹降海洄游和脂质代谢的关系。

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Changes of tissue indices and lipid composition of wild adult Chinese mitten crab (Eriocheir sinensis) during catadromous migration
FENG Qiangmei1,2,3, LONG Xiaowen1,2,3, JIANG Xiaodong1,2,3, CHENG Yongxu1,3,4, WU Xugan1,3,4     
1. Centre for Research on Environmental Ecology and Fish Nutrition, Ministry of Agriculture and Rural Affairs, Shanghai Ocean University, Shanghai 201306, China;
2. Key Laboratory of Freshwater Aquatic Genetic Resources, Ministry of Agriculture and Rural Affairs, Shanghai Ocean University, Shanghai 201306, China;
3. Shanghai Engineering Research Center of Aquaculture, Shanghai Ocean University, Shanghai 201306, China;
4. National Demonstration Centre for Experimental Fisheries Science Education, Shanghai Ocean University, Shanghai 201306, China
Abstract: The study was conducted to compare hepatosomatic index (HSI), gonadosomatic index (GSI) in wild adult Chinese mitten crab (Eriocheir sinensis) during their catadromous migration, which were sampled from freshwater region near Zhenjiang city (defined as freshwater group) and brackish water area near Chongming Island in the estuary of Yangtze River(defined as brackish water group). After the dissection and biochemical analysis, total lipids and fatty acid composition in the hepatopancreas and muscle were further compared between the two groups. The results showed that: (1) The HSI of the males and the GSI of the females in the freshwater group were significantly higher than those of the corresponding brackish water group (P < 0.05); despite of males or females, the freshwater group had the significantly higher total lipids in the hepatopancreas than the brackish water group (P < 0.05), and there was no significant difference in total lipids in the muscle between the two groups (P> 0.05); (2) As for the hepatopancreatic fatty acid compositions, both male and female crabs from the freshwater group had the significantly higher levels of C20:1n9, C18:2n6 and C18:3n3, but the lower percentages of C18:1n7, C20:5n3 and C22:5n3 in the hepatopancreas than the brackish water group, which resulted in the fact that the freshwater group had the significantly lower levels of total n-3 polyunsaturated fatty acids (∑n-3 PUFA) and the ratio of ∑n-3 PUFA/ total n-6 polyunsaturated fatty acids (∑n-6 PUFA), but the higher ∑n-6 PUFA levels in the hepatopancreas than the brackish water group (P < 0.05); (3) As for the fatty acid compositions in the muscle, the males from freshwater group had significantly higher levels of C17:0, C18:0, C20:1n9, C18:2n6, C18:3n3 and C20:2n6, but lower percentages of C16:1n7, C20:5n3 and C22:5n3 than that of the brackish water group; thus the males from brackish water group had higher percentages of ∑n-3 PUFA and the ratio of ∑n-3 PUFA/ ∑n-6 PUFA than males from freshwater group (P < 0.05);The changing patterns of female fatty acid profiles in the two groups were consistent with those in the males, but the levels of C17:1n7 and C20:4n6 in female muscle of the freshwater group were significantly higher than those of brackish water group (P < 0.05). In conclusion, when the wild adult E. sinensis migrated from freshwater area to the brackish water area in the estuary of Yangtze River, the significant decreases were found in HSI and total lipids in the hepatopancreas, but the crabs sampled from brackish water generally had the higher percentage of the total n-3-long chain PUFA (∑n-3-LC-PUFA, ≥C20:2n) in the hepatopancreas and muscles than the freshwater group.
Key words: Eriocheir sinensis     Yangtze River wild adult crab     catadromous migration     lipid composition     hepatosomatic index     gonadosomatic index