Monogenea Class Characteristics Quiz

Helminths: Parasitic Worms

• Helminths are large and multicellular, including several phyla that are unrelated but have superficial similarities.
• They are vermiform or "worm-like" in form.

Phyla and Classes

• Phylum Nematoda (roundworms)
• Phylum Platyhelminthes (flatworms)
  • Class Cestoda (tapeworms)
  • Class Trematoda (flukes: endoparasites)
  • Class Monogenea (skin flukes: ectoparasites)
  • Class Turbellaria (free-living flatworms, some parasitic forms)
• Phylum Acanthocephala (thorny-headed worms)
• Phylum Annelida (segmented worms) not parasitic (leeches)

Acanthocephala: Thorny-Headed Worms

• Acanthocephalans are highly adapted parasites that have lost many organs and structures through evolutionary processes.
• They are vermiform or "worm-like" in form.
• The earliest recognisable description of Acanthocephala was made by Italian author Francesco Redi in 1684.
• Recent genomic data suggests that Acanthocephala are a sister group to rotifers: freshwater zooplankton.
• Invertebrate intermediate host, adult stage almost always in the digestive tract, not usually pathogenic.
• Acanthocephalans are sexually dioecious (an individual organism is either male or female).
• All have complex life cycles, many are not host-specific, and they infect invertebrates, birds, amphibians, fish, and mammals.
• Diagnosis is by observation of eggs in feces.

Monogenea: Ectoparasitic Flatworms

• Part of the Phylum Platyhelminthes
• Ectoparasites: especially on the skin, fins, and gills of fish, also in exotics like frogs, reptiles, and hippos (in the eye)
• No true body cavity or coelom
• Can be quite large (2-3 cm) or microscopic (e.g., Gyrodactylus)
• Simple digestive system: mouth, pharynx, and intestine, no anus (flame cells are specialized excretory cells found in the simplest freshwater invertebrates, including flatworms)
• Poorly developed suckers, attachment is via hooks (classification)
• Hermaphroditic, mostly producing eggs, some are viviparous
• Ancestrally related to Turbellaria (free-living flatworms) and closest to modern cestodes

Myxozoa: Parasitic Jellyfish (Cnidaria)

• Obligate parasites with a two-host life cycle, producing two different spore forms.
• All have polar capsules, which are used to help infect a new host, related to the stinging cells (nematocysts) in non-parasitic Cnidaria.
• Some cause very serious diseases in commercial fish species, both farmed and wild.
• Problematic species are typically histozoic, being found inside host tissues.
• Coelozoic forms are generally not pathogenic, just interesting.
• They also infect some birds, amphibians, reptiles, and mammals.
• Some species of Kudoa are known to cause human gastrointestinal problems when eaten in undercooked fish.

Myxosporea: Class of Myxozoa

• Phylum Cnidaria
• Myxosporea are highly adapted parasitic cnidarians.
• Indirect life cycle: fish host, myxospore, actinospore, annelid host (oligochaete / polychaete).
• Examples: Myxobolus cerebralis and whirling disease, Hepatic biliary group of myxosporeans.

Tetracapsuloides bryosalmonae: A Myxozoan Parasite

• Causes Proliferative Kidney Disease (PKD), one of the most serious parasitic diseases of salmonid populations in Europe and North America.
• Collapse of arctic char population in Iceland due to this parasite, as water temperatures have risen about 1°C, and the parasite becomes pathogenic.