A sample of yellowfin tuna, Thunnus albacares (Bonnaterre, 1788), and bigeye tuna, Thunnus obesus (Lowe, 1839), caught in the western North Pacific Ocean near the Izu Islands off central Japan, was examined for parasitic copepods. Three species of caligid copepods were collected: Euryphorus brachy-pterus (Gerstaecker, 1853) from the inner surface of the operculum of both tuna species, and Caligus bonito Wilson, 1905 and Caligus coryphaenae Steenstrup and Lütken, 1861 from the gills of bigeye tuna. These copepods are known to parasitize ocean-migrating pelagic fishes, and the present collection includes the first record of C. bonito from bigeye tuna in Japan. Based on the literature published between the years 1894 and 2018, a list of the parasitic copepods of tunas (the tribe Thunnini) from Japan is given: 13 nominal species (3 spp. in Unicolax [Bomolochidae]; 6 spp. in Caligus; 2 spp. in Euryphorus [Caligidae]; 1 sp. in Brachiella [Lernaeopodidae]; and 1 sp. in Pseudocycnus [Pseudocycnidae]) and three or four unidentified species (Caligus sp./spp., Parapelatus sp.? [Caligidae], and Pennella sp. [Pennellidae]) have so far been reported from the Japanese tunas.

Caligid copepods parasitic on yellowfin tuna, Thunnus albacares, and bigeye tuna, Thunnus obesus, in the western North Pacific Ocean off central Japan, with a list of parasitic copepods of tunas (Auxis spp., Euthynnus affinis , Katsuwonus pelamis, and Thunnus spp.) in Japan (1894–2018) (Kazuya Nagasawa , Hiroshi Ashida , Takuya Sato)