Phaeodarea or Phaeodaria is a group of amoeboidcercozoan organisms. They are traditionally considered radiolarians,[3] but in molecular trees do not appear to be close relatives of the other groups, and are instead placed among the Cercozoa.[4] They are distinguished by the structure of their central capsule and by the presence of a phaeodium, an aggregate of waste particles within the cell.
The term "Radiozoa" has been used to refer to radiolaria when Phaeodarea is explicitly excluded.[5]
Phaeodarea produce hollow skeletons composed of amorphous silica and organic material, which rarely fossilize. The endoplasm is divided by a cape with three openings, of which one gives rise to feeding pseudopods, and the others let through bundles of microtubules that support the axopods. Unlike true radiolarians, there are no cross-bridges between them. They also lack symbiotic algae, generally living below the photic zone, and do not produce any strontium sulphate.
Characteristics
Cell structure
Phaeodarea are unicellularprotists that grow a capsule with a thick, double-layered wall containing two kinds of pores or openings: the large type, known as "astropylum" or oral pore, from which a massive strand of cytoplasm protrudes; and the smaller type, known as "parapylae" or lateral pores, from which thinner strands of cytoplasm protrude. External to the capsule there is a large, often darkly pigmented, mass of granular cytoplasm called "phaeodium" which contains undigested or partially digested food or debris. Their mitochondrial cristae are tubular.[6][1]
Most Phaeodaria have a siliceous skeleton composed of amorphous silica joined by organic matter. They may contain more organic matter than Polycystinea (Radiolaria). Skeletons of some species are composed of hollow bars, instead of solid rods as in Polycystinea. Others have ornate spicules scattered through the external cytoplasm in a variety of forms, including geodesic frameworks, spheres or polyhedra. Other species have porous shells that are either bivalved, resembling clams, or vase-like with ornamentations around the opening. Other species have highly branched antler-llike spines stemming from a central shell. Unlike Polycystinea, the capsular wall surrounding the denser endoplasm of Phaeodarea lacks fusules.[6][1]
Their cell size ranges from hundreds of micrometres to a few millimetres, roughly depending on the family. For example, species of Tuscaroridae exceed 3 mm, while Challengeriidae are generally smaller than a few hundred μm. Some phaeodarians with spherical skeletons are similar to Polycystinea and Acantharea (Radiolaria), although they are more porous and fragile. Some species of the family Challengeriidae resemble marine dinoflagellates, but can be correctly distinguished by the presence of a phaeodium and absence of grooves.[7]
Nutrition
The continuous, massive strand of cytoplasm in the astropyle of Phaeodarea provides a pathway to carry digested prey matter into the endoplasm, similarly to some testate amoebae and foraminifera. Among the limited evidence of phaeodarian predation, it was reported in 1986 that a mesopelagic phaeodarian had absorbed microflagellate and metazoan prey.[8] When introduced in the laboratory cultures, copepods and salps also were snared by phaeodaria.[1]
Reproduction
Sexual reproduction of Phaeodaria has not been confirmed, but the release of motile swarmers that are likely gametes is widely documented. In the species Coelodendrum ramosissimum, dispersal starts with the disappearance of the phaeodium, followed by the dissolution of the capsule and the creation of small plasmodial spheres in the ectoplasm; each of the spheres produces hundreds of multinucleated amoebae that eventually form swarmers with two undulipodia (flagella).[1]
As a protist group often broken by normal sampling methods and not very abundant in comparison with other organisms in the euphotic zone, Phaeodaria have attracted little attention from plankton researchers. The majority of taxonomic and ecological information regarding Phaeodaria, studied by German scientists, ended after World War I, and it has been little updated until today. Because of the presence of an organic central capsule and "ray-like pseudopodia", Phaeodaria were historically regarded as Radiolaria, along with Polycystinea and Acantharea.[7] The marine Radiolaria were, along with the freshwater Heliozoa, assigned to the phylum Actinopoda due to their elaborate siliceous skeletons surrounding the central capsule with pores from which axopodia emerge.[4]
Through phylogenetic analyses it has been discovered that Phaeodarea is a monophyletic clade, but the historical orders and families comprising it aren't. Instead, the clade consists of 11 subclades defined by morphological and phylogenetic values that do not correspond with the traditional orders and families:[9]
Despite this, the current taxonomy by Cavalier-Smith maintains the original classification of suborders[10] divided between two new orders:[11][12]
Eodarida, containing phaeodarea with no silica skeleton or with a skeleton made of simple radiating spicules. Contains two suborders: Phaeogymnocellina and Phaeocystina.
Opaloconchida, containing phaeodarea with a highly perforated, shell-like opaline silica skeleton. Contains the remaining five suborders: Phaeosphaeria, Phaeocalpia, Phaeogromia, Phaeoconchia and Phaeodendria.
Modern classification
The modern classification is the following, with the subclass containing a total of 2 orders, 7 suborders,[11][12] 16 families and 39 genera.[13]
Family CoelodendridaeHaeckel 1887 – Coelodendrum, Coelographis
References
^ abcdefgBoltovskoy D, Anderson OR, Correa NM (2017). "Radiolaria and Phaeodaria". In Archibald JM, Simpson AGB, Slamovits CH (eds.). Handbook of the Protists (2nd ed.). Springer International Publishing AG. pp. 731–763. doi:10.1007/978-3-319-28149-0_19. ISBN978-3-319-28149-0.
^Moreira D, von der Heyden S, Bass D, López-García P, Chao E, Cavalier-Smith T (2007). "Global eukaryote phylogeny: Combined small- and large-subunit ribosomal DNA trees support monophyly of Rhizaria, Retaria and Excavata". Molecular Phylogenetics and Evolution. 44 (1): 255–66. doi:10.1016/j.ympev.2006.11.001. PMID17174576.
^ abcdNakamura, Yasuhide; Suzuki, Noritoshi (2015). "Chapter 9. Phaeodaria: Diverse Marine Cercozoans of World-Wide Distribution". In Ohtsuka, Susumu; Suzaki, Toshinobu; Horiguchi, Takeo; Suzuki, Noritoshi; Not, Fabrice (eds.). Marine Protists: Diversity and Dymanics. Springer Japan. pp. 223–249. doi:10.1007/978-4-431-55130-0_9. ISBN978-4-431-55130-0.
^Swanberg N, Bennett P, Lindsey JL, Anderson OR (1986). "The biology of a coelodendrid: a mesopelagic phaeodarian radiolarian". Deep Sea Research Part A. Oceanographic Research Papers. 33 (1): 15–25. doi:10.1016/0198-0149(86)90105-6.
^ abCavalier-Smith T, Chao EE (2012). "Oxnerella micra sp. n. (Oxnerellidae fam. n.), a Tiny Naked Centrohelid, and the Diversity and Evolution of Heliozoa". Protist. 163 (4): 574–601. doi:10.1016/j.protis.2011.12.005. ISSN1434-4610. PMID22317961.
^Takahashi K, Anderson OR (2000). "Class Phaeodaria"(PDF). In Lee JJ, Leedale GF, Bradbury P (eds.). Illustrated Guide to the Protozoa. Vol. II (2nd ed.). Society of Protozoologists, Lawrence, Kansas. pp. 981–994.
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