The Japanese scad, Decapterus maruadsi (Temminck & Schlegel, 1843), is a commercially important carangid fish caught in coastal waters of the Indo-West Pacific (Froese & Pauly, 2016; FAO FishFinder, 2016) . This species occurs in waters around central-western Japan (Senou, 2013) and is known to be infected by the cymothoid isopod Ceratothoa carinata (Bianconi, 1869) in the western North Pacific off Kanagawa Prefecture (Nunomura, 2006, as the Sagami Sea) and Mie Prefecture (Saito, 2009, 2014)

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... the western Seto Inland Sea off Oita Prefecture (Nagasawa et al., 2014) (Fig. 1). This fish also has been reported to harbor unidentified cymothoids in Okayama and Kochi prefectures (Saito, 2009). Recently, we collected further specimens of C. carinata from D. maruadsi in the western North Pacific off central-western Japan. In total, 59 individuals of D. maruadsi were obtained in fishing ports or at fish markets in central-western Japan from February 2012 to May 2016: these fish were commercially caught in the Kii Channel (locality 4 in Fig. 1 ) and its southern Pacific waters (locality 5 in Fig. 1 ) off Wakayama Prefecture in August and February 2012 (n=20, 202-306 [mean 244] mm fork length [FL] and n=18, mm FL), respectively; Tosa Bay (locality 6 in Fig. 1 ) off Kochi Prefecture in May 2016 (n=17, 173-228 [198] mm standard length [SL]); and the Hyuga-nada Sea (locality 7 in Fig. 1 ) off Miyazaki Prefecture