Radiolaria and Phaeodaria

Polycystina (~400–800 living species and several thousand extinct forms) and Phaeodaria (~400–500 living species) are exclusively marine, open-ocean planktonic protists, most of which possess elaborate siliceous skeletons. The cytoplasm is divided into an internal part (endoplasm) separated from the external, more vacuolated one (ectoplasm) by a perforated membrane – the central capsule. The Polycystina protrude long and slender cytoplasmic projections (axopodia) supported internally by a rigid central rod (axoneme); while the Phaeodria have anetwork ofperipheral finely interconnectedpseudopodia.Afew Polycystina are colonial, but most, as well as all Phaeodaria, are solitary, around 40 μm to almost 2 mm in size. Most polycystine species peak in abundance between 0 and 100 m, whereasphaeodarianstendtolivedeeper,oftenbelow300m.Polycystineshavea rich fossil record dating from the Cambrian and are important for stratigraphic, paleoecologic, and evolutionary studies. The world-wide biogeography and diversity of radiolarians is chiefly governed by water temperature. Radiolarian prey includes bacteria, algae, protozoa, and microinvertebrates. Many surfacedwelling species of Polycystina possess symbiotic algae and photosynthetic cyanobacteria that provide nourishment to the host. Some colonial radiolaria reproduce by binary fission of the central capsules. Sexual reproduction of polycystines or Phaeodaria has not been confirmed, but the release of motile swarmers, likely gametes, has been widely documented. In species with a radial symmetry (Spumellaria) shell-growth is centrifugal, whereas in the Nassellaria the internal cephalic elements and the cephalis appear first. Individual longevity is estimated to range between 2 and 3 weeks and 1–2 months