Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Protozoa
Ciliates, or Ciliophora

General characteristics

Ciliates comprise over 6,000 species of the most highly organized unicellular organisms, inhabiting marine and freshwater environments, soils, and specific Organs of various invertebrate and vertebrate animals. The body shape of ciliates varies, but in many free-swimming species adapted to locomotion, it is elongated and streamlined (Fig. 8). Body length varies widely, ranging from 30–40 µm to 1 mm or more. The Organelles of locomotion in ciliates are cilia, which are typically numerous (numbering in the thousands).

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Fig. 8

Slipper animalcule (Paramecium caudatum):

1 — cilia; 2 — collecting canal of the contractile vacuole; 3 — reservoir of the contractile vacuole; 4, 5 — digestive vacuoles; 6 — trichocysts; 7 — macronucleus; 8 — micronucleus; 9 — peristomal depression; 10 — cytostome (Cell Mouth); 11 — cytopharynx (cell Pharynx); 12 — formation of a digestive vacuole; 13 — expulsion of indigestible food waste through the cytopyge (cell anus).

The Ciliate cell contains at least two functionally distinct nuclei: a large (vegetative) macronucleus and a small (generative) micronucleus. The Cytoplasm is always clearly divided into ectoplasm and endoplasm. The outer layer of the ectoplasm secretes a tough, elastic pellicle, which not only gives the animals a definite shape but also allows it to change slightly.

In the ectoplasm of many ciliate species, there are contractile fibers, nerve and supporting filaments, and some also possess trichocysts—organelles of offense and defense.

When ciliates are stimulated, trichocysts discharge a liquid outward that instantly solidifies into an elastic filament, which penetrates the body of an enemy or prey.

The digestive organelles of ciliates begin with The Cell mouth, which is an opening in the pellicle. In Paramecium, the mouth is located at the bottom of the peristomal depression. The mouth leads into the cell pharynx—a channel sometimes lined with cilia. At the inner edge of the pharynx, a vesicle (digestive vacuole) forms, into which food particles enter. This digestive vacuole detaches from the pharynx and circulates through the body. Indigestible food remnants are expelled through an opening in the pellicle known as the cytopyge.

Freshwater ciliates possess one, two, or several contractile vacuoles. They are located at the boundary between the endoplasm and ectoplasm. Each vacuole consists of feeding canals, the contractile vacuole itself, and an excretory pore leading through the pellicle.

The nuclear apparatus is located in the endoplasm of ciliates. The macronucleus varies in shape: ribbon-like, moniliform, but most commonly spherical or oval. The micronucleus (which may number one or more) is usually spherical or oval. These nuclei differ not only in size but also in their Functions. The large Nucleus is responsible for the cell's vital activity, while the small nucleus governs sexual reproduction.

Ciliates reproduce both asexually and sexually. Asexual reproduction begins with the division of both nuclei, followed by The formation of a transverse constriction that divides the body in two.

The Sexual process in ciliates is called conjugation. Typically, two ciliates align with each other along their oral sides, the pellicle dissolves in specific areas, and the individuals become connected by a cytoplasmic bridge. The macronucleus breaks down and dissolves in the cytoplasm. The micronuclei divide twice to form four nuclei, of which three degenerate and one persists. The remaining nucleus divides once more to yield two nuclei: the first is stationary, and the second is mobile and migratory. The individuals exchange migratory nuclei via the cytoplasmic bridge. Afterward, the migratory nuclei fuse with the stationary ones, re-establishing one nucleus in each cell. Conjugation concludes, and the individuals separate. Following Separation, The Nucleus divides, resulting in the formation of new macro- and micronuclei. During conjugation, the heredity of the ciliates is enriched through the exchange of nuclear material from other individuals. Under unfavorable conditions, ciliates form cysts, much like other Protozoans.

The lifestyles of ciliates are diverse. Some are planktonic, freely moving in the Water Column; others are benthic, crawling along the bottom of water bodies; a third group consists of sessile forms attached to underwater objects; a fourth group has adapted to Life in the thin water films of soil; and a fifth group inhabits the bodies of various animals. For instance, The Stomach of ruminants contains A large number of loricate ciliates from the order Entodinomorpha. They assist in Cellulose Digestion and therefore act as symbionts. Paramecia, Stylonychia, and others inhabit freshwaters (Fig. 9).

Fig. 9 Ciliates:

1 — Bursaria; 2 — Stentor; 3 — Ophryoscolecida; 4 — Dendrocometes; 5 — Vorticella.

Parasitic ciliates are diverse and numerous. We shall briefly focus only on the most important representatives. Among the many ciliate species that parasitize fish, the holotrichous ciliate of the genus Ichthyophthirius is of particular significance. It invades the Skin of fish, forming numerous ulcers. This severe disease can result in mass fish mortality, a phenomenon frequently observed in fish-farming ponds.

A representative of the order Peritrichida belonging to the genus Trichodina frequently parasitizes the gills and skin of fish, causing considerable harm to juvenile fish.

In the Large Intestine of humans, the holotrichous ciliate Balantidium coli sometimes acts as a parasite, causing a severe form of colitis. The source of human infection is typically pigs, in whose intestines Balantidium coli resides as a parasite.

Significance of Ciliates

Ciliates, like other unicellular organisms, participate in the biogeochemical cycling of substances in the biosphere. Soil ciliates take part in soil-forming processes. Many ciliate species serve as bioindicators of the sanitary condition of water bodies. A significant number of ciliate species, alongside other protozoans, form part of the activated sludge used in biological wastewater Treatment.

Symbiotic loricate ciliates in the rumen of ruminants are of great importance, as are the parasites that cause dangerous diseases and even the death of various animals.

The Classification of ciliates cannot currently be considered definitively established, as systems proposed by different scientists often conflict. One widely accepted system divides the phylum Ciliophora into two classes: Ciliata and Suctoria.

Review and Self-Assessment Questions:

1. What Characteristic Features of the high level of Organization in ciliates are known to you?

2. Name the Methods of reproduction in ciliates and specify the Functions of the macronucleus and micronucleus.

3. How does The process of conjugation occur in ciliates?

4. What is the diversity and significance of ciliates?

Phylogeny of the Subkingdom Protozoa

Protozoa are unicellular organisms possessing fully differentiated organelles and a true Cell Nucleus that divides by mitosis. They are complex organisms representing the outcome of a very prolonged pre-cellular evolution that preceded The Emergence of the true Introduction/5.html">Eukaryotic Cell. Among eukaryotic animal organisms, THE CELLULAR LEVEL of organization should be regarded as more primitive, and it is believed that in The Development of life on Earth, protozoa appeared earlier than other animal phyla, giving rise to more complex forms of organization. As a result of evolution, members of the subkingdom Protozoa have successfully adapted to environmental conditions, and many protozoan groups (such as ciliates) are biologically progressive, despite remaining at the cellular level of organization. When assessing the relationships between individual protozoan groups, the question arises as to which of them should be considered the most ancient. Sporozoans and ciliates cannot be considered primitive: the former due to their parasitic mode of life, and the latter because of their highly complex Structure. The question regarding sarcodines and flagellates is more difficult to resolve. Morphologically, sarcodines exhibit the greatest structural simplicity in many respects, such as the absence of permanent (in terms of number and position) organelles like a cytostome (cell mouth), cytopyge (cell anus), pellicle, and all fibrillar structures, as well as possessing a changeable body shape, etc. Nevertheless, many zoologists have considered not sarcodines, but flagellates and their ancestors, to be the most primitive protozoa. The similarity of flagellates to such primitive organisms as Bacteria argues in favor of their primitiveness. The latter possess a constant body shape and flagella. Many authors advocating the primitiveness of flagellates point out the fact that many sarcodines pass through flagellated stages during their developmental cycle (e.g., the Gametes of radiolarians and foraminifers). This fact is interpreted as a recapitulation in the developmental cycle of certain sarcodines of a structure characteristic of their ancestors. Finally, numerous instances of flagellates transitioning into an amoeboid state to feed have been described. All this suggests that Sarcodina likely represents a group descended from ancient heterotrophic flagellates. If this view is accepted, Mastigophora should be placed at the ROOT of the Phylogenetic Tree of Protozoa, and consequently of the entire animal kingdom. However, this viewpoint, which was accepted until recently by many biologists, currently encounters A number of difficulties. Electron microscopic studies show that the flagellum of Mastigophora has a very complex structure. This structure, as it turns out, has nothing in common with bacterial flagella, which are arranged much more simply. Thus, one of the key arguments supporting the primitiveness of flagellates and their connection with bacteria is invalidated. It is difficult to assume that the complexly organized flagellum of Mastigophora is a feature of the most primitive eukaryotic organisms.

Perhaps it is more correct to assume that the classes of flagellates and sarcodines originate from an ancient, primitive, and now extinct group of eukaryotic heterotrophic organisms that possessed a flagellar-like type of movement but lacked complexly organized locomotor organelles.

In the subsequent phylogeny of protozoa, flagellates played an exceptionally important role. It is indisputable that the roots of ciliate origins trace back to flagellates, since cilia share the same ultrastructure as flagella. In the course of evolution from flagellates to ciliates, there occurred a multiplication (polymerization) of locomotor organelles and a complex transformation of the nuclear apparatus (the appearance of nuclear dualism and polyploidy of the macronuclei). The phylum Sporozoa is also likely related in its origin to flagellates. This is evidenced by the close structural similarity between the gametes of many Sporozoa (coccidia, Blood sporozoans) and typical flagellates. There are also many common features in the life cycles of these two groups.

The Importance of the class Mastigophora is further underscored by the fact that it served as the source from which all Multicellular animals—the Metazoa—evolved via colonial flagellates.

The phyla Cnidosporidia and Microsporidia likely have a different origin and should be associated with sarcodines. This is supported by the striking similarity between the amoeboid germs of cnidosporidians and amoebas, as well as the complete absence of flagellated stages in their developmental cycle.

In Conclusion, it should also be noted that the class Mastigophora has an undeniable phylogenetic connection with lower Algae. Green flagellates (Phytomonadina, Dinoflagellata, Euglenoidea, Chrysomonadina) can be attributed with equal justification to both Protozoa and lower algae. Many other groups of algae have originated from these groups.