Anderson, O. Roger Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York.
Harley, John P. Department of Biological Sciences, Eastern Kentucky University, Richmond, Kentucky.
Last reviewed:August 2019
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A taxon of marine protozoans belonging to the Acantharea. Acanthometrida (also known as Arthracanthida) is a taxonomic grouping (sometimes considered as an order or subclass) placed within the Acantharea (Acantharia) taxon. All members are unicellular planktonic marine protozoa with axopodia (rigid, linear cellular projections). Long slender pseudopodia (temporary projections) are found in certain members as well. Mitochondria are always present and contain flattened cristae. No cilia are present in the trophic phase. Members have a skeleton, made up of strontium sulfate (celestite), that is limited to 20 radially arranged rods that extend from the center, forming a characteristic pattern in which angles are quite exact, as in Acanthometra (see illustration). Because strontium sulfate is soluble in seawater, no acantharean fossils exist. The central endoplasm contains numerous nuclei and other organelles. The cytoplasm surrounding the spines contains a conical array of contractile microfilaments (myophrisks or myonemes that are calcium-activated) containing the contractile protein actin. The microfilaments expand or contract the gelatinous sheath surrounding the cell, which allows for changes in the level of flotation. Electron microscopic research shows that the microfilaments are arranged in two sets of bands; one set runs parallel to the axis of the spine, and the other runs crosswise, thus producing a cross-fibrillar network. Cysts and flagellated swarmers represent two stages of the life cycle, which is not completely known at present. The endoplasm often contains zooxanthellae (small dinoflagellates living in the protozoan's endoplasm). See also: Acantharea; Celestite; Protozoa; Strontium
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