Phylum Protozoa- Definition, Characteristics, Classification and Examples

PHYLUM PROTOZOA

(Gr., protos = first ; zoon = animals)

Phylum Protozoa is a group within the kingdom Protista consisting of single-celled, eukaryotic organisms that can be free-living or parasitic. These organisms are characterized by their ability to move independently using structures such as cilia, flagella, or pseudopodia. Protozoa are diverse in form and function and can reproduce sexually or asexually. They are found in a variety of environments, including freshwater, marine, and soil habitats. Some protozoans are known to cause diseases in humans and other animals.

General characteristics of Phylum Protozoa

• Protozoa are microscopic and unicellular animals.
• Solitary and colonial.
• Body naked or covered by pellicle but in some forms the body is covered with shells and often provided with internal skeleton.
• Cytoplasm is differentiated into an outer ectoplasm and inner endoplasm.
• Protozoan nuclei are vesicular or massive. Vesicular nuclei are commonly spherical, oval or biconvex, consist of a central body – the endosome (nucleolus) encircled by a zone of nuclear sap.
• Locomotory organs are pseudopodia in Rhizopoda, flagella in Flagellata, cilia in Ciliates and absent in Sporozoa.
• Nutrition may be holophytic (plant-like), holozoic (animal-like), saprozoic or parasitic.
• Respiration occurs by diffusion through the general body surface.
• Excretion through the general body surface but in some forms through a temporary opening in the ectoplasm or through the permanent pore – the cytopyge.
  The contractile vacuoles also serve to remove the soluble waste matter.
• Reproduction sexual or asexual.
  Sexual by means of copulation or syngamy and conjugation.
  Asexual by binary fission, budding, multiple fission and plasmotomy.
• Life cycle often exhibits alteration of generation, i.e., it includes asexual and sexual phase.
• The formation of cyst or protective wall under unfavourable conditions or during reproduction commonly occurs among the freshwater and parasitic Protozoans.
• No physiological division of labour. All the physiological activities are performed by a single cell.
• Examples:- Paramecium, Euglena, Amoeba, Noctiluca, Plasmodium, Trypanosoma, Leishmania, Entamoeba histolytica

Classification of Phylum Protozoa

Note: The classification is adopted from Hyman, L.H., (1940) with certain modifications from Parker and Haswell, (1965).

Subphylum I. PLASMODROMA

1. Locomotion either by pseudopodia or flagella.
2. Nucleus one or many, generally alike.
3. Sexual reproduction by complete fusion of gametes, i.e., syngamy.
4. Asexual reproduction by binary fission or multiple fission.
5. Complicated life cycle exhibits an alternation of generation.
6. Found in soil, fresh and salt-water.

CLASS 1. MASTIGOPHORA

1. Protozoa with distinct or constant body form.
2. The body is usually covered by a firm and elastic pellicle. Cases or shell of cellulose or silica may be present.
3. Locomotion and food capturing organs are one to many flagella. Pseudopodia also common.
4. The cytoplasm cannot be differentiated into ectoplasm and endoplasm.
5. Nucleus is generally single and vesicular with a central endosome. Meganucleus is absent.
6. Reproduction almost by longitudinal fission (asexual).
7. Life cycle may be simple or dimorphic or polymorphic.
8. Solitary or colonial.
9. Free-living parasitic.
10. Some forms are pigmented and exhibit both protozoan and algal characters.

Order 1. Chrysomonadina

1. Small simple amoeboid flagellates with a thin pellicle.
2. Flagella one or two.
3. Gullet or transverse groove absent.
4. Having one to several, yellow or brown chromoplasts or chromatophores.
5. Locomotion and food capturing by pseudopodia.
6. Nutrition holophytic in coloured forms and holozoic in amoeboid forms.
7. Reproduction usually occurs by fission.
8. The chief characteristic feature of the order is the secretion of a unique endogenous siliceous cyst or test.
9. Solitary or colonial.
10. Examples: 1. Chrysamoeba, 2. Dinobryon.

Order 2. Cryptomonadina

1. Small oval typically biflagellate forms provided with a firm cuticle.
2. A vertical gullet present from which two flagella originate.
3. Colourless or with green, brown or yellow chromatophores.
4. Nutrition holophytic or holozoic or saprozoic.
5. Reserve food material is in the form of starch, fat and oil.
6. Freshwater and marine forms.
7. Examples: 1. Cryptomonas, 2. Chilomonas.

Order 3. Dinoflagellata

1. The shape of the body often irregular, either naked or covered by a simple membrane.
2. Flagella two, one trailing in the body axis and the other transverse to the axis.
3. Girdle and sulcus are characteristic of flagellate stage.
4. Nucleus single, massive and complex.
5. Single stigma and numerous small yellow, brown or greenish chromatophores.
6. Nutrition holophytic or holozoic.
7. Life cycle simple to polymorphic.
8. Some forms are phosphorescent.
9. Mostly marine and few freshwater.
10. Examples: 1. Noctiluca, 2. Ceratium.

Order 4. Chloromonadina

1. Small group of moderate size and amoeboid forms.
2. Two flagella, one trailing.
3. Chromatophores pale green.
4. Nutrition holophytic or saprophytic.
5. Reserve food in the form of oil or glycogen. Starch never formed.
6. A number of mucous globules and trichocysts are present.
7. Freshwater forms.
8. Example: Coelomonas.

Order 5. Euglenoidina

1. Flagellates of moderate size and definite shape.
2. Body covered with soft or rigid pellicle which consists of the epicuticle and thick cuticle.
3. Another end is provided with cytopharynx or gullet.
4. Flagella one or two arising from the cytopharynx.
5. Single vesicular nucleus with a central endosome.
6. Stigma single and red and one or two contractile vacuoles present.
7. Chromatophores few to numerous or absent in some forms.
8. Nutrition holophytic or saprozoic or holozoic.
9. Reserve food materials are paramylum bodies and droplets.
10. Asexual reproduction by longitudinal fission. No sexual reproduction.
11. Examples: 1. Euglena, 2. Paranema.

Order 6. Phytomonadina

1. Small oval or elongated flagellates, enclosed in a semi-cellulose membrane.
2. Flagella 1 to 8.
3. Single large cup-shaped green chromatophore and two small contractile vacuoles.
4. Red stigma and simple vesicular nucleus is usually present.
5. Nutrition holophytic or saprozoic.
6. Reserve food material in the form of starch and oil.
7. Reproduction asexual and sexual.
8. Chiefly freshwater, solitary or colonial forms.
9. Examples: 1. Chlamydomonas, 2. Volvox, 3. Pandorina.

Order 7. Protomonadina

1. Small colourless, mostly naked, amoeboid or euglenoid flagellates.
2. Flagella usually one or two.
3. Nutrition mostly holozoic in free-living and saprozoic in parasitic forms.
4. Reproduction occurs by longitudinal or multiple fission. No sexual reproduction.
5. Solitary, colonial or parasitic.
6. Examples: 1. Trypanosoma, 2. Leishmania, 3. Proteromonas, 4. Leptomonas.

Order 8. Polymastigina

1. Minute oval or elongated, covered with delicate pellicle.
2. Flagella three to eight but generally four.
3. Mouth elongated slit-like.
4. Nuclei one to many accompanied by parabasal bodies and axostyles.
5. Nutrition is holozoic or saprozoic in parasites.
6. Reproduction occurs by longitudinal fission.
7. Commensal or parasites in the intestine of arthropods and vertebrates.
8. Examples: 1. Chilomastix, 2. Trichomonas, 3. Giardia.

Order 9. Hypermastigina

1. Small or large, oval, complex flagellates.
2. Flagella numerous in one, two or more tufts or over the entire body.
3. Single nucleus accompanied by many elongated parabasal bodies and axostylar filaments.
4. Nutrition saprozoic or holozoic.
5. Parasites in the intestine of termites and cockroaches.
6. Examples: 1. Lophomonas, 2. Trichonympha.

Order 10. Rhizomastigina

1. Colourless amoeboid flagellates.
2. Single long flagellum is usually present.
3. The position of nucleus varies and may or may not be accompanied by blepharoplasts.
4. Pseudopodia slender.
5. Examples: 1. Mastigamoeba, 2. Mastigella, 3. Mastigina.

CLASS 2. RHIZOPODA OR SARCODINA

1. Body naked without definite pellicle or provided with shells.
2. The cytoplasm is usually divisible into ecto-and endoplasm.
3. Rhizopoda exhibits spherical symmetry or irregular or deoid of symmetry.
4. Pseudopodia are chief organs of locomotion and food catching.
5. Nuclei are one to many and one to several simple contractile vacuoles in freshwater forms.
6. Nutrition is holozoic.
7. Asexual reproduction chiefly by binary fission or multiple fission.
8. Mostly solitary and free-living rarely colonial and parasitic forms.

Order 1. Lobosa

1. Amoeboid forms with lobose or filose, i.e., short and blunt pseudopodia.
2. Cytoplasm can be differentiated into ecto-and endoplasm.
3. Nuclei one to many with central endosome and one to several simple contractile vacuoles in freshwater forms.
4. Nutrition is holozoic or saprozoic in parasitic forms.
5. Reproduction usually occurs by binary fission. Encystment is the common feature.

Suborder I. Gymnamoebaea or Nuda

1. Body naked or enclosed in a thin membrane.
2. Pseudopodia arising at any point.
3. Examples: 1. Amoeba, 2. Entamoeba, 3. Pelomyxa.

Suborder II. Thecamoebaea or Testacea

1. Body enclosed in a single chambered shell.
2. Shell with a single aperture through which the pseudopodia protrude.
3. Fine branched pseudopodia which do not fuse near the base.
4. Examples: 1. Arcella, 2. Difflugia, 3. Euglypha.

Order 2. Foraminifera

1. Body naked amoeboid or enclosed in a single or many chambered shell with single or many openings.
2. The shell is made up of gelatinous secretion, sand grains or calcium carbonate in majority of forms.
3. Reticulopods or branching slender pseudopodia is the chief distinguishing feature.
4. Cytoplasm is viscous and granular, lacks distinction into ectoplasm and endoplasm having one to many nuclei.
5. Contractile vacuoles present in freshwater forms but absent in marine forms.
6. Reproduction occurs exclusively by multiple fission exhibiting the alternation of generation.
7. Mostly marine forms.
8. Examples: 1. Polystomella, 2. Globigerina, 3. Allogromia, 4. Microgromia.

Order 3. Heliozoa

1. Body naked or enclosed in gelatinous covers.
2. Cycloplasm is distinctly marked into an outer highly vacuolated ectoplasm and an inner granular endoplasm.
3. Pseudopodia fine stiff and radiating.
4. Nutrition is holozoic.
5. Mostly freshwater, free or attached by stalks.
6. Examples: 1. Actinospherium, 2. Actinophrys.

Order 4. Radiolaria

1. Body spherical and always provided with skeletons.
2. The body is divided into central capsule and extra-capsular cytoplasm.
3. Pseudopodia are axopodia or filopodia.
4. Contractile vacuoles are absent.
5. Nutrition is holozoic.
6. Asexual reproduction by binary fission.
7. Examples: 1. Lithocircus, 2. Collozoum, 3. Acanthometra.

CLASS 3. SPOROZOA

1. The body is covered by a firm pellicle, oval or rounded or often elongated and worm-like showing amoeboid, euglenoid or gliding movements.
2. The adult Sporozoa are without external organs of locomotion.
3. Cytoplasm is having single nucleus in young stages and becomes multinucleate at the time of multiple fission.
4. Contractile vacuoles are absent.
5. Nutrition is saprozoic by absorption through the outer surface of the body.
6. Both asexual and sexual reproduction occurs.
7. Life cycle includes an alternation of generation.
8. These are exclusively parasitic forms living either intercellular or intracellular parasites of vertebrates and invertebrates.

Subclass I. TELOSPORIDIA

1. The adult trophozoite is uninucleate.
2. Sporozoites are elongated and without polar capsules in the spores.
3. Spore cases are simple and each containing several sporozoites.
4. Life cycle includes both sexual and asexual phases.

Order 1. Gregarinida

1. The trophozoites are usually elongated worm-like, free and motile.
2. The cytoplasm can be distinguished into ectoplasm consisting of four layers and endoplasm containing paraglycogen bodies, prole and fat spheres.
3. Single nucleus in young stages and numerous nuclei before multiple fission.
4. Intracellular parasites of invertebrates, e.g., Annelida, Mollusca, Arthropoda and Echinodermata. Examples: 1. Monocystis, 2. Gregarina, 3. Schizocystis.

Order 2. Coccidia

1. Trophozoites minute, intracellular parasites of epithelial cells of invertebrates and vertebrates.
2. Young sporozoites and merozoites are elongated and motile.
3. Life cycle complicated exhibits an alternation of schizogony and sporogony sometimes with change of host.
4. Examples: 1. Eimeria, 2. Isospora.

Order 3. Haemosporidia

1. Trophozoite are small, amoeboid and intercellular.
2. Sporozoites are naked.
3. Zygote is motile.
4. Life cycle complicated alternating schizogony in the vertebrate host and sporogony in the invertebrate host.
5. Parasites of the blood of vertebrates
6. Example: Plasmodium.

Subclass II. NEOSPORIDIA

1. Adult trophozoite is multinucleate.
2. Sporozoites are amoeboid with polar capsules in spores.
3. Spore cases are complex, each having single sporozoite.

Order 1. Myxosporidia

1. Trophozoites are amoeboid and not intracellular.
2. Spores are enclosed in a bivalve membrane.
3. Spore consists of one, two, or four polar capsules.
4. Parasites of fishes, inhabits gall-bladder and urinary-bladder, etc.
5. Mostly harmless to the host.
6. Examples: 1. Myxidium, 2. Nosema.

Order 2. Sarcosporidia

1. Trophozoites are elongated.
2. Spore cases are absent.
3. Young parasite forms large cyst in the muscles known as sarcocyst.
4. Parasites in the muscles of reptiles, birds and mammals.
5. Example: Sacrocystis.

Subphylum II. CILIOPHORA

1. Locomotion by cilia either throughout life or in the young condition.
2. Two types of nuclei, i.e., mega-and micronuclei.
3. Asexual reproduction by binary fission and budding.
4. Sexual reproduction by conjugation.
5. Simple life cycle and does not exhibit the phenomenon of alternation of generation.

CLASS 4. CILIATA

1. Body is usually provided with a firm pellicle.
2. Cytoplasm is distinguishable into ecto-and endoplasm. Endoplasm is fluid and granular containing food vacuoles and food reserves.
3. Cilia are the chief locomotory and food catching organelles present throughout life.
4. Nutrition in holozoic.
5. Contractile vacuoles are usually present.
6. Usually two kinds of nuclei occur, larger meganucleus or macronucleus and smaller micronucleus.
7. Asexual reproduction occurs by binary fission and sexual reproduction by conjugation.
8. Mostly solitary, free-swimming, some sessile and some ecto-and endoparasitic.

Subclass I. PROTOCILIATA

1. Body is simple, oval, flattened or cylindrical and uniformly covered with cilia.
2. Mouth and contractile vacuoles are absent.
3. Nuclei two or many but all are similar.
4. Asexual reproduction occurs by plasmotomy and sexual by gametes.
5. Ectocommensals or intestinal parasites of amphibians.
6. Example: Opalina.

Subclass II. EUCILIATA

1. Cilia of unequal size, not uniformly distributed.
2. Mouth or cytostome is usually present.
3. Contractile vacuoles are present.
4. Both meganucleus and micronucleus are present.
5. Asexual reproduction by binary fission and sexual reproduction by conjugation.

Order 1. Holotricha

1. Usually simple, cilia uniformly distributed.
2. Mouth usually present.
3. Sometimes with gullet and undulating membrane.
4. Examples: 1. Paramecium, 2. Didinium.

Order 2. Spirotricha

1. Usually provided with an adoral zone of membranelles beginning on the right side of the peristome and passing along its left edge into the cytopharynx.
2. Peristome is disc-like or triangular or elongated depression.
3. Undulating membrane generally present.
4. Examples: 1. Balantidium, 2. Nyctotheras, 3. Stentor.

Order 3. Peritricha

1. Body is typically bell-shaped attached by a short or long stalk.
2. Body is naked or provided with cilia only in adoral zone or in some a posterior circlet of cilia.
3. Gullet provided with an undulating membrane.
4. Anterior end forms a broad circular flat or bulging peristome.
5. Reproduction occurs by binary fission.
6. Mostly sessile, few colonial.
7. Example: Vorticella.

Order 4. Chonotricha

1. Body is vase-shaped and provided with a spirally coiled funnel-shaped ciliated peristome.
2. Adoral zone is present.
3. Asexual reproduction occurs by the formation of lateral buds.
4. Parasitic on the gills of the amphipods.
5. Example: Spirochona.

CLASS 5. SUCTORIA

1. Body is rounded, oval or conical.
2. Cilia or other locomotory organelles are absent in adult stages.
3. Mouth absent but provided with tentacles for capture and intake of food.
4. Single oval elongated meganucleus and one to several micronuclei.
5. Contractile vacuole one to several.
6. Feed on small protozoans and rotifers.
7. Usually sessile.
8. Examples: 1. Acineta, 2. Podophrya, 3. Ephelota.