Protocotyle euzetmaillardi n. sp. (Monogenea: Hexabothriidae) from the bigeye sixgill shark Hexanchus nakamurai Teng (Elasmobranchii: Hexanchidae) off New Caledonia.

Protocotyle euzetmaillardi n. sp. is described from the gills of the sixgill shark Hexanchus nakamurai Teng caught in deep-sea off New Caledonia, South Pacific. The new species is compared with the two other species of the genus (both from the only other species in this shark genus, H. griseus (Bonn.)), namely P. grisea (Cerfontaine, 1899) Euzet & Maillard, 1974, redescribed from vouchers, and P. taschenbergi (Maillard & Oliver, 1966) Euzet & Maillard, 1974, redescribed from its type-specimens. The anatomy of the reproductive system is detailed; all three species have a characteristic oötype with longitudinal cells ('ootype côtelé' of Euzet & Maillard). The following unique combination of characters differentiates the new species from its two congeners: posterior lobe of seminal vesicle absent, diverticulum of oviduct present and small body size. Furthermore, its tubular ovary does not include a region with sperm, which is present in both of the other species, and its eggs have only one filament, whereas eggs in the uterus have one or two filaments in P. grisea and one filament in P. taschenbergi. The latter features differ from existing diagnoses of Protocotyle, in which eggs with two filaments and the presence of a tubular ovary dilated with sperm are key characteristics.