Phaeodarians are unicellular marine protists characterized by the "central capsule" containing the nucleus, the "phaeodium", or mass of brown particles, and a siliceous skeleton called the "scleracoma". Phaeodaria have long been classifi ed as a member of the Radiolaria; however, this protist group now belongs to the phylum Cercozoa. The ancestor of phaeodarians is thought to have appeared in the upper Triassic Epoch according to the fossil record. They reproduce by cell division and swarmer production. These plankton are heterotrophic, and they presumably feed on organic materials suspended in the water column or capture other plankton. Although this group is widely distributed in the world ocean from the surface to deep waters, they have attracted little attention from marine researchers partly because their abundance has long been underestimated. Recent study, however, revealed that phaeodarians are more numerous than expected, and their high abundance is sometimes reported. Considering their occasional high biomass and the fact that their scleracoma is made chiefl y from silica, this plankton group plays an important role in local ecosystems and has a large impact in the silica cycle of the ocean. Knowledge of phaeodarian is indispensable for future oceanography; therefore the hitherto-known information on this marine protist is comprehensively reviewed in this chapter.

Phaeodarians are a group of widely distributed marine cercozoans. These plankton organisms can exhibit a large biomass in the environment and are supposed to play an important role in marine ecosystems and in material cycles in the ocean. Accurate knowledge of phaeodarian classification is thus necessary to better understand marine biology, however, phylogenetic information on Phaeodaria is limited. The present study analyzed 18S rDNA sequences encompassing all existing phaeodarian orders, to clarify their phylogenetic relationships and improve their taxonomic classification. The monophyly of Phaeodaria was confirmed and strongly supported by phylogenetic analysis with a larger data set than in previous studies. The phaeodarian clade contained 11 subclades which generally did not correspond to the families and orders of the current classification system. Two families (Challengeriidae and Aulosphaeridae) and two orders (Phaeogromida and Phaeocalpida) are possibly polyphyletic or paraphyletic, and consequently the classification needs to be revised at both the family and order levels by integrative taxonomy approaches. Two morphological criteria, 1) the scleracoma type and 2) its surface structure, could be useful markers at the family level.

No abstract

No abstract

Zooplankton samples from the deep water of the Sea of Japan often contain yellowish semitransparent spheres (1.0-1.5 mm in diameter). We recognized these spheres as a single phaeodarian species (Cercozoa, Rhizaria) and described them as Aulographis japonica sp. nov. (family Aulacanthidae) in this paper. This species has a high abundance in the Japan Sea Proper Water (JSPW) and occasionally higher biomass than that of copepods. Molecular analysis based on 18S SSU rDNA revealed that Aulacantha scolymantha, which belongs to the same family as A. japonica, is closer to Aulosphaera trigonopa and Protocystis spp., which belong to different orders, than to the present species. The distribution of A. japonica is apparently restricted to low temperature water. Its biomass was the highest in the uppermost layer of JSPW, and this phaeodarian species was the second most important zooplankton below 250 m depth in terms of biomass among the total zooplankton groups. This is probably due to its generalist type of feeding. Considering its large biomass, A. japonica possibly plays an important role in matter cycles within the Sea of Japan.