Grazing experiment with Acartia tonsa

Marine copepods provide the major food-web link between primary producers and higher trophic levels, and their feeding ecology is of acute interest in light of global change impacts on food-web functioning. Recently, quantitative polymerase chain reaction (qPCR) protocols have been developed, which can complement classic diet quantification methods, such as stable isotope or fatty acid analyses tools. Here, we present first results of feeding experiments assessing sex- and stage-specific food intake by the ubiquitous calanoid copepod Acartia tonsa by 18S targeted qPCR and microscopic grazing assessment. In triplicated mixed-diet feeding treatments, three suitable A. tonsa diets, the cryptophyte Rhodomonas balthica, the haptophyte Isochrysis galbana, and the diatom Thalassiosira weissflogii, were offered in equal biomass proportions under constant conditions. Prey uptake substantially varied between different algal species, as did the extent of sex- and stage-specificity of prey uptake. Male adult copepods had higher R. balthica gut contents than females, and nauplii contained more of this prey source than copepodites or adult copepods in mixed treatments. A trend towards higher amounts of ingested T. weissflogii in adult females than in males and in nauplii than in other stages was detected. Genetic gut content quantifications indicated low feeding on I. galbana, and no consistent sex- or stage-specific differences of I. galbana content in A. tonsa. Our results highlight diet-specific feeding differences between Acartia life stages and sexes, which can have implications on food-web dynamics and specific nutrient transfer to higher trophic levels in copepod populations of varying age composition under changing environmental parameters, such as rising temperatures and increasing ocean acidification.

海洋桡足类是连接初级生产者与更高营养级的核心食物网环节，鉴于全球变化对食物网功能的影响，其摄食生态学备受学界关注。近年来，定量聚合酶链式反应（quantitative polymerase chain reaction，qPCR）技术方案得以开发，可作为稳定同位素、脂肪酸分析等经典饵料定量方法的补充手段。
本研究首次报道了通过18S靶向qPCR与显微滤食评估，对广布性哲水蚤目物种汤氏纺锤水蚤（Acartia tonsa）的性别与发育阶段特异性摄食率开展的摄食实验结果。在三重重复的混合饵料摄食处理组中，我们为汤氏纺锤水蚤提供了三种适配饵料：隐藻门的巴尔特红胞藻（Rhodomonas balthica）、定鞭金藻门的球等鞭金藻（Isochrysis galbana）以及硅藻门的威氏海链藻（Thalassiosira weissflogii），各饵料按等生物量比例在恒定环境条件下投喂。
不同藻类的猎物摄取量存在显著差异，其摄食的性别与发育阶段特异性程度亦各不相同。混合饵料处理组中，雄性成体桡足类的巴尔特红胞藻肠道内含物水平高于雌性，无节幼体的该饵料摄入水平亦高于桡足幼体或成体。研究还检测到，成年雌性个体摄入的威氏海链藻量高于雄性，且无节幼体的摄入量高于其他发育阶段的趋势。基于分子标记的肠道内含物定量结果显示，汤氏纺锤水蚤对球等鞭金藻的摄食水平较低，且未发现该饵料的肠道含量存在一致的性别或发育阶段特异性差异。
本研究结果揭示了汤氏纺锤水蚤不同发育阶段与性别间的饵料特异性摄食差异，这类差异或对食物网动态产生影响，并在温度升高、海洋酸化等环境参数变化的背景下，影响不同年龄组成的桡足类种群中营养物质向更高营养级的特异性转移。