monogene

Monogenea, commonly called monogeneans, are a class of small parasitic flatworms within the phylum Platyhelminthes. They are predominantly ectoparasites of fishes, attaching to external surfaces such as the gills, skin, and fins. Most species have a direct life cycle, with no intermediate hosts, and can complete their development on a single fish host.

Anatomy and attachment: Monogeneans are dorsoventrally flattened and possess a syncytial tegument. A hallmark feature is the opisthaptor, a posterior attachment organ that bears clamps, hooks, or suckers for secure adhesion to the host. They are typically hermaphroditic (monoecious), with reproductive organs that allow cross-fertilization between individuals.

Life cycle and transmission: Eggs or free-swimming larvae called oncomiracidia hatch from eggs and must locate a suitable fish host. After attachment, adults feed on host tissues, mucus, or blood, depending on the species. The direct life cycle means transmission usually occurs between hosts without an intermediate host.

Ecology and significance: Monogeneans are common parasites of wild and cultured fish. Heavy infections can cause tissue damage, respiratory or osmoregulatory stress, and increased mortality, contributing to economic losses in aquaculture and fish health management.

Taxonomy and diversity: The class is traditionally divided into two major groups, Monopisthocotylea and Polyopisthocotylea, distinguished by the structure and number of clamps on the opisthaptor. Notable genera include Dactylogyrus (gill parasites) and Gyrodactylus (often direct-life-cycle parasites).