Actinophryid
Encyclopedia
The actinophryids are small, familiar group of heliozoa
n 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. Each actinophryid are unicellular and roughly spherical in shape, without any shell or test, and with many pseudopodia supported by axopods radiating outward from the cell body, which adhere to passing prey and allows it to roll or float about. The outer portion of the cell, or ectoplasm, is distinct and is filled with many tiny vacuole
s, which assist in flotation. This is very similar to the process of osmosis. The movement of water from inside the cell to the outside is not because of water concentration in this case. It is the cell pushing the excess water out. A few contractile vacuoles around the periphery of the cell expel excess water, and are visible as clear bulges when full.
There are two genera included here. Actinophrys have a single, central nucleus. Most are around 40-50
μm in diameter, with axopods up to 100 μm in length, though this varies. Actinosphaerium are several times larger, from 200-1000
μm in diameter, with many nuclei, and are found exclusively in fresh water. Two other genera, Echinosphaerium and Camptonema, have been described but appear to be synonyms.
Reproduction takes place by fission, with open mitosis
. Under unfavourable conditions, the organism will form a cyst, which is multi-walled and covered in spikes. While encysted it may undergo a peculiar process of autogamy or self-fertilization, where it goes through meiosis
and divides to form two gametes, which then fuse together again. This is the only form of sexual reproduction
that occurs within the group, though it is really more genetic reorganization than reproduction.
The axopods are supported by microtubule
s arranged in a unique and characteristic double-coil pattern. In Actinophrys, these arise from the nuclear membrane, while in Actinosphaerium some do and others don't. Other heliozoa where the microtubules arise from the nucleus have been considered possible relatives, and it now appears that the actinophryids developed from axodine
s such as Pedinella
. These are specialized heterokont
algae, related to golden algae, diatom
s, brown algae, and the like, which have microtubule-supported tentacles.
As far as the diet of the Actinophyrys goes, the protist feeds on small flagellates, diminutive cilates, microscopic algae, etc.
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. 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. Each actinophryid are unicellular and roughly spherical in shape, without any shell or test, and with many pseudopodia supported by axopods radiating outward from the cell body, which adhere to passing prey and allows it to roll or float about. The outer portion of the cell, or ectoplasm, is distinct and is filled with many tiny vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
s, which assist in flotation. This is very similar to the process of osmosis. The movement of water from inside the cell to the outside is not because of water concentration in this case. It is the cell pushing the excess water out. A few contractile vacuoles around the periphery of the cell expel excess water, and are visible as clear bulges when full.
There are two genera included here. Actinophrys have a single, central nucleus. Most are around 40-50
1 E-5 m
To help compare different orders of magnitude, this page lists lengths between 10−5 m and 10−4 m .Distances shorter than 10 µm* 10 µm — width of cotton fibre...
μm in diameter, with axopods up to 100 μm in length, though this varies. Actinosphaerium are several times larger, from 200-1000
1 E-4 m
To help compare different orders of magnitude this page lists lengths between 10−4 m and 10−3 m .Distances shorter than 100 µm* 100 µm – 1/10 of a millimetre* 100 µm – 0.00394 inches...
μm in diameter, with many nuclei, and are found exclusively in fresh water. Two other genera, Echinosphaerium and Camptonema, have been described but appear to be synonyms.
Reproduction takes place by fission, with open mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
. Under unfavourable conditions, the organism will form a cyst, which is multi-walled and covered in spikes. While encysted it may undergo a peculiar process of autogamy or self-fertilization, where it goes through meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
and divides to form two gametes, which then fuse together again. This is the only form of sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
that occurs within the group, though it is really more genetic reorganization than reproduction.
The axopods are 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 arranged in a unique and characteristic double-coil pattern. In Actinophrys, these arise from the nuclear membrane, while in Actinosphaerium some do and others don't. Other heliozoa where the microtubules arise from the nucleus have been considered possible relatives, and it now appears that the actinophryids developed from axodine
Axodine
The axodines are a group of unicellular heterokont algae.They include pedinellids, silicoflagellates, and Rhizochromulinales.Other terms include "Actinochrysophyceae" and "Dictyochophyceae".-Characteristics:They characteristically have a...
s such as Pedinella
Pedinella
Pedinella is a small, unicellular planktonic or attached, flagellated Heterokont first described in 1888 by A. V. Vysotskij. The genus is monospecific, and the single species is Pedinella hexacostata Vysotskij...
. These are specialized heterokont
Heterokont
The heterokonts or stramenopiles are a major line of eukaryotes currently containing more than 100,000 known species. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton...
algae, related to golden algae, diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s, brown algae, and the like, which have microtubule-supported tentacles.
As far as the diet of the Actinophyrys goes, the protist feeds on small flagellates, diminutive cilates, microscopic algae, etc.