Oculotrema hippopotami Stunkard parasitizes the eye of the hippopotamus and is the only monogenean known from a mammalian host. Eggs from the uterus of O. hippopotami hatch in water in about 30 days at 24 to 26°C. The ciliated larva resembles the larvae of other polystomatid monogeneans, apart from the absence of hamuli and the unequal lengths of the two intestinal caeca.
Up to 62 eggs are stored within the uterus of the mature worm. They are not expelled when the worm is transferred from the eye of the hippopotamus to a tube of water. Development starts when the eggs are placed in fresh-water. There is no development in saline, and no evidence to suggest that larvae develop in situ around the eye, although very small worms have been collected from this site.