monogena

Monogenea are a class of parasitic flatworms (phylum Platyhelminthes) that primarily infest aquatic vertebrates, especially fishes. They are ectoparasites with direct life cycles, typically lacking intermediate hosts, and are often highly host-specific.

Morphology and attachment: Most monogeneans are small and dorsoventrally flattened. A distinguishing feature is the haptor, a posterior attachment organ bearing hooks or clamps that secures the parasite to the host, commonly on the gills or skin. Monogenea are divided into two main subgroups: Monopisthocotylea, with a single posterior attachment organ (haptor), and Polyopisthocotylea, with multiple clamps or suckers distributed across the haptor.

Life cycle and reproduction: The life cycle is direct. Most species are simultaneous hermaphrodites and reproduce sexually on the host. Eggs or free-swimming larvae called oncomiracidia are released and must locate a suitable host. Notably, some lineages in Gyrodactylidae are viviparous, giving birth to live young which can immediately begin parasitizing the same host, enabling rapid population growth and frequent outbreaks in aquaculture.

Ecology and impact: Monogeneans inhabit the gills, skin, mouth, or fins and can cause irritative lesions, increased mucus production, and osmoregulatory stress. Heavy infections can lead to reduced growth, diminished fitness, and mortality, posing problems in fish farming and ornamental fisheries.

Taxonomy and examples: Monogenea is a class within Platyhelminthes. Notable genera include Dactylogyrus and Cichlidogyrus (family Dactylogyridae) and Gyrodactylus (family Gyrodactylidae). They are widespread in freshwater and marine systems and exhibit considerable host specificity.