INVERTEBRATE ZOOLOGY - G. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

MYXOSPORIDIANS

PHYLUM MYXOZOA

CLASS MYXOSPOREA

Structure and Life cycle. The invasive stage of myxosporidians—the spore—is externally covered by a shell consisting of two to six Valves joined by tight sutures. At one pole of the spore, there are from two (most commonly) to six stinging, or polar, capsules (Fig. 51). Inside each capsule lies a spirally coiled filament that, upon irritation, is ejected outwards and serves to anchor the spore to the villi of the host's intestine, which the spore enters with food or Water. The spore contains an amoeboid germ with two haploid nuclei. Under the action of digestive Enzymes, the spore valves open, and the amoeboid germ emerges into the intestinal lumen (Fig. 52). Through the intestinal walls, it penetrates the Blood and is carried by the bloodstream to its site of localization.

Inside the amoeboid germ, the haploid nuclei merge into a single diploid one (a process of autogamy). Through mitotic nuclear division, the amoeboid germ transforms into a multinucleate plasmodium that acts as a vegetative body containing generative Cells. The plasmodium grows by feeding via pinocytosis, or it forms numerous tiny outgrowths—microvilli—through whose Plasmalemma enzymes are secreted to digest the surrounding host Tissues (parietal Digestion). In some species, phagocytosis of host cells occurs via the plasmodium's lobopodia.

The plasmodium is capable of asexual reproduction by budding and forming new plasmodiums. Prior to sporogenesis, each generative Cell (or several cells) becomes surrounded by a membrane, forming a pansporoblast, inside of which the nuclei divide repeatedly, with one of the divisions being reductional (Meiosis). Cytoplasmic areas segregate around the haploid nuclei, and the pansporoblast becomes multicellular. Most commonly, two spores are formed within it, each consisting of six cells: two form the valves, two form the polar capsules, and another two form the amoeboid germ with two haploid nuclei. Through ruptures in the plasmodium walls, the spores escape to the outside (if the plasmodium forms subcutaneous tumors) or are eliminated from the host's body with excrement, urine, etc.

Thus, The life cycle of myxosporidians proceeds with gametic reduction: the nuclei of the plasmodiums and generative cells are diploid, meiosis occurs only during spore formation, and the spore nuclei become haploid. After the binucleate amoeboid germ penetrates the host Organism, its nuclei fuse, and it becomes diploid.

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Fig. 52. Life Cycle of myxosporidians

(adapted from Hausmann, with modifications): a — spore; b — amoeboid germ emerged from the spore; c — multinucleate plasmodium: 1, 2 — nuclei;

3 — generative cells; 4 — pansporoblast; 5 — development of spores;

6 — spore with a haploid and 7 — with a diploid amoeboid germ; 8 — discharged spore

For some species of myxosporidians, a change of hosts during the life cycle has been proven, with fish serving as the definitive hosts and oligochaetes and sipunculans as the intermediate hosts. For instance, the spores of Myxosoma cerebralis, a parasite of salmonid Cartilage, are actively ingested in polluted water by oligochaete worms of the genus Tubifex, while fish become infected by eating the worms. Within the worms' bodies, the spores penetrate the intestinal epithelium, where they form a special morphological type of spore known as actinosporeans.

Many species of myxosporidians cause significant damage to commercial fish farming. For example, Myxosoma cerebralis causes mass mortality in trout and salmon fry. The parasite affects cartilage, leading to spinal curvature and damage to the Organ of Equilibrium, which causes the fish to swim in circles ("whirling disease in trout").

M. cyprini parasitizes the gills, Muscles, and Internal Organs of carps, causing malignant anemia. Myxobolus pfeifferi, a Muscle parasite of the barbel, causes muscular degeneration and The formation of large tumors under the Skin (Fig. 53).

Fig. 53. Barbel with tumors caused by the myxosporidian Myxobolus pfeifferi (from Döhle)