The occurrences of three ectoparasites on the flathead <i>Platycephalus</i> sp. 2, the “Magochi” from Tokyo Bay, Japan

The flathead Platycephalus sp. 2 (“Magochi”) is a commercially and recreationally important marine fish in Japanese coastal waters. During the course of our surveys on the occurrence of metazoan parasites of this fish, we found that individuals caught from Tokyo Bay are commonly infected with the parasitic copepod Taeniacanthus platycephali (Yamaguti, 1939) and a monogenean parasite Platycephalotrema ogawai Kritsky & Nitta, 2019 on their gill filaments, and in a few cases can also be infected with another species of parasitic copepod Acanthochondria tchangi Yü, 1935 on the operculum and buccal cavity. Infection levels and microhabitat use of these parasites were therefore investigated, and the relationship with host condition was tentatively assessed for these two common gill parasites. As a result of examinations of the host obtained from seven coastal sites, the present study suggested that (1) P. ogawai is the numerically dominant-ectoparasite, (2) the abundance of the parasitic copepod T. platycephali increases significantly with host body size, (3) host body condition declines as P. ogawai abundance increases, and (4) P. ogawai represents a niche specificity in that it is abundant in the anterior gills, whereas T. platycephali does not follow such a bias.