How do radiolarians feed

It is thought that the evolution of diatoms in the Cretaceous may have had a significant effect on radiolarian evolution due to competition for silica diatoms also use silica to build their skeleton ; it is commonly accepted that radiolarian skeletons have become finer and less robust from this time. Extant radiolaria are classified using features of both the preservable skeleton and the soft parts, which makes the classificaiton of fossil forms extremely difficult.

Most workers in this field today use classification schemes based on Nigrini and Moore's and Nigrini and Lombari's works on modern and Miocene radiolarians.

A major problem with radiolarian classification is that separate classifications have been established for the Palaeozoic, Mesozoic and Cenozoic, and little has been done to integrate them.

The two suborders, the spumellarians and the nassellarians are subdivided into informal groups which equate to family level. Radiolarian assemblages often contain species so they can potentially be very useful biostratigraphic and palaeoenvironmental tools. They have an unusually long geological range, from latest Pre-Cambrian to Recent. Because Radiolaria have a skeleton composed of silica and have an extremely long geological range they have become useful in the study of sediments which lack calcareous fossils, either because of deposition below the CCD Carbonate Compensation Depth or because the strata being examined are too old.

Cherts and particularly nodules within chert bands are often good sources for Radiolaria. Ophiolites and accretionary terrains often include chert bands and Radiolaria may be the only palaeontological aid available in these situations and as such have proved invaluable in the study of these geological settings.

Despite being single-celled protozoans Radiolaria are quite complex, sophisticated organisms. The body is divided into a central capsule which contains the endoplasm and nucleus or nucleii and the extracapsulum which contains peripheral cytoplasm composed of a frothy bubble-like envelope of alveoli and a corona of ray-like axopodia and rhizopodia.

They feed on other zooplankton, phytoplankton and detritus using their axopodia and rhizopodia in a similar fashion to foraminifera, except that Radiolaria seldom possess pseudopodia and their rhizopodia are not as branching or anastomosing as in foraminifera.

Symbiotic algae including dinoflagellates often occur in the extracapsulum. The central capsulum is separated from the extracapsulum by the central capsular wall, cytoplasmic strands called fusules link the central capsulum and extracapsulum via pores in this wall. Fusules are unique to Radiolaria and their close relatives the Acantharia. Because Radiolaria are heterotrophic they are not limited to the photic zone and have been found at water depths as great as m.

However, because many living Radiolaria contain symbiotic photosynthesising algae they must spend at least daylight hours within the photic zone.

Pseudopodia surround the projecting skeletal spines, protecting them from dissolution in sea water. The pseudopods may be quite active, capturing prey and disposing of wastes, and reacting to external stimuli. They are able to cling to and move the individual about surfaces of laboratory vessels, and may thus cling to foreign objects in nature. But so far attachment has not been observed in nature, and there are no known benthonic forms.

Digestion and waste disposal functions occur in the ectoplasm. Individual radiolarians are normally in the size range of hundredths to tenths of millimeters, but some reach dimensions of a millimeter or more, large enough to be seen with the naked eye. Some species are amassed into colonies, which may reach sizes of centimeter and even meter scale. Radiolarians are part of the marine plankton. They occur in all oceans, including shallow seas, bays, fjords, etc. They are most abundant in the upper few hundred meters of the open oceans, but have been reported at all depths, including deep trenches of the Pacific, with different species often inhabiting different depth horizons.

Surface and subsurface geographic distributions of species are influenced by ocean climatic variables, with biogeographic provinces characteristically mirroring surface and subsurface water masses. The classification of Radiolaria recognizes two major extant groups: 1 the polycystines, with solid skeletal elements of simple opaline silica, and 2 the Phaeodarians, with hollow skeletal elements of a complex and as yet poorly understood siliceous composition that results in rapid dissolution in sea water and consequent rare preservation in sediments.

The Phaeodarians also possess a unique anatomical feature, a mass of tiny pigmented particles called the phaeodium. The polycystines, which are the radiolarians best known to geologists, are subdivided into two major groups: the basically spherical-shelled Spumellaria, and the basically conical-shelled Nassellaria. Pseudocubus and Arachnocorallium , are advantageous in collecting tiny prey including microflagellates and bacteria. Solitary spumellarians, represented by Diplosphaera , Spongosphaera , and Spongaster , also gather tiny prey.

Colonial radiolarians such as Collozoum and Buccinosphaera seem to live exclusively on symbiotic algae. The wide variation in feeding behaviour means that radiolarians occupy several kinds of ecological niches in marine environments.

Assuming that the radiolarian skeletal morphology indicates their feeding strategy, living forms can give some light on the feeding mechanisms of similar Mesozoic forms. By contrast, the unique curved conical skeleton of the Paleozoic Albaillellaria, unknown after the end-Permian mass extinction, suggests a different feeding behaviour. You can also search for this author in PubMed Google Scholar. Correspondence to Atsushi Matsuoka. Reprints and Permissions.

Matsuoka, A. Living radiolarian feeding mechanisms: new light on past marine ecosystems. Swiss j geosci , —