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 Major Groups | Protozoa (protozoans)

Protozoa (protozoans)

Descriptive Features:

  • unicellular or acellular
  • body a constant rounded, oval or elongated shape, or an ever changing irregular outline, consists of nuclear material contained in cytoplasm, not divided into cells
  • membrane bounding body is thick and rigid (pellicle) or very thin and delicate (plasmalemma)
  • a non living covering may be secreted or acquired, soft and mucilaginous or hard and calcareous
  • most protozoans are colourless and translucent, pigment granules in food, usually green, brown and red, may impart colour
  • 1 or 2 flagella present anteriorly
  • cilia, shorter and more numerous than flagella, often arranged in definite patterns
  • cilia may be fused to form special lamellae (membranelles) or tufts of fused cilia (cirri)
  • Total length: small to microscopic, less than 0.5 mm

    Ciliata Paramecium bursaria

    Distribution: Australia wide

    Sensitivity Rating: none

    Functional Feeding Group: gathering collector



    Ecology: Protozoans occur in littoral areas of lakes, natural ponds, waste treatment ponds, dams, small pools and streams. They are found on submerged surfaces or in open water. Individual protozoans of most species are solitary, but some species may aggregate to form colonies with thousands of individuals that reach large sizes and are visible. Protozoan species are free living or ectocommensal upon crustaceans or parasitic upon terrestrial and aquatic animals. They are either sessile, adhering to submerged surfaces by a distinct stalk, or free floating. Free floating protozoans move by the actions of pseudopodia, flagella or cilia. Those with pseudopodia are restricted to submerged surfaces, whereas those with cilia or flagella can swim freely. Pseudopodial movement involves changes in the internal consistency of the animal and expansion and contraction of the membrane. Flagella drive the protozoan through the water with whip-like threshing movements. Cilia produce a more steady directional movement. Free living protozoans ingest bacteria, small algae, other protozoans, small metazoans and particulate organic detritus by the action of special tentacles, cilia or by engulfing movements of pseudopodia. Some species of the Mastigophora obtain energy from sunlight by a process of photosynthesis. Other species of Mastigophora may be saprophytic in darkness and photosynthetic in sunlight. Most protozoans reproduce by fission, whereby an individual splits into two or more new individuals. Ciliata undergo budding in which new individuals are formed without the fusion of nuclear material. Budding may be either external or internal into a brood chamber. Protozoans form resistant cysts at the onset of unfavourable environmental conditions.


    Information Sources: Williams 1980 


    Protozoans occur in large numbers in waste treatment ponds where they aid the breakdown process.