Biology of widely distributed marine protists, Phaeodaria (Rhizaria, Cercozoa)

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  • Nakamura Yasuhide
    Plankton Laboratory, Graduate School of Fisheries Sciences, Hokkaido University
  • Suzuki Noritoshi
    Institute of Geology and Paleontology, Graduate School of Science, Tohoku University

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Other Title
  • 海洋に広く分布する浮遊性原生生物，フェオダリア類の生物学（総説）
  • カイヨウ ニ ヒロク ブンプ スル フユウセイ ゲンセイ セイブツ,フェオダリアルイ ノ セイブツガク(ソウセツ)

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<p>Phaeodarians are unicellular marine plankton characterized by the presence of 1) “central capsule” containing the nucleus, 2) “phaeodium”, or mass of brown particles, and 3) a siliceous skeleton called the “scleracoma”. This protist group has long been classified as a member of Radiolaria, but now they belong to the phylum Cercozoa of Rhizaria, which also contain Radiolaria and Foraminifera. The subclass Phaeodaria is divided into 7 orders and 18 families under the current classification scheme, and the group consists of ca. 80 genera and ca. 200 species. These heterotrophic plankton presumably feed on organic materials suspended in the water column or capture other plankton. They are thought to reproduce by cell division and swarmer production. This group is widely distributed in the world ocean from the surface to deep waters. The number of phaeodarian species is the largest in the Pacific and its marginal seas (ca. 150 species), and the smallest in the Arctic regions (ca. 15 species). The abundance of this group tends to be the highest in the lower epipelagic or mesopelagic zone, and their distributional depth is largely controlled by the water temperature, the amount of food, and the quantity of dissolved silica. The seasonality of their vertical flux depends on the region. The biomass of phaeodarians sometimes becomes as large as that of major plankton, such as copepods. Considering their occasional high biomass and the fact that they produce minipellets, this group possibly plays an important role in the material cycles of the ocean.</p>

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