Radiolarians are single-celled protozoa, measuring less than 0.1 – 0.2 mm in diameter, that produce intricate shells (skeletons) of amorphous silica. They float as part of the zooplankton in the first 200 meters of water in the Earth’s oceans (photic zone). They are mostly heterotrophic but some species are in symbiosis with photosynthetic organisms. Radiolarians occur at all latitudes, although different taxa are adapted to tropical waters, while others prefer arctic oceans. These organisms are some of the largest producers of siliceous ooze that accumulate on abyssal plains and, therefore, represent the primary producers of sequences of radiolarian cherts, which are very common in the sedimentary record. The earliest known radiolarians date back to the Early Cambrian period as part of the so-called ‘small shelly fauna’.

Above: radiolarian with endosymbionts (Diploconus) by Alvaro E. Migotto.