Radiolarians are marine planktonic protozoans characterised by transparent opaline skeletons. These skeletons show a great variety of shapes but most are generally based on spherical, conical, or rocket-shaped forms. Their size is usually within the range 20 to 400µm. Most of them are living as isolated individuals, but some species can form very long colonies (1m). Rare in nearshore waters, they usually live at all depths of the ocean, but they are mostly abundant in the upper 200m of the water column. There are no freshwater or benthonic forms. They are particularly abundant and diverse in equatorial latitudes, especially in areas of upwelling. They are also common in subpolar seas.

Radiolarians are major contributors to deep-sea siliceous oozes, mostly in the equatorial Pacific where radiolarian oozes can be found. They have a very long stratigraphical range (from the Cambrian 550 MA to Recent) and are valuable stratigraphic markers.
Today the specific composition of their assemblages is used to reconstruct palaeoenvironmental changes in high fertility areas (coastal upwelling systems and equatorial Belt).
The picture features a recent entactinarian form Hexacontium clevei from the temperate latitudes (120µm wide).