Heterotrophic protists within the taxa Acantharia, Radiolaria and Foraminifera (collectively referred to as ‘sarcodines’) are among the largest and most complex single-celled organisms in existence. These three groups were once thought to be closely related (together with amoebae and a few other groups) because they share a common feature, the pseudopod, cytoplasmic extensions that are used to engulf prey. In addition, the production of elaborate, delicate skeletal structures of calcium carbonate (Foraminifera), silica (Radiolaria) and strontium sulfate (Acantharia) make some of species similar in gross morphology.
Planktonic sarcodines live almost exclusively in oceanic ecosystems. These species form extensive pseudopodial networks that superficially resemble spiderwebs. These networks are used to subdue prey that are often much larger than the sarcodines themselves. Thus, these species are some of the most voracious predators among single-celled organisms. In addition, most of the surface-dwelling species within these taxa harbor intracellular algae in a mutualistic relationship. These algae contribute to the nutrition of the host, and presumably gain nutrients for growth in return.
Planktonic sarcodines have lengthy life spans, as life spans for protists go (a few weeks to perhaps a year) but these are only estimates as no planktonic sarcodine species has been cultured through successive generations in the laboratory. This fastidious nature, and the fossilizable skeletons that many sarcodines produce, have made many of these species invaluable tools for paleoclimatological studies.
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