INVERTEBRATE ZOOLOGY - G. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION SPIRALIA (SPIRALIA)

PHYLUM FLATWORMS (PLATHELMINTHES)

SUBPHYLUM NEODERMATA (NEODERMATA)

SUPERCLASS CERCOMEROMORPHA (CERCOMEROMORPHA)

CLASS MONOGENEA (MONOGENEA)

Most species of monogeneans are ectoparasites of vertebrates; a small number of species are endoparasites found in the Oral Cavity, posterior intestine, excretory system (Ureters of fish, Urinary Bladder of frogs), or Circulatory system. They predominantly parasitize freshwater and marine fish, and much less frequently amphibians and reptiles. Only a single species, Oculotrema hippopotami, is a parasite of the conjunctival sac of hippopotamuses. The Class includes about 2,000 species (with over 200 species recorded in Ukraine).

Monogeneans, or monogenean flukes, are characterized by the presence of an attachment organ—a cercomer equipped with hooks or suckers—certain structural details of the digestive, excretory, and reproductive systems, and a simple life cycle involving metamorphosis but no change of hosts.

Morphology. The body of monogeneans is elongated and leaf-shaped, ranging from 0.02 mm to 3 cm in length. It is divided into the trunk proper and an attachment disc—the cercomer (opisthaptor)—bearing chitinoid hooks, which the parasite uses to anchor itself in the host's Tissues (Fig. 133). In more specialized species, the cercomer also bears suckers or Valves. Less commonly, the entire attachment disc transforms into a single large sucker, which may secondarily be divided by septa into separate loculi (e.g., in Nitzschia). Occasionally, the cercomer acquires a rather bizarre shape; for instance, in Heterobothrium affinis, it features finger-like outgrowths that resemble a hand seemingly clasping the host's gill lamellae (Fig. 133, c). Mobile HEAD lobes may be located at the anterior end of the body, On the surface of which open the ducts of unicellular glands containing an adhesive secretion.

Fig. 133. Digestive System and attachment apparatuses of monogeneans (from Strelkov):

a — Dactylogyrus; b — Hexabothrium; c — Heterobothrium affinis:

1 — Pharynx; 2 — midgut; 3 — attachment disc; 4 — hooks; 5 — suckers; 6 — attachment valves; 7 — sickle-shaped hooks

Skin-muscular sac. Beneath the typical tegument lie layers of circular and longitudinal Muscles, sometimes with a layer of diagonal muscles situated between them. In some species, numerous cutaneous glands are associated with the tegument, with their ducts opening onto the body surface; their Functions remain unclear.

The digestive system is similar to that of trematodes. The midgut is sac-like or bilobed, occasionally forming a closed ring or bearing lateral outgrowths. In larger species, the main intestinal branches form numerous ramifications resembling a dense network.

Primitive Representatives of the class feed on mucus and host epithelial Cells. In doing so, species of the genus Gyrodactylus tear apart the host's epithelium using the outgrowths of pyramidal pharyngeal cells. Members of the family Polystomatidae are obligate hematophages. The Glands of the anterior regions of their intestine secrete anticoagulants that prevent the host's Blood from clotting.

The excretory system is protonephridial, represented by paired canals, each opening via an independent pore.

The Nervous system is a typical orthogon. Innervation of the attachment disc is well developed.

Sense Organs are poorly developed. Sensilla—occurring singly or gathered in groups—are located on the body surface. Inverted eyes mostly function only during the larval stage; in adults, they are rudimentary or entirely absent.

The Reproductive System of monogeneans is hermaphroditic. The Male Reproductive System consists of Testes, vas deferens, accessory glands, and a copulatory apparatus, which is often complex in Structure. The number of testes varies considerably. The structural details of the FEMALE REPRODUCTIVE SYSTEM also vary among different species. There is always a single Ovary, and vitelline glands are well developed. A Vagina is present, through which the partner's sperm enters the ootype. Monogeneans lay complex eggs.

Reproduction. Fertilization is usually cross-fertilization, though self-fertilization is also possible. A unique adaptation to ensure cross-fertilization is found in parasites of cyprinid fish belonging to the genus Diplozoon. While still in the larval stage, two individuals fuse together in such a way that the male reproductive duct (vas deferens) of one individual opens into the female duct (vagina) of the other (Fig. 134). Without this, they do not reach sexual maturity.

Fig. 134. Diplozoon paradoxum (after Strelkov):

1 - oral opening; 2 - buccal suckers; 3 - pharynx; 4 - midgut; 5 - vitellaria; 6 - reproductive system complex; 7 - attachment clamps

The life cycle of monogeneans, with rare exceptions, proceeds without ALTERNATION OF GENERATIONS or change of hosts. Viviparity is uncommon (e.g., in the genus Gyrodactylus). The simplest life cycles are observed in monogeneans of the genus Dactylogirus, where eggs hatch into ciliated larvae bearing two pairs of eyes and a haptor (opisthaptor) at the posterior end of the body. These larvae attach to fish gills and develop into adults.

A more complex life cycle is characteristic of the frog bladder fluke, Polystomum integerrimum (Fig. 135), whose cyclical nature is closely tied to the biology of its host. Adult worms parasitize the urinary bladder of frogs and feed on blood. The timing of egg-laying in Polystomum coincides with the period when frogs spawn in Water. The hatched larvae infect newly hatched frog tadpoles, settling not in the urinary bladder, but on their gills. Individuals of the ectoparasitic generation are significantly smaller than those of the endoparasitic generation; they have a less developed haptor and lack a vagina. They reach sexual maturity on the tadpole's gills, release a small number of eggs into the water, and subsequently die. The larvae emerging from these eggs (the next generation) reinfect the tadpoles. However, they do not have time to complete their development on the gills, as the tadpole's gills regress and the gill chamber closes. Consequently, the polystomes migrate across The surface of the tadpole's body and enter the urinary bladder via the cloaca, where The Development of the endoparasitic generation is completed. Frogs are infected only once in their lifetime, during the tadpole stage. These parasites live for up to six years (for comparison, fish ectoparasites of the family Dactylogyridae live for only 25–40 days).

Fig. 135. Life Cycle of Polystomum integerrimum:

a - parasite from the frog urinary bladder; b - egg; c - larva; d - gill form of the parasite on a tadpole; e - hatching of the larva from the egg;

f - tadpole at the end of metamorphosis with second-generation parasite larvae; g - migration of the larva to the frog urinary bladder;

h - immature form of the parasite from the frog urinary bladder:

1 - oral opening; 2 - pharynx; 3 - midgut; 4 - vas deferens; 5 - ovary; 6 - Testis;

7 - sperm duct; 8 - sucker; 9 - hooks; 10 - vitellaria;

11 - vitelline ducts; 12 - ootype surrounded by Mehlis' gland; 13 - vagina;

14 - Uterus; 15 - genital pore; 16 - eyes; 17 - ciliary bands

Monogeneans cause the greatest damage to pond fish farming, particularly representatives of the families Dactylogyridae and Gyrodactylidae. Fry and juvenile fish suffer the most. By destroying the gill epithelium, the parasites disrupt gas exchange and induce inflammation. In cases of high infection intensity (up to 500 parasites per individual), dactylogyrids can cause fish mortality, sometimes on a mass scale.

There are documented cases where monogeneans have caused mass mortality of hosts in natural ecosystems as well. For instance, following an attempt to acclimatize the Volga sturgeon (sevryuga) in the Aral Sea, the parasites of this sturgeon species, Nitzschia sturionis, shifted to parasitize the native Aral barbel sturgeon (ship), triggering its mass mortality.