Abstract / Introduction / Summary:
A pair of an ovigerous female and an adult male of the cymothoid isopod Ceratothoa oxyrrhynchaena Koelbel, 1878 were found in the buccal cavity of a yellowback sea-bream, Dentex hypselosomus Bleeker, 1854 (Sparidae), from Tosa Bay (western North Pacific Ocean), Kochi Prefecture, Shikoku, Japan. This represents the first record of C. oxyrrhynchaena from Kochi Prefecture, and the species is briefly described. The female and the male were attached to the tongue and beside the female, respectively, in the buccal cavity. Four individuals of polyopisthocotylean monogenean (Platyhelminthes) were found on the brood pouch of the female. The biology of C. oxyrrhynchaena in Japanese waters is summarized based on the literature published between 1878 and 2020.