|
What are Radiolarians? ![]() The central capsule is enclosed in a membrane. It contains not only the nucleus but mitochondria (respiratory organelles) and Golgi bodies (secretory organelles), as well as vacuoles, lipid droplets, and food reserves. Reproduction, respiration, and biochemical synthesis are functions carried out in the central capsule. Communication to the calymma is through pores in the membrane.
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.
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. A few polycystine groups lack a skeleton altogether. Characteristics of the central capsular membrane also distinguish these major divisions of the Radiolaria. Some major groups of extinct radiolarians differ substantially from both Spumellaria and Nassellaria, and may be ranked at the same taxonomic level as those groups (see links to Mesozoic and Paleozoic radiolarians). Spumellarians come in various shapes ranging from spherical to ellipsoidal to discoidal. These are the ones typically with radial symmetry. It is common for the Spumellarians to have several concentric shells connected by radial bars. The colonial radiolarians are spumellarians, some with spherical shells and others whose skeletons are instead an association of loose rods, and yet others without skeletons. Nassellarian shapes derive from geometrically simple spicules (resembling saw horses, "D"-shaped rings, and the like) to which are added (from taxa to taxa) latticed cover to form a chamber, then additional chambers expanding axially into the conical forms typical of the group.
Nutrition of radiolarians involves a large variety of materials, including many zooplankton groups such as copepods, crustacean larvae, ciliates, and flagellates, and such phytoplankton groups as diatoms, coccolithophores, and dinoflagellates. They may also consume bacteria and organic detritus. Algal symbionts play an important role in at least the polycystines, not only for the nutrition of the host, but also for primary production of the area. Experimental evidence shows that organic carbon fixed by algae is assimilated by the radiolarian host. Fascinating schemes have been observed in which the symbionts spread far out in pseudopodial networks during daylight and are retracted at night. |