Monogeneans, members of the class Monogenea, are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish.

Infected skin where the parasite is attached may show areas of scale loss and may produce a pinkish fluid.

[2] In salt water fish, Monogeneans can infect the skin and gills, resulting in irritations to the host.

[3] Monogeneans lack respiratory, skeletal, and circulatory systems but they do have posterior attachment structures in the form of adhesives, clamps, hamuli and suckers.

In addition, they have a head region that contains concentrated sense organs and nervous tissue (brain).

Generally, monogeneans also are hermaphroditic with functional reproductive organs of both sexes occurring in one individual.

The following cladogram depicts the phylogenetic relationships of the different monogenean orders:[5][6] Monocotylidea Capsalidea Lagarocotylidea Montchadskyellidea Gyrodactylidea Dactylogyridea Polystomatidea Chimaericolidea Diclybothriidea Mazocraeidea The ancestors of Monogenea were probably free-living flatworms similar to modern Turbellaria.

Polyopisthocotyleans are almost exclusively gill-dwelling blood feeders, whereas monopisthocotyleans may live on the gills, skin, and fins.

They have no intermediate hosts and are ectoparasitic on fish (seldom in the urinary bladder and rectum of cold-blooded vertebrates).