Raphidiophrys cells usually are encountered as rounded, axopodial amoebae, either suspended in the water column or loosely adhering to substrate. In most species, the cells are solitary, but in a few the cells may form loose, unorganized colonial aggregates.
|The trophic cells range from 10 to 150 micrometers in length; individual species have characteristic size ranges within these limits. No flagella are present. The cells appear coarsely "furry" due to a covering of flattened silica scales. The scales may adhere to each other, to the cell body (which may have a gelatinous coat around it), and to the axopodia radiating from the cell body; but they do not project at right angles to the cell surface. Depending on the species, the scales may be circular, ellipsoidal or nearly linear.|
Axopodia, unlile the scales, radiate from the cell body at right angles to
the cell surface. They
are longer and much more slender than the scales and may bear prominent
a structurally distinct type of extrusome). Axopodia are used to capture
prey and to move cells
along a substrate. Disturbed cells may retract the axopodia.|
The axopodia radiate from a prominent body, the centroplast, located in the geometric center of the cell. Both the centroplast and the axopodial microtubules radiating from it are strongly birefringent, easily visible in plane-polarized light and optical systems (such as differential interference contrast = Nomarski optics) based on plane-polarized light.
The single nucleus is eccentrically located. Most other organelles are located in the peripheral cytoplasm; these organelles typically include one or more contractile vacuoles. In several species, the peripheral cytoplasm may harbor endosymbiotic coccoidal green algae ("zoochlorellae"); the species illustrated here was named pallida ("pale", "pallid") precisely because zoochlorellae are absent, and this lack was considered unusual at the time the species was described.
The centrally-located, birefringent centroplast separates centrohelid heliozoa from other radially-symmetrical, axopodial amoebae. The absence of "spine" scales projecting at right angles from the cell surface separates Raphidiophrys from other centrohelids.
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