INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 1 - G.Y. Shcherbak - 1995

SUBKINGDOM PROTOZOA (PROTISTA)

PHYLUM APICOMPLEXA

This phylum encompasses over 6,000 species of intracellular and, less commonly, coelomic parasites that infect various animals. A defining characteristic of the group is the specialized structural plan of the active life-cycle phases—merozoites and sporozoites (Fig. 28). These are elongated Cells covered by a pellicle consisting of a Cell/33.html">Plasma Membrane and an underlying layer of microtubules that extend from the anterior, apical (apex) end to the posterior end of The Cell. At the anterior pole lies the apical complex, which contains specialized Organelles designed for parasite invasion into host cells, visible only under an Electron microscope. The apical complex comprises a truncated cone-shaped conoid formed by spirally coiled, tightly adjacent fibrils, with a pair of compact polar (preconoidal) bodies at its apex.

In the anterior third of the cell lie the club-shaped rhoptries and thread-like micronemes. The conoid serves as a skeletal support Structure. The rhoptries are presumably filled with lytic Enzymes that break down the host cell surface. The microtubules running beneath the pellicle throughout the cell provide structural support, maintaining a constant cell shape. At one or more points, the cellular pellicle is interrupted and the membrane becomes single-layered, forming a micropore. This structure may function as a cellular Mouth through which pinocytosis occurs. Because the micropore is visible only via Electron Microscopy, it is often referred to as an ultracytostome; certain groups possess multiple ultracytostomes.

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Fig. 28. Diagram of the ultrastructure of a sporozoite (a), showing details of the micropore (b) and conoid (c) based on electron microscopy: 1 — conoid; 2 — micronemes; 3 — rhoptries; 4 — micropore; 5 — Golgi apparatus; 6 — Nucleus; 7 — mitochondrion; 8 — Endoplasmic reticulum; 9 — pellicle

All apicomplexans are characterized by metagenesis with zygotic Meiosis; in some species, the sexual process remains unknown. The infective stage for members of this phylum is the sporozoite, enclosed within a protective oocyst wall, inside of which there may be four, eight, or more sporozoites,

which in turn can become enveloped in additional membranes to form spores. Within the host Organism, the sporozoites emerge from the spore and sporocyst walls, penetrate host cells, and subsequently feed and grow. This constitutes the trophozoite stage. Most commonly, they reproduce inside the host cell through multiple fission, or schizogony (Fig. 29). This division yields merozoites that escape the cell, infect new cells, and repeat the schizogony cycle to form new merozoites. After several generations of merozoites, gamonts are produced, which subsequently develop into Gametes.

Apicomplexans exhibit various types of sexual reproduction, including isogamy and anisogamy. Anisogamous forms include heterogamy—the fusion of motile gametes of different sizes (micro- and macrogametes)—and oogamy—the fusion of a motile microgamete with a non-motile macrogamete. The zygote becomes surrounded by a wall (oocyst), inside which meiosis takes place, occasionally followed by several mitotic divisions to produce sporozoites, which are frequently encased in protective walls (spores).

The phylum comprises two classes: Sporozoea and Perkinsea, the latter containing the single genus Perkinsus (parasites of marine Mollusks), whose life cycle remains poorly understood.

Fig. 29. Generalized diagram of the Morphology/12.html">ALTERNATION OF GENERATIONS in Apicomplexa