Inter-individual variability of stable-isotope ratios in marine organisms

利用氮和碳稳定同位素比值分析营养水平比通过肠道含量检查进行传统的饮食习惯分析有利，因为前者反映了单个动物的长期饮食史，而后者仅提供了捕获前的食物组成。本研究采用氮碳稳定同位素比值法和常规肠道含量分析法，对相模湾及邻近海域中上层鱼类、中深海鱼类和虾类食性的个体间变异进行了研究。在日本凤尾鱼中，幼鱼和成鱼的个体稳定同位素比值都聚集在两组，氮的变化约为1.7%。在中深海和深海对虾中，Bentheogennema borealis（一种非洄游的深海对虾）的δ ^<13>C值显著低于两种垂直洄游对虾，δ ^<15>N值显著高于两种垂直洄游对虾。在后者中，Sergiaprehensilis在体型较大的个体中显示出更高的δ ^<15>N值，这与肠道含量分析的结果一致（强食肉性）。另一种候鸟——四足棘藻（Acanthephyra quadrispinosa）的δ ^<15>N值与卷叶棘虫相似，但大小之间的差异不太明显，表明四足棘藻从相对年轻的阶段开始就摄入了更高营养水平的食物。垂直洄游鱼类毛利鱼（Maurolicus muelleri）的δ ^<15>N的个体间差异约为三种非洄游中上层鱼类的1.5倍，表明毛利鱼同时利用上层和中上层的食物资源。综上所述，目前的研究结果表明，三角洲值提供了个体动物食物特征和食物历史的有用信息，这些特征与个体的其他生态特征（如生长、垂直迁移和分布深度）密切相关。

Analysis of trophic levels by using nitrogen and carbon stable-isotope ratios is advantageous to conventional food-habit analysis by gut-content examination, in that the former reflects a long-term food history of an individual animal, while the latter gives a food composition only immediately before capture. The present studies examined inter-individual variability of food-habits of pelagic fishes, meso-and bathypelagic fishes and shrimps in Sagami Bay and adjacent waters, by applying nitrogen and carbon stable-isotope ratio method along with conventional gut-content analysis.In Japanese anchovy, the individual stable-isotope ratios of both larvae and adults were clustered into two groups, with a variation of ca. 1.7% in nitrogen. In meso-and bathypelagic shrimps, Bentheogennema borealis, a non-migrant, deep-sea species, showed significantly lower delta^<13>C and higher delta^<15>N values than the two vertical migrants. Of the latter, Sergiaprehensilis showed higher delta^<15>N values in larger individuals, coinciding to the result of gut-content analysis (strong carnivory). Acanthephyra quadrispinosa, another migrant, showed delta^<15>N values similar to S.prehensilis but difference between sizes was less marked, suggesting that A.quadrispinosa take foods of higher trophic levels from relatively young stages. Maurolicus muelleri, a vertical migrant, showed inter-individual variations of delta^<15>N of ca. 1.5 times those of the three non-migrating mesopelagic fishes of the genus Cyclothone, suggesting that the former utilizes food resources from both the epi-and mesopelagic layers.In summary, the present results demonstrated that the delta values provide useful information on food characteristics and food history of individual animals and that these characteristics are closely related to other ecological characteristics of individuals, such as growth, vertical migration, and distributional depths.