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Radiolarian tests (salt water), animated coloured scanning electron micrograph (SEM). The hard skeletons are composed of silica or strontium sulphate. The Radiolaria (also called Radiozoa) are protozoa that produce intricate mineral skeletons. They are found as zooplankton throughout the ocean and their skeletal remains make up a large part of the cover of the ocean floor as silica ooze. Radiolarians can often contain symbiotic algae, especially zooxanthellae, which provide most of the cell's energy. Some of this organization is found among the heliozoa, but those lack central capsules and only produce simple scales and spines.