radiolarians

Radiolarians are single-celled marine protists known for their intricately patterned silica skeletons, or tests. They are primarily planktonic, drifting in open oceans, though a few species occur in coastal and deeper-water environments. Radiolarians belong to the broader group Rhizaria and feed by extending slender pseudopodia, called axopodia, through their porous tests to capture prey.

Morphology and biology are dominated by their siliceous tests, which range from delicate lattices to elaborate spherical or discoid forms. The axopodia radiate outward through central pores, supported by microtubules, and are used to engulf microscopic algae and other protists. The cell body lies within the test, and nutrient uptake occurs at the tips of the axopodia as prey is trapped and ingested.

Ecology and distribution: Radiolarians inhabit a wide range of depths, from the photic zone to mid and deeper waters, and can be abundant in nutrient-rich surface waters as well as in some low-oxygen environments. Their silica tests contribute to biogenic sediment as radiolarian ooze, particularly in regions with high productivity. They interact with other planktonic communities as both grazers and prey for larger organisms.

Fossil record and significance: Radiolarians have a rich fossil record dating back to the early Paleozoic, with tests that fossilize well in deep-sea sediments. Their assemblages are widely used in biostratigraphy and paleoceanography to reconstruct past ocean conditions, including changes in temperature, productivity, and water mass movements. Reproduction is primarily asexual through cell division, with occasional reports of sexual stages in some species.