Piscinquilinidae

Ichthyophaga is a monotypic genus of flatworm belonging to the monotypic family Piscinquilinidae; its sole described species is Ichthyophaga subcutanea. When described in 1949, this species was one of the few turbellarians known to be a parasite of vertebrates, along with Micropharynx, though I. subcutanea is an endoparasite. The type specimens, the largest of which were 4.4 mm (0.17 in) long, were recovered by one V. A. Dogel from within the tissues of Cottoid fish caught around Vladivostok; the fish species parasitized being Bero elegans and Hexagrammos decagrammus. It was historically placed within the family Genostomatidae of Alloeocoela due to a similar morphology and parasitic lifestyle, but is now placed within its own monotypic family after an analysis of flatworm 18S and 28S rRNA in 2017.

I. subcutanea is brick red, while a turbellarian parasite also assigned to Ichthyophaga were yellow-tinted to off-white; this undescribed species causes black spot disease (sometimes known as "black ich") to Naso unicornis tang.