Protozoa: Definition, Characteristics, Classification, Examples

Protozoa Definition

Protozoa are “microscopic acellular animalcules that exist single or in colonies, are devoid of tissue and organs, and have one or more nuclei.”

Some of the characteristics are:

  1. Approximately 50,000 species of Protozoa are known.
  2. Protozoans primarily demonstrate two modes of existence: free-living (aquatic, freshwater, and marine) and parasitic (ectoparasites or endoparasites). Also, they are habitat buddies.
  3. Typically, they are so small that they cannot be observed without a microscope.
  4. They are the most fundamental and fundamental of all beings.
  5. They possess a simple body structure. In other words, having a protoplasmic level of structure.
  6. The body has only one cell (without tissue and organs).
  7. They contain one or more monomorphic or dimorphic nuclei.
  8. In some forms, the body may be covered by shells, and it is frequently equipped with an internal skeleton.
  9. Either they are solitary (living alone) or they are colonial (individuals are alike and independent).
  10. Body shape variations may be round, oval, elongated, or flattened.
  11. The symmetry of the body may be unilateral, bilateral, radial, or spherical.
  12. In most cases, the shape of the human body is stable; yet, in many others, it varies with environment or age.
  13. The protoplasm of the body is subdivided into an outside ectoplasm and an interior endoplasm.
  14. The single-cell body does fundamental and essential tasks which constitute the animal body; therefore, subcellular physiological division of labour is the only kind of division of labour.
  15. The locomotory organs consist of pseudopodia, flagella, and cilia, or none at all.
  16. Nutrition can be holozoic (similar to animals), holophytic (similar to plants), saprozoic, or parasitic.
  17. The intracellular digestion of food happens within the food vacuoles.
  18. Respiration is accomplished through diffusion across the whole body surface.
  19. In most cases, excretion occurs through the major body surface, but in rare instances, through a short opening in the ectoplasm or a permanent pore termed the cytopyge.
  20. Contractile vacuoles provide osmoregulation and aid in the removal of excretory products in freshwater organisms.
  21. Asexual reproduction (binary or multiple fission, budding, sporulation) or sexual reproduction (conjugation (hologamy), game creation (syngamy)).
  22. Alternating asexual and sexual periods frequently complicate the life cycle (alternation of generation).
  23. Encystment often happens to withstand adverse food, temperature, and moisture conditions and also aids in dissemination.
  24. The undifferentiated single-celled organism is consequently free from natural death, which is the body’s price.
  25. Protozoans primarily demonstrate two modes of existence: free-living (aquatic, freshwater, and marine) and parasitic (ectoparasites or endoparasites). They are also habitat companions.
  26. Examples: Euglena, Amoeba, Plasmodium, Paramecium, Podophyra, etc.

Classification of Phylum Protozoa

Classification of the phylum protozoa is complicated since it is a huge and diverse group.

On the basis of locomotion organs, the usual classification scheme adopted by Hyman (1940), Hickman (1961), and Storer (1965), et al., recognises two subphyla and five classes as follows:

Sub-Phylum A: Plasmodroma

  • Locomotor organelles are flagella, pseudopodia, or none.
  • Nuclei are homogeneous.

Class 1: Mastigophora

  • Move by a few too many flagella.
  • Example: Euglena.

Class 2: Sarcodina

  • Food is moved and captured by pseudopodia.
  • Example: Amoeba

Class 3: Sporozoa

  • Without locomotor organs
  • All parasitic.
  • Spore formation is prevalent.
  • Example: Plasmodium.

Sub-Phylum B: Plasmodroma

  • Cilia are locomotory organelles, as are sucking tentacles.
  • There are two kinds of nuclei.

Class 4: Ciliate

  • Move with cilia
  • Example: Paramecium.

Class 5: Suctoria

  • Move through cilia as a juvenile and tentacula as an adult.
  • Example: Podophyra.

A second categorization is based on the scheme provided by the Committee on Taxonomy and Taxonomic Problems of the Society of Protozoologists, which was primarily presented by BM Honigberg and others (1964).

It classifies protozoa into four distinct subphyla.

Subphylum I: Sarcomastigophora

Subphylum II: Sporozoa

Subphylum III: Cnidospora

Subphylum IV: Ciliophora

Subphylum I: Sarcomastigophora

  • Locomotor organelles are pseudopodia or flagella.
  • The nucleus is a unique type (monomorphic).
  • There is no spore development.
  • Syngamy is present during reproduction.

Superclass A: Mastigophora

  • They are often referred to as flagellates.
  • Flagella are locomotory organelles in adults.
  • A pellicle covers the body.
  • The fission process is longitudinal.
  • The majority are free-living, although a few are parasitic.
  • Autotrophic or heterotrophic nutrition, or both, may exist.

Class 1: Phytomastigophorea

  • Existence of chlorophyll-containing chromatophores
  • Nutrition is mostly derived from holophytes as a result of phototrophy.
  • Reserve food consists of starch and paramylon.
  • Typically, they have only one or two flagella.
  • The nucleus has a vesicular structure.

Order 1: Chrysomonadina

Chrysamoeba, Chromulina, Ochromonas, Dinobryon, Synura, etc.

Order 2: Coccolithophorida.

Examples: Coccolithus, Rhabdosphaera, etc.

Order 3: Heterochloride

Examples: Heterochloris, Myxochloris, etc.

Order 4: Cryptomonadida

Examples: Chilomonas, Cryptomonas, etc.

Order 5: Dinoflagellida.

Examples: Noctiluca, Ceratium, etc.

Order 6: Euglenida.

Examples: Euglena, Phacus, Copromonas, Peranema, etc.

Order 7: Volvocida (Phytomonadida).

Examples: Volvox, Chlamydomonas, Eudorina, etc.

Order 8: Chloromonadida

Examples: Vacularia, Coelomonas, Gonyostomum, etc.

Class 2: Zoomastigophorea

  • Absence of chlorophyll and chromatophores
  • mostly parasitic
  • Food is stored as glycogen.
  • Flagella many to one
  • There is a membrane with undulations.

Order 1: Choanoflagellida.

Example: Proterospongia.

Order 2: Rhizomastigida

Examples: Mastigoamoeba, Dimorpha, etc.

Order 3: Hypermastigida.

Trichonympha, Lophomonas, Leptomonas, etc. are a few examples.

Order 4: Diplomonadida

Examples: Giardia, Hexamita, etc.

Order 5: Kinetoplastida.

Suborder 1: Bodonina.

Examples: Bodo.

Suborder 2: Trypanosomatina.

Examples: Trypanosoma, Leishmania, etc.

Order 6: Bicosoecida

Examples: Salpingoeca, Poteriodendron, etc.

Order 7: Retortamonadida.

Example: Chilomonas.

Order 8: Oxymonadida

For example, Oxymonas and Pyrsonympha.

Order 9: Trichomonadida.

Trichomonas, for example

Superclass B: Opalinata

  • Many organelles imitating cilia are organised in oblique rows over the whole surface of the animal.
  • The absence of a cytostome
  • Existence of two or more monomorphic nuclei
  • The process of binary fission is interkinetic.
  • Syngamy exists in flagellated anisogametes.
  • Each parasitizes primarily frogs and toads.
  • Examples: Opalina, Protoopalina, Zelleriella, Protozelleriella, and Cepedea.

Superclass C: Sarcodina

  • Locomotory organelles are pseudopodia.
  • The primary form is amoeboid.
  • Some have a tough exterior.
  • Typically, they do not produce spores.
  • It is usual for gametes and flagellated offspring to develop.
  • Nutrition is holozoic or saprozoic.

Class 1: Rhizopodea

  • Locomotory organelles are pseudopodia (lobopodian or filopodia but never axopodia).
  • These are often slithering forms.

Subclass a: Lobosia

  • Pseudopodia as lobopodian.

Order 1: Amoebida.

  • Examples: Amoeba, Entamoeba, Pelomyxa, etc.

Order 2: Arcellinida. 

  • Examples: Arcella, Diffugia, Euglypha, etc.

Subclass b: Filosia

  • Their filopodia are tapered and branched.
  • Examples: Gromia, Allogromia, Penardia (naked).

Subclass c: Granuloreticulosia

  • These rhizopodia feature coarsely granular reticulose (reticulopodia).

Order 1: Foraminiferida

  • Examples: Globigerina, Elphidium, etc.

Subclass d: Mycetozoia

  • The amoeboid trophic stage progresses into either a multicellular aggregation or a genuine multinucleate plasmodium.
  • The life cycle is intricate and sexual reproduction is present.
  • Typically, sporangia are produced that release spores.
  • The nutritional process is phagocytic.
  • Example: Plasmodiophora

Class 2: Actinopodea

  • Pseudopodia are mostly composed of axopodia with axial filaments emanating from a spherical body.
  • The majority of their forms are sessile or floating.
  • Typically, gametes are flagellated.
  • Reproduction can occur both sexually and asexually.

Subclass a: Radiolaria

  • One or more pores perforate the central capsule.
  • They have spicules, or a skeleton made of silica.
  • Filopodia or axopodia are found.
  • The capsule distinguishes between ectoplasm and endoplasm.
  • All are maritime.
  • Thalassicola, Collozoum, and Lithocircus are a few examples.

Subclass b: Acantharia

  • The capsule centre is imperforate, non-chitinoid, and devoid of pores.
  • The skeleton of strontium sulphate is anisotropic.
  • Axopodia is present.
  • Marine
  • Acanthometra is one such example.

Subclass c: Heliozoia

  • The absence of a central capsule
  • Body contoured with radiating axopodia.
  • If a skeleton is present, it is composed of scales and spines constructed of siliceous material.
  • Both axopodia and filopodia are present.
  • There may be many nuclei, especially in freshwater.
  • Actinophrys, Actinosphaerium, Clathrulina, etc. are examples.

Subclass d: Proteomyxidia

  • Mostly marine and freshwater algae and higher plant parasites.
  • In certain species, filopodia and reticulopodia are seen.
  • Examples: Vampyrella, Pseudospora, etc.

Classification 3: Piroplasma

  • Small, spherical or amoeboid parasites in the red blood cells of vertebrates.
  • Example: Babesia.

Subphylum II: Sporozoa

  • Absent locomotory organelles
  • Usually, spores are present.
  • Exclusively endoparasites.
  • There may be cilia or flagella in gametes.
  • Following the process of syngamy, many spores are produced.
  • The spores are simple and include between one and many sporozoites.
  • Sporozoites constitute the infectious stage.
  • The nucleus is a single-type structure.

Class 1: Telosporea

  • Pseudopodia are frequently missing.
  • Movement by gliding or bodily flexion.
  • Spores are generated, and some of them include flagellated microgametes.
  • Spores lack polar capsules and filaments; they are bare or encysted.
  • Reproduction both sexually and asexually.

Subclass a: Gregarinia

  • Mature trophozoites are big and extracellular.
  • Sporophyte reproduction is purely sexual.
  • There are eight sporozoites per spore.
  • They are parasites of invertebrates’ digestive tracts and body cavities.
  • Gregarina, Monocystis, and Nematocystis are a few examples.

Subclass b: Coccidia

  • Mature trophozoites are often intracellular and tiny.
  • Each oocyst generates several sporozoites.
  • Parasites of the gastrointestinal tract or blood of vertebrates
  • Gametocytes have two forms.
  • Sporozoites proliferate in tissue cells via schizogony.
  • Examples: Eimeria, Isospora, Plasmodium, etc.

Order 1: Eucoccida

Schizogony takes place.

There are both sexual and asexual stages.

They parasitize the blood and epithelial cells of invertebrates and vertebrates.

Suborder 1: Eimeriina

  • Macrogamete and microgametocyte development are separate.
  • The absence of syzygy
  • The macrogametocyte generates several microgametes.
  • The zygote lacks motility.
  • Oocyst size does not grow during sporogenesis.
  • The sporocyst encases the sporozoites.
  • Example: Eimeria.

Suborder 2: Haemosporina

  • Macrogamete and microgametocyte development are separate.
  • The absence of syzygy
  • Few microgametes are produced by microgametocytes.
  • Zygote are often motile.
  • During sporogony, oocysts grow in size.
  • Sporozoites are naked.
  • Schizogony occurs in vertebrates, whereas sporogony occurs in invertebrates.
  • Host cell haemoglobin creates pigment.
  • Example: Plasmodium.

Class 2: Toxoplasmea

  • There are no spores.
  • At no stage are flagella or pseudopodia present.
  • Reproduction using Asexual Reproduction (binary fission).
  • Cysts are created that contain several sporozoites in their naked state.
  • Examples: Sarcocystis, Toxoplasma, etc.

Class 3: Haplosporea

  • Spores are found.
  • There may be pseudopodia, but flagella are absent.
  • Reproduction is exclusively asexual.
  • Schizogony takes place.
  • Examples: Caelosporidium, Ichthyosporidium, etc.

Subphylum III: Cnidospora

  • Spores have many cells, one or more polar filaments, which are coiling threads which may move, and one or more sarcoplasms or sporoplasms (analogous to sporozoites).
  • Everyone is parasitic.
  • The zygote gives rise to one or more trophozoites in the absence of sporogony.

Class 1: Myxosporidea

  • Large spores are of multicellular origin.
  • There are two or three valves on one or more sporoplasms.
  • They are fish parasites.
  • Examples: Myxobolus, Myxidium, Ceratomyxa, etc.

Class 2: Microsporidea

  • Spores are minute and derived from a single cell.
  • Through a single valve of sporoplasms arises from a single, long, tubular polar filament.
  • They are intracellular parasites (cytozoa) of arthropods and vertebrates.
  • Example: Nosema.

Subphylum IV: Ciliophora

  • For motility, they contain basic ciliary organelles; infraciliature is uncommon.
  • A trophic macronucleus and a reproductive micronucleus are present.
  • The process of binary fission is perkinetal.
  • Fusion of nuclei results in conjugation; autogamy and cytogamy also occur.
  • There are never any gametes for free.
  • Nutrition is either heterotrophic or mixotrophic.
  • Typically, they have a cytostome.

Class 1: Ciliata

  • They possess cilia or composite ciliary structures as organelles for locomotion or food acquisition.
  • There is an infraciliary system that consists of basal granules under the cell surface and longitudinal fibrils that link them.
  • Most ciliates have a cytostome, or cell mouth.
  • Anal aperture (cytopyge): permanent.
  • One is vegetative (macronucleus) while the other is reproductive (micronucleus) (micronucleus).
  • Fission is transverse.
  • Never does sexual reproduction include the creation of free gametes.
  • One or more contractile vacuoles are prevalent even in marine and parasitic kinds.

Subclass 1: Holotricha

  • The cilia of the body are simple and consistent.
  • Buccal cilia are uncommon.

Order 1: Gymnostomatida.

  • Examples: Coleps, Dileptus, Didinium, Prorodon, Nassula, etc.

Order 2: Trichostomatida.

  • Examples: Colpoda, Balantidium, etc.

Order 3: Chonotrichida.

  • Examples: Spirochona, Lobochona, Chilodochona, etc.

Order 4: Apostomatida.

  • Example: Hyalophysa.

Order 5: Astomatida.

  • Examples: Anoplophyra, Maupasella, Hoplitophyra, etc.

Order 6: Hymenostomatida.

  • Examples: Colpidium, Tetrahymena, Paramecium, etc.

Order 7: Thigmotrichida.

  • Thigmophyra, Boveria, etc. are examples.

Subclass 2: Peritricha

  • Adults devoid of cilia.
  • The apex has buccal cilia.

Order 1: Peritrichida.

  • Examples: Vorticella, Carchesium, Trichodina, etc.

Subclass 3: Suctoria

  • Sessile and elongated bodies
  • The young have cilia, whereas the adults have suctorial tentacles.

Order 1: Suctorida.

  • Examples: Acineta, Ephelota, Podophyra, etc.

Subclass 4: Spirotrichia

  • Reduced cellular cilia.
  • Buccal cilia are clearly defined.

Order 1: Heterotrichida.

  • Stentor, Bursaria, Spirostomum, and Nyctotherus are a few examples.

Order 2: Oligotrichida.

  • Examples: Halteria, Strombidium.

Order 3: Tintinnida.

  • Examples: Codonella, Favella, etc.

Order 4: Entodinomorphida.

  • Examples: Entodinium, Cycloposthium, etc.

Order 5, Odontostomatida.

  • Example: Saprodinium.

Order 6: Hypotrichida.

  • For instance, Euplotes, Stylonychia, Urostyla, and Oxytricha.


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