INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 1 - H. Y. Shcherbak - 1995

SUBKINGDOM PROTOZOA, OR UNICELLULAR ANIMALS

GENERAL CHARACTERISTICS

Protozoans are distributed globally, inhabiting both aquatic environments and soils. They encompass both free-living and parasitic species. To date, over 30,000 species have been described, though it is believed that the vast majority of protozoan species remain unknown to science. Protozoans play a crucial role in The formation of the Earth's crust. Aquatic protozoans serve as a vital food component for numerous hydrobionts, particularly fish fry. Parasitic species cause dangerous diseases in humans and domestic animals, yet some show promise for The Development of biological pest control Methods.

Protozoans are eukaryotic organisms at THE CELLULAR LEVEL of Organization, morphologically and physiologically complete. They include unicellular and colonial organisms, multinucleated plasmodia, and multicellular forms.

The term "Protozoa" (meaning "first animals") does not accurately reflect their complex internal Structure; however, this complexity pertains to cellular structures rather than multicellular Tissues, unlike true Multicellular animals (Metazoa). Protozoan sizes range from 2 µm to 5 cm (averaging 5–250 µm). The smallest known species is the intracellular cattle parasite *Babesia bovis*, measuring no more than 2–2.5 µm. Conversely, colonial radiolarians can reach sizes of up to 25 cm.

Protozoans exhibit a remarkable diversity of Symmetry types, including spherical, radial, and bilateral symmetry. A significant number of protozoans are asymmetrical. The most diverse forms of symmetry are found among planktonic marine radiolarians, which possess mineral skeletons that frequently form intricate, regular geometric patterns. Rapidly moving flagellates and Ciliates feature a specialized type of symmetry known as helical symmetry.

Like any Introduction/5.html">Eukaryotic Cell, a protozoan cell contains universal organoids such as the membrane, nucleus, Mitochondria (absent in Microspora and certain flagellates), Ribosomes, Endoplasmic reticulum, Golgi apparatus, Lysosomes, etc. However, because a protozoan cell Functions as an independent Organism, it also possesses specialized Organelles that vary across different protozoan taxa. A protozoan cell is enclosed by a membrane, or Plasmalemma, which features the mosaic structure typical of Biomembranes. Externally, the cytoplasmic membrane is covered by a glycocalyx composed of mucopolysaccharides. This layer is linked to The Cell's signaling system and contains specialized receptor molecules. Through the glycocalyx, Cells can selectively accumulate various substances from their environment for subsequent internalization. In addition to the glycocalyx, many protozoans form various extracellular surface structures, such as scales, cell walls, or testaceous shells (tests). In most protozoans, supportive structures form beneath the membrane to maintain a constant cell shape. These structures vary among different taxa and will be discussed in the respective sections.

The Cytoplasm of protozoans can be differentiated into two layers: the outer, denser, gelatinous ectoplasm (in a gel state) and the inner, granular, more fluid endoplasm (in a sol state). These layers are capable of interconverting.

All protozoans possess a nuclear apparatus, typically consisting of a single nucleus, although multinucleated forms are also common. As in Multicellular Organisms, the protozoan nucleus synthesizes mRNA, which plays a key role in Protein Synthesis.

Protozoans contain various fibrillar structures, namely microfilaments and microtubules. Microfilaments (4–10 nm in thickness) are contractile structures responsible for cell mobility. They drive the contraction of the cell body or its individual parts (such as the stalks of peritrichous ciliates) and participate in Cell Division. Microtubules are relatively rigid elements that frequently act as a Cytoskeleton, fixing specific organelles in defined cellular positions. By interacting with microfilaments, microtubules facilitate protozoan locomotion.

Protozoans move either by means of temporary cytoplasmic extensions known as pseudopodia or through specialized locomotor structures such as flagella and cilia.

The cytoplasm of freshwater protozoans contains contractile vacuoles, usually one, but occasionally two or 15–20. The primary function of contractile vacuoles is osmoregulation. Due to the high intracellular concentration of salts, Amino Acids, and sugars, the cytoplasm maintains a higher osmotic pressure than the surrounding environment, causing Water to continuously enter the cell. By actively expelling excess water, the contractile vacuole maintains constant intracellular solute concentrations and eliminates Metabolic waste products.

Protozoans exhibit diverse feeding strategies. The vast majority are heterotrophic, feeding via phagocytosis or pinocytosis. Phagocytosis, discovered by

I. I. Mechnikov in 1882, is an active mechanism for capturing microorganisms and solid particles using pseudopodia or a cell Mouth (cytostome). The ingested food is enclosed within digestive vacuoles in the cytoplasm, where it undergoes enzymatic breakdown, and the nutrients are absorbed into the cytoplasm across the vacuole membrane. Phagocytosis is also characteristic of many specialized cells in multicellular animals (e.g., Blood Leukocytes, digestive cells of Sponges and Cnidarians). Pinocytosis involves the uptake of fluid through fine channels formed by the invagination of The cell membrane (without pseudopodia formation). Pinocytotic vesicles pinch off from these channels into the cytoplasm, where their contents are digested. Pinocytosis is likewise observed in the intestinal epithelial cells and nephrons of vertebrates.

Most plant-like flagellates feed autotrophically via Photosynthesis and possess chlorophyll-containing chromatophores.

The Increasing complexity of a unicellular organism can occur through The Emergence of new types of organelles (e.g., supportive structures such as the axostyl or radiolarian skeletons) or through an increase in the number of pre-existing ones (such as multiple flagella). V. A. Dogel termed this multiplication of homologous structures polymerization. Any organelle, including nuclei, can undergo polymerization. If multiple identical nuclei are formed, each participates in protein synthesis within the cytoplasm. Such a functional unit—a cytoplasmic territory containing a single nucleus and a corresponding set of organelles—is called an energid. A cell containing multiple energids is termed polyenergid (e.g., most radiolarians), whereas a cell with a single nucleus is monoenergid.

In certain multinucleated protozoans (such as ciliates) that possess structurally and functionally differentiated nuclei, only one nucleus (the macronucleus) directs protein synthesis; therefore, they

remain monoenergid regardless of the number of generative micronuclei.

In some protozoans, a single nucleus is retained, but polymerization manifests as an increase in the number of homologous Chromosomes (polyploidy), which accelerates PROTEIN SYNTHESIS AND, consequently, promotes rapid cell growth. Monoenergid cells can form colonies or even multicellular structures with rudimentary cellular differentiation (e.g., Volvox).

Protozoan reproduction occurs via both Selection/8.html">Asexual and sexual modes.

Several modes of asexual reproduction are known. In binary fission (monogamy), a cell divides into two daughter cells, which subsequently grow and regenerate all necessary organelles. Palintomy involves a series of rapid successive divisions without intervening cell growth until all divisions are complete (reminiscent of zygote Cleavage in multicellular animals). Multiple fission, or schizogony (syntomy), is a process in which The Nucleus divides repeatedly before cytoplasmic cleavage occurs; subsequently, a portion of cytoplasm surrounds each daughter nucleus, causing the parent cell to break apart into numerous daughter cells. A specialized type of asexual reproduction is endodyogeny, in which nuclear division and organelle duplication occur beneath the maternal cell membrane, with the newly formed daughter cells remaining connected for a period.

The Sexual process in protozoans involves either copulation—the fusion of two sex cells (Gametes)—or conjugation, a temporary union of two individuals accompanied by the reciprocal exchange of nuclear material.

Unlike multicellular organisms, which are characterized by ontogeny (individual development), protozoans undergo a regular succession of individual unicellular generations known as a life cycle.

In the simplest case, The life cycle extends from one cell division to the next, with individuals of all generations remaining morphologically identical (as seen in many sarcodines and flagellates). More complex life cycles primarily involve the alternation of morphologically distinct cell types (for example, flagellates of the genus Leishmania alternate between two morphological cell types As a result of parasitizing vertebrate cells and insect organ cavities, despite reproducing by simple fission). The most complex life cycles are found in protozoans that alternate between sexual and asexual reproduction (metagenesis), accompanied by an alternation of Haploid and Diploid nuclear phases (nuclear cycle). Gametes are invariably haploid, whereas the zygote is diploid. Depending on the timing of reductional division (Meiosis), three distinct types of nuclear cycles are recognized (Fig. 2).

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Fig. 2. MAIN TYPES OF nuclear life cycles — with zygotic reduction (a), gametic (b), and intermediate (c): 1 — Stages of the haploid generation; 2 — stages of the diploid generation; 3 — zygote; 4 — agametes; 5 — gametes (R! — site of reduction division) (reduction division leads to their formation) — in heliozoans, many animal flagellates, ciliates, myxosporidians, and multicellular organisms.

1. With zygotic reduction: the major part of the cycle consists of haploid cells, and only the zygote is diploid (undergoes reduction division to form haploid cells) — in plant flagellates, some animal flagellates, and Sporozoans.

2. With gametic reduction: the major part of the cycle consists of diploid cells, with only gametes being haploid

3. With intermediate reduction: each haploid and diploid stage accounts for approximately half of the cycle, comprising two generations: the diploid generation (agamonts), which produce agametes via meiosis, and the haploid generation (gamonts), which produce gametes through mitotic division (in foraminifers).

An important biological feature of protozoans is their ability to encyst. During this process, their cells round up, shed or retract locomotory organelles, secrete a thick protective wall around themselves, and enter a dormant state. In the encysted state, protozoans can withstand drastic changes in Temperature and moisture while maintaining viability.

When conditions favorable for the species return, protozoans excyst and transform back into vegetative stages.

Below is the Classification of protozoans adopted in this textbook