radiolarios

Radiolaria, commonly known as radiolarians, are a diverse group of primarily marine, single-celled eukaryotes within the Rhizaria. They are best known for their delicate siliceous skeletons, or tests, which form intricate latticework and often persist as fossils long after the organism has died. The tests are typically composed of opaline silica and come in a range of shapes, from spherical to conical and highly ornate.

Most radiolarians float in the ocean's upper to mid-water column as part of the plankton. They feed by extending axopodia, slender pseudopodia supported by microtubules, to capture small prey such as bacteria, algae, and other microzooplankton. The axopodial network also helps with buoyancy and balance in the water column.

Classification among radiolarians is based largely on test morphology. The two principal lineages are Spumellaria, which have spherical or near-spherical tests, and Nassellaria, which possess conical or lattice-like tests. Acantharia is often treated as a separate lineage with skeletons made of different minerals, illustrating the diversity within Radiolaria.

Ecologically, radiolarians are important components of marine zooplankton and contribute to silica cycling in oceanic ecosystems. Their silica skeletons sink after death, forming radiolarian ooze and contributing to deep-sea sediment. The fossilized tests provide valuable records for biostratigraphy and paleoceanography, helping to reconstruct past ocean conditions and climates. Radiolarians reproduce both asexually and sexually, and some species form resting stages to survive unfavorable conditions.