Radiolaria (Challenger) Plate. Arthracanthida, a subclass of

Acantharea The Acantharea (Acantharia) are a group of radiolarian protozoa, distinguished mainly by their strontium sulfate skeletons. Acantharians are heterotrophic marine microplankton that range in size from about 200 microns in diameter up to several ...

, is a group of marine

protozoan Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...

s. They consist mainly of a gelatinous sheath filled with

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...

and a skeleton of up to 20 radially placed

spicule Spicules are any of various small needle-like anatomical structures occurring in organisms Spicule may also refer to: *Spicule (sponge), small skeletal elements of sea sponges *Spicule (nematode), reproductive structures found in male nematodes ( ...

s made of celestine (

strontium sulfate Strontium sulfate (SrSO4) is the sulfate salt of strontium. It is a white crystalline powder and occurs in nature as the mineral celestine. It is poorly soluble in water to the extent of 1 part in 8,800. It is more soluble in dilute HCl and nitric ...

). While mostly found in the upper areas of the ocean, they are able to move vertically by using

microfilament Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other pr ...

s attached to the spicules to expand and contract the sheath. They are plentiful in the

Gulf Stream The Gulf Stream, together with its northern extension the North Atlantic Current, North Atlantic Drift, is a warm and swift Atlantic Ocean, Atlantic ocean current that originates in the Gulf of Mexico and flows through the Straits of Florida a ...

during the summer months, but little is known about their overall distribution.

References

*L. H. Hyman, The Invertebrates, vol. 1, McGraw-Hill, 1940 *T. Cavalier-Smith, Kingdom Protozoa and its 18 phyla, Microbiol. Rev., 57(4):953–994, 1993 *K. Hausmann and N. Hulsmann, Protozoology, Georg Thieme Verlag, New York, 1996 {{Taxonbar, from=Q4797630 Radiolarians