radiolaria

Radiolaria, commonly called radiolarians, are a large group of single-celled marine protists notable for their complex siliceous skeletons. Most species are planktonic, inhabiting open-ocean waters from the surface to several thousand meters deep, though some occur in other habitats. They are predominantly heterotrophic, feeding on small planktonic organisms.

Cell organization and skeletons: Each organism consists of cytoplasm encased by a tests (skeleton) made of opaline silica. Within the test a central capsule separates inner and outer compartments. The cell extends slender axopodia—pseudopodia reinforced by microtubules—through pores in the skeleton to capture prey and aid buoyancy. The external skeleton can be highly elaborate, forming latticed spheres, cones, or other shapes with intricate pore patterns that aid classification.

Feeding and ecology: Radiolarians feed on bacteria, algae, and other small protists, using their axopodia to entangle and transport prey to the cell body. They are a key component of marine zooplankton in many open-ocean ecosystems and can influence nutrient transfer within the planktonic food web.

Skeleton diversity and taxonomy: Skeletons vary from delicate latticed shells to more robust forms. The diversity of test shapes, chamber arrangements, and pore patterns underpins their taxonomic classification. Modern groups include several orders such as Polycystinea and Acantharia, and radiolaria are placed within the larger supergroup Rhizaria.

Fossil record and significance: Radiolarians have a long fossil record dating from the early Paleozoic to the present, with abundant silica-based skeletons that accumulate as radiolarian ooze in deep-sea sediments. Their well-preserved tests make them valuable for biostratigraphy and studies of past ocean conditions, temperatures, and circulation.