Monogenea (adj. monogenean) are a group of largely ectoparasitic members of the flatworm phylum Platyhelminthes. They are especially common on the skin, fins and gills of fishes and there are thousands of described and undescribed species. Less commonly, they can be found in the urinary bladder and rectum of cold-blooded vertebrates. None infect birds, but one (Oculotrema hippopotami) infects mammals, parasitizing the eye of a hippopotamus. They have direct life-cycles with no asexual reproduction (unlike the Digenea) and in those that lay eggs, a larval stage (generally ciliated) called an oncomiracidium that is responsible for transmission from host to host. As adults, they eat the blood, mucus, and epithelial cells of their host. Adults have both a prohaptor (for feeding) and an opisthaptor (for attachment).

Some parasitologists divide Monogenea into two subclasses based on the complexity of their haptor: Monopisthocotylea have one main part to the haptor, often with hooks or a large attachment disc, whereas Polyopisthocotylea have multiple parts to the haptor, typically clamps. These groups are also known as Polyonchoinea and Heteronchoinea, respectively. Polyopisthocotyleans are almost exclusively gill-dwelling blood feeders, whereas Monopisthocotyleans may live on the gills, skin and fins.

Monopistocotylea include:

• Genus Gyrodactylus, which has no eyespots and is viviparous.
• Genus 'Dactylogyrus, which has four eyespots and lays eggs. This is one of the largest metazoan genera, with at least 970 species.
• Genus Neobenedenia, which is much larger and lives on the skin of many tropical marine species, causing problematic disease in marine aquaria.

All of which can cause epizootics in freshwater fish when raised in aquaculture.

Polyopisthocotylea include:

• Genus Diclidophora, which is primarily found in marine fish and primitive freshwater fish like sturgeons and paddlefish.

Flatworms | parasites | Fish diseases

Hakensaugwürmer | Monogenea | Monogenea | Skrzelowce | Monogenea | Motolicovce