The Importance of Small Planktonic Copepods and Their Roles in Pelagic Marine Food Webs
The Importance of Small Planktonic Copepods and Their Roles in Pelagic Marine Food Webs
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作者：
J. T. Turner
小型浮游海洋桡足类（长度<1毫米）是地球上最丰富的后生动物。包括哲水蚤属的成虫和桡足类，如副哲水蚤属、黄哲水蚤属和蜱螨属;剑水蚤属，如长刺水蚤属、Oncaea属和Corycaeus属;小刚毛属的无节幼体;以及几乎所有桡足类物种的无节幼体。尽管小型桡足类丰富，但由于使用网眼大于200 - 333微米的网，它们在历史上一直被采样不足。然而，最近的研究表明，当使用100微米或更小的适当网目时，小型桡足类的丰度和生物量远远超过大型桡足类。未能充分考虑小型桡足类可能会导致严重低估浮游动物的丰度和生物量，桡足类放牧对浮游植物初级生产的影响，浮游动物介导的化学品和材料的通量，以及海洋中的营养相互作用。小型桡足类的摄食生态学不如大型桡足类（如哲水蚤属）的成体那样为人所知。此外，大多数小型桡足类的摄食信息是沿海属，如Acartia，而不是近海类群。虽然一般认为小型桡足类，包括无节幼体，主要以小型浮游植物细胞为食，但大多数此类信息来自有限实验室饮食的饲养或喂养研究。很少有人研究过真正的桡足类以海洋中发现的天然浮游植物和微型浮游动物的混合食物为食，但其中一些已经产生了惊喜。例如，Oithona和Paracalanus的一些物种，甚至北极哲水蚤的无节幼体。可能主要以捕食异养原生生物为生，而不是作为浮游植物的食草动物。此外，各种热带桡足类物种的无节幼体已被证明以浮游细菌为食。因此，许多基本的问题仍然是喂养生态和放牧/捕食影响的小型桡足类在海洋中。尽管对小型桡足类的食物了解有限，但很明显，许多高营养级的消费者吃它们。许多研究表明，桡足类无节幼体，Oithona spp.，和其他小型桡足类是鱼苗和其他食虫动物的重要猎物。小型桡足类表现出多种多样的繁殖策略，以弥补由于捕食而造成的种群损失。这些优势包括：在不受食物不足限制的情况下，繁殖力和生长率高;在较温暖的温度下繁殖和生长率高;运动有限，呼吸率低，从而可以在繁殖中投入更多的能量;寿命延长，从而最大限度地提高一生的生殖产出。因此，小型桡足类是海洋食物网中的重要环节，是浮游植物的主要食草动物，是微生物环的组成部分，也是浮游鱼类和其他大型远洋食肉动物的猎物。我们目前对真正的丰度、生物量、营养生态学以及小型桡足类在生源通量中的作用的认识不足，妨碍了对海洋生态学的正确认识。
Small planktonic marine copepods (＜ 1 mm in length) are the most abundant metazoans on Earth. Included are adults and copepodites of calanoid genera such as Paracalanus, Clausocalanus, and Acartia; cyclopoid genera such as Oithona, Oncaea, and Corycaeus; planktonic harpacticoids of the genus Microsetella; and nauplii of almost all copepod species. Despite the abundance of small copepods, they have historically been undersampled due to the use of nets with meshes ＞ 200- 333 μm. Recent studies have shown, however, that when appropriate net meshes of 100 μm or less are used, small copepods vastly exceed the abundance and sometimes the biomass of larger ones. Failure to adequately account for small copepods may cause serious underestimations of zooplankton abundance and biomass, the copepod grazing impact on phytoplankton primary production, zooplankton-mediated fluxes of chemicals and materials, and trophic interactions in the sea. The feeding ecology of small copepods is less well-known than that of adults of larger copepod species, such as members of the genus Calanus. Further, most feeding information for small copepods is for coastal genera such as Acartia, rather than for offshore taxa. Although it is generally assumed that small copepods, including nauplii, feed primarily upon small-sized phytoplankton cells, most such information comes from rearing or feeding studies on limited laboratory diets. There have been few examinations of actual copepod feeding on mixed diets of natural phytoplankton and microzooplankton found in the sea, but some of those have produced surprises. For instance, some species of Oithona and Paracalanus and even nauplii of Arctic Calanus spp. may feed primarily as predators upon heterotrophic protists, rather than as grazers of phytoplankton. Also, nauplii of various tropical copepod species have been shown to feed upon bacterioplankton. Thus, numerous basic questions remain as to the feeding ecology and grazing/predation impact of small copepods in the sea. Despite limited knowledge of what small copepods eat, it is clear that many higher-trophic-level consumers eat them. Numerous studies have shown that copepod nauplii, Oithona spp., and other small copepods are important prey of fish larvae and other planktivores. Small copepods exhibit a variety of reproductive strategies to compensate for losses to their populations due to predation. These include having high fecundity and growth rates, when not limited by insufficient food; having high reproduction and growth rates at warmer temperatures; having limited motion and low respiration rates, allowing the investment of more energy in reproduction; and having extended longevity to maximize lifetime reproductive output. Thus, small copepods are important links in marine food webs, serving as major grazers of phytoplankton, as components of the microbial loop, and as prey for ichthyoplankton and other larger pelagic carnivores. Our present inadequate understanding of the true abundance, biomass, trophic ecology, and role of small copepods in biogenic fluxes precludes proper understanding of the ecology of the sea.