Radiolarian

Radiolarians are single-celled heterotrophs (protozoa). They move in an amoeboid manner and produce skeletons, most commonly with a central capsule dividing the cell into inner and outer sections, called the endoplasm and ectoplasm.

They are found as plankton throughout the ocean. They float in the ocean's currents. Some common radiolarian fossils include Actinomma, Heliosphaera and Hexadoridium. They reproduce asexually and probably sexually also.

Anatomy


Radiolarians have hard skeletons usually made of silica. The skeletons are usually spherically symmetrical and structurally complicated, containing very interesting patterns. Their skeletons tend to have arm-like extensions. They tend to resemble spikes, which are used to increase surface area for buoyancy and to capture their food.

Parts of the skeleton of radiolaria are covered with a layer of cytoplasm which is rapidly withdrawn if the organism is disturbed in any way.

Individual radiolarians are normally in the size range of hundredths to tenths of millimeters. Some of them reach dimensions of a millimeter or more, which is large enough to be seen with the unaided eye.

Reproduction
They can reproduce sexually or asexually. Asexual reproduction is most common assumed because that's what has been observed most often. Asexual fission of radiolarian cells has also been observed. Sometimes expelling from the swellings of the cell are "swarmers" which means there is possibly gametogenesis.

Basically the reproduction of radiolarians is not well known. Binary fission, multiple fission, and budding have been reported. That is most common.

Ecology
Radiolarians live in large quantities as part of the ocean's plankton. They move around by spreading their arms and catching the ocean's currents.

They may be filter feeders or predators. Radiolarian can go several weeks with out food. When feeding as predators, they may capture tintinnids, other zooplankton, copepods, crustacean larvae, ciliates, phytoplankton, detritusdiatoms, and other calcareous organisms by ingesting them into their central cavity(Calcareous organisms refer to calcium carbonate). They may also consume bacteria.

Radiolaria appear to be most abundant in warm waters near the equator. As latitude increases the population of radiolarians decreases. Chlorophyll is also a big factor to the number of radiolarians in the water. Many scientists believe this is important for the survival of the radiolarians, but that is just an assumption.

They are most abundant in the upper few hundred meters of the open oceans, but they have been found at water depths as great the deepest trenches of the Pacific.

Radiolarians can be found in abundance floating in the waters of the California Current. They are found less in the Gulf of Mexico and the Gulf Stream, and even less in shallow waters suffering from eutrophication, which means the water has been over-enriched with nutrients.

Related References

 * Radiolarian Reference
 * Introduction to RadiolariansUCMP
 * What are radiolarians? Stanley A. Kling and Demetrio Boltovskoy
 * Miracle Radiolaria UCL