Two New Species of Gyrocotyle (Monogenea) from Hydrolagus affinis (Brito Capello) (Holocephali)

Two new species of Gyrocotyle have been found in the intestine of the deepwater chimaera Hydrolagus affinis of the northwest Atlantic. Gyrocotyle had not previously been recorded from this host species.Descriptions are provided for ♂-mature and ♀-mature specimens of Gyrocotyle major n.sp. and ♂-mature specimens of Gyrocotyle abyssicola n.sp. The systematic status, distribution, and some host relationships of these species are discussed. Within the subgenus Amphiptyches, two groups of species are recognized: the urna-group, including G. major, and the confusa-group including G. abyssicola. Geographical variation is shown by G. major. Allometric growth of different parts of the body in G. major is described. These Gyrocotyle specimens are the largest yet recorded, G. major attaining a maximum length in our collection of 200 mm and G. abyssicola of 138 mm. The size of G. major increases with size of host.