Radiolaria
A group of marine protozoa, regarded as a subclass of Actinopodea in older classifications, but not recognized as a natural group in some modern systems owing to its heterogeneity. In certain modern systems, the Radiolaria are subdivided into two classes, Polycystinea and Phaeodarea. Radiolarians occur almost exclusively in the open ocean as part of the plankton community, and are widely recognized for their ornate siliceous skeletons produced by most of the groups (illus. a–c). Their skeletons occur abundantly in ocean sediments and are used in analyzing the layers of the sedimentary record (biostratigraphy).
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A characteristic feature of the group is the capsule, a central mass of cytoplasm bearing one or more nuclei, food reserves, and metabolic organelles. This is surrounded by a perforated wall and a frothy layer of cytoplasm known as the extracapsulum, where food digestion generally occurs and where numerous axopodia (stiffened strands of cytoplasm) and rhizopodia radiate toward the surrounding environment. Algal symbionts including dinoflagellates, green algae, and golden-brown pigmented algae occur profusely in the extracapsulum. The algal symbionts living within the protection of the extracapsulum provide photosynthetically derived food for the radiolarian host.
In the order Spumellarida (class Polycystinea) the central capsule is perforated by numerous pores distributed evenly on the surface of the capsular wall. These pores, containing strands of cytoplasm, provide continuity between the cytoplasm in the central capsule and the surrounding extracapsulum. The skeletons of the Spumellarida are characteristically developed on a spherical organizational plan, but some are spiral-shaped (resembling snail shells) or produce elongate skeletons composed of numerous chambers built one upon another. In some genera, such as Thalassicolla (illus. e), there is no skeleton; in others there are rods or spicules, or often a single or multiple concentric latticework skeleton (illus. c).
Multicellular aggregates (colonies of spumellaridans), measuring several centimeters in diameter (or even several meters in some rare elongate forms), consist of numerous radiolarian central capsules enclosed within a gelatinous envelope and interconnected by a web of rhizopodia that bears abundant algal symbionts. A thin halo of feeding rhizopodia protrudes from the surface of the colony and is used to capture prey. Reproduction is poorly understood. In some colonial forms, daughter colonies are produced by asexual reproduction (fission). Flagellated swarmers (illus. f) released from mature central capsules of some species are possibly gametes and contain a large crystal inclusion of strontium sulfate.
In the Nasselarida (Polycystinea), the central capsule is often ovate and the pores are localized at one pole (illus. d). The axopodia and rhizopodia emerge from this pore field and are supplied by a conelike array of microtubules within the central capsule. The skeleton, when present, is often shaped like a dome or helmet.
Radiolarians in the class Phaeodarea possess a central capsule with two types of pore areas, a larger one (astropyle) that serves as a kind of cytopharynx where food is carried into the central capsule, and two accessory openings (parapylae) where smaller strands of cytoplasm emerge. The skeleton exhibits a wide range of shapes, including geodesic-like lattice spheres and small porous clam-shaped shells.
Radiolarians have a fossil record that extends back to early Paleozoic time, about a half billion years ago. Compared to other groups of shell-bearing marine microplankton, they are highly diverse, several hundred species having inhabited the oceans at any given time. Because they are planktonic and have undergone continuous evolutionary change, radiolarians are particularly useful for determining time equivalence (and geological ages) of marine sedimentary deposits at widely separated localities. The Cenozoic record of radiolarians in sediments, particularly on the deep-sea floor, is sufficiently complete to show the course of evolutionary change in considerable detail.
Assemblages of fossil radiolarians also provide clues to oceanic conditions during the geological past. Each of the major oceanic water masses has its characteristic radiolarian fauna, and so changes in the distribution or composition of these assemblages can be interpreted in terms of changes in the pattern of water masses, or in their oceanographic properties.
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