Polycystinea

Polycystinea, also called Polycystina, is a major lineage of radiolarians, single-celled marine protists within the supergroup Rhizaria. Members are characterized by siliceous, glassy skeletons that form a test, often composed of multiple interlocking chambers. The group includes the bulk of radiolarian taxa with intricate silica tests, most notably the orders Spumellaria and Nassellaria.

Cell organization: Polycystines possess a central capsule that partitions the cytoplasm into inner endoplasm and outer ectoplasm. Pseudopodia extend through pores in the skeleton to capture prey, sense the environment, and aid in locomotion. The silica tests are formed intracellularly and secreted to the exterior.

Habitat and ecology: They are predominantly marine plankton, widespread in oceans from surface to midwater, with high diversity in nutrient-rich regions. They are heterotrophic, feeding on smaller plankton and detritus with their radiating filopodia. Reproduction is typically asexual, though sexual stages and resting cysts occur in some species.

Fossil record and significance: Siliceous tests fossilize well, making Polycystinea a major component of siliceous ooze and a valuable resource for biostratigraphy and paleoceanography. Their distribution and morphology help reconstruct past ocean temperatures, chemistry, and productivity. Taxonomic classification within Radiolaria has evolved; Polycystinea is sometimes treated as a class or subclass, with Spumellaria and Nassellaria as major orders.