Gymnosphaerid
Encyclopedia
The gymnosphaerids are a small group of heliozoa
n protists found in marine environments. They tend to be roughly spherical with radially directed axopods
, supported by microtubule
s in a triangular-hexagonal array arising from an amorphous central granule
.
s, which also have microtubules in a triangular-hexagonal array, but are set apart from the others by the structure of the central granule and the mitochondria
, which have tubular crista
e. The two groups have been treated as separate orders (Axoplasthelida and Centroplasthelida) in a common class, but this has lost support. Instead the gymnosphaerids may be allied with the desmothoracid
s, and on account of this have been placed in the Cercozoa
, but this is somewhat tentative.
Heliozoa
Heliozoa are phagotrophs. They are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface. These give them the characteristic sun-like appearance for which they are named, and are variously used for capturing food,...
n protists found in marine environments. They tend to be roughly spherical with radially directed axopods
Pseudopod
Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
, supported by microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s in a triangular-hexagonal array arising from an amorphous central granule
Granule (cell biology)
In cell biology, a granule is a small particle. It can be any structure barely visible by light microscopy. The term is most often used to describe a secretory vesicle.-Leukocytes:...
.
Genera
There are only three genera, each with a single species: Gymnosphaera albida, Hedraiophrys hovassei, and Actinocoryne contractilis.- Gymnosphaera albida is free-living, usually benthic in shallow water. The cells are round and naked, around 70-100 μm in diameter, and resemble the unrelated ActinosphaeriumActinophryidThe actinophryids are small, familiar group of heliozoan protists. They are the most common heliozoa in fresh water, and are especially frequent in lakes and rivers, but a few are found in marine and soil habitats as well...
. The outer cytoplasm, or ectoplasm, forms a distinct layer containing large vesicles.
- Hedraiophrys hovassei is larger and lives attached to algae and other objects. The cells have a conical base, and are covered with long siliceous spicules. The ectoplasm is distinct and frothy, and typically contains bacterial and algal endosymbiontEndosymbiontAn endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
s.
- Actinocoryne contractilis is benthic. When feeding, it has a multinucleateMultinucleateMultinucleate cells have more than one nucleus per cell, which is the result of nuclear division not being followed by cytokinesis. As a consequence, multiple nuclei share one common cytoplasm. This can be the consequence of a disturbed cell cycle control Multinucleate (also multinucleated,...
base and a contractile stalk up to 150 μm in length, supporting a relatively small uninucleate head, where the central granule and axopods are located. It may move about in a headless amoeboid form. Reproduction is either by budding off the head or fragmentation of the headless form, producing small free-living cells similar to Gymnosphaera, which then attach themselves and regrow the stalk and base.
Classification
Gymnosphaerids were originally considered centrohelidCentrohelid
The centrohelids or centroheliozoa are a large group of heliozoan protists. They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth.-Characteristics:...
s, which also have microtubules in a triangular-hexagonal array, but are set apart from the others by the structure of the central granule and the mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
, which have tubular crista
Crista
Cristae are the internal compartments formed by the inner membrane of a mitochondrion. They are studded with proteins, including ATP synthase and a variety of cytochromes. The maximum surface for chemical reactions to occur is within the mitochondria...
e. The two groups have been treated as separate orders (Axoplasthelida and Centroplasthelida) in a common class, but this has lost support. Instead the gymnosphaerids may be allied with the desmothoracid
Desmothoracid
The desmothoracids are a group of heliozoan protists, usually sessile and found in freshwater environments. Each adult is a spherical cell around 10-20 μm in diameter surrounded by a perforated organic lorica or shell, with many radial pseudopods projecting through the holes to capture food...
s, and on account of this have been placed in the Cercozoa
Cercozoa
The Cercozoa are a group of protists. They are sometimes described as a kingdom.-Characteristics:The group includes most amoeboids and flagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome or mouth as found in...
, but this is somewhat tentative.