In the course of a parasitological survey carried out on Greenland halibut, Reinhardtius hippoglossoides (Pleuronectiformes) from the eastern part of the Bering Sea (North Pacific) a single specimen of a monogenean fluke was found. The parasite, later identified as Pseudacanthocotyla williamsi (Price, 1938) Yamaguti, 1963 has never been found on a Greenland halibut. This fish species has been considered an atypical host for all monogeneans of the family Acanthocotylidae. The present work contains a detailed redescription of the parasite found.
An ultrastructural study has been made of the egg assembly apparatus of the monopisthocotylean monogenean skin parasites Entobdella soleae and E. hippoglossi from the common sole, Solea solea, and the halibut, Hippoglossus hippoglossus, respectively. The ootype consists of a distal tetrahedral chamber where the egg capsule is assembled and a proximal ootype tube where the egg appendage is made. Two types of Mehlis' gland (alpha and beta) open at the proximal end of the ootype tube, which has a non-secretory lining. In both species, the tetrahedral ootype chamber has a syncytial lining, which apparently is not secretory and possesses on its luminal surface stud-like projections, each with vacuolated cytoplasm and an electron-dense core. The ootype chamber is enclosed by a single layer of muscle fibres and is embedded in a spongy "connective tissue". The uterus in E. soleae has a cellular secretory lining, with densely packed luminal microvilli.