Rhizaria

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  • Rhizaria
    • Cercozoa (amoeba with fine, hairline pseudopods)
      • Cercomonads
      • Euglyhids
      • Phaeodarea
      • Heteromitids
      • Thaumatomonads
    • Chlorarachniophytes
    • Phytomyxids (Plasmodiomorphs, plant parasitic slime molds)
    • Haplosporidia
    • Foraminifera (amoeba with branched fine pseudopods and carbonate shells)
    • Radiolaria (amoeba with needle-like pseudopods and silica shells)
    • Acantharia (amoeba with needle-like pseudopods and strontium sulfate shells)

About this Superkingdom

This is a very large, abundant and diverse group of unicellular eukaryotes. Most are heterotrophic amoeboids, with very thin pseudopods, which are either thread-like (filose) or reticulose (branched or networked). and often with shells. Mitochondria have tubular cristae. Most Rhizaria have a distinct division of the cytoplasm into an inner granular “endoplasm” containing the usual organelles (including the nucleus), vesicles, and ribosomes, and a peripheral “ectoplasm” packed with vacuoles and lipid droplets. These apparently function to maintain buoyancy counteracting the ballast of the mineral shell.

Foraminifera

Ernst Haeckel's drawing of foraminifera
Ernst Haeckel's drawing of foraminiferan tests

Forams are common marine amoeboids with carbonate shells punctured by openings for their numerous reticulate pseudopods. The shells (tests) are chambered, with connecting holes. Unlike most rhizaria, pseudopods are used for motility as well as feeding. Most are microscopic (although just barely), but many are several millimeters in diameter. Many contain plastids, which are typically kleptochloroplasts (chloroplasts scavenged from food algae and retained for use in photosynthesis), or endosymbiotic dinoflagellates (zooxanthella).

Because foramin tests fossilize readily, and make up a large fraction of ocean sediment and limestones, the fossil record of this group is essentially limitless. Analysis of foramin populations in geologic strata can be used to precisely date limestones, and establish detailed histories of ocean depth, temperature, primary production and climate.

Example species : Rotalipora globotruncanoides

Rotalipora globotrunacoides
from Dr. Richard E. Zeebe University of Hawaii at Manoa

R. globotruncanoides is an abundant modern planktonic species that first appeared approximately 100 million years ago, and it’s appearance in the fossil record defines the end of the early cretaceous and the beginning of the late cretaceous epoch.

Radiolaria

Ernst Haekel's radiolarians
Ernst Haeckel's drawing of some radiolarian tests

Radiolarians are also common marine amoeboids with silica shells (tests). Some are relatively single spicule-like cones or vases, but most are elaborate, including complex concentric spheres and 3-dimensional snowflakes-like structures. The test does not usually surround the cell, but permeates it, i.e. the inside of the test is imbedded in the cytoplasm. Their pseudopods are rigid and needle-like, being supported by microtubule bundles (axopods). The endoplasm is separated from the ectoplasm by a perforated membrane. They usually contain endosymbiotic dinoflagellates (zooxanthella).

Like Forams, radiolarian tests fossilize readily, and make up a large fraction of marine sediment and limestones, the fossil record of this group is essentially limitless. Analysis of radiolarian populations in geologic strata can be used to precisely date limestones, and establish detailed histories of ocean depth, temperature, primary production and climate.

Example species : Hexacontium giganthium

Heliocontium giganthiumhelioconium giganthium test

from Jande Dolven : http://tolweb.org/Polycystina/121189

These are large modern radiolarians of the North Sea, ca. 0.1mm in diameter. The test is comprized of three concentric spheres, connected by 8-12 large three-sides spines that originate in the innermost sphere and form radial bars inside the cell. The pores of the outermost (cortical) sphere are surrounded several small spines (byspines). The thin rigid pseudopods (axiopods) protrude from the pores in the cortical sphere.

Cercozoa

Cercozoans are amoeba and flagellates with filose pseudopods used for feeding, rather than motility. Unflagellated species are motile by gliding. Most have silica or organic plates assembled in a regular pattern to form a simple shell. An opening at the anterior end allows the single thread-like pseudopod to protrude and search for food.

Example species : Euglyphia strigosa

Euglyohia strigosa
from A. Coruña http://picasaweb.google.com/Ado.Out/AmoebozoaRhizaria#5313012633635393490

Euglypha have flattened oval tests composed of 150-300 scale-like plates. Cells are 50-100um in length. Silica spines project from the test. The oval anterior opening has a ring of specialized plates with tooth-like projections protecting the entrance. Inhabitants of rich soil, mosses, and peats.