Platycephalotrema n. gen. (Dactylogyridae) is proposed for four new species and 5 previously described species parasitizing the gills of flatheads (Scorpaeniformes: Platycephalidae) as follows: Platycephalotrema ogawai n. sp. (type species) from Platycephalus sp. 1 (type host) and Platycephalus sp. 2, both of Nakabo & Kai (2013) (locally known as “Yoshino-gochi” and “Ma-gochi,” respectively) (Japan); Platycephalotrema austrinum n. sp. from Platycephalus endrachtensis Quoy & Gaimard (type host) and Platycephalus sp. (Australia); Platycephalotrema bassensis (Hughes, 1928) n. comb. from Platycephalus bassensis Cuvier (Australia); Platycephalotrema koppa n. sp. from Platycephalus fuscus Cuvier (Australia); Platycephalotrema macassarensis (Yamaguti, 1963) n. comb. from Platycephalus indicus (Linnaeus) (China, Macassar); Platycephalotrema mastix n. sp. from P. fuscus and P. endrachtensis (Australia); Platycephalotrema platycephali (Yin & Sproston, 1948) n. comb. from P. indicus (China) and P. fuscus (Australia); Platycephalotrema sinensis (Yamaguti, 1963) n. comb. from Cociella punctata (Cuvier) (China); Platycephalotrema thysanophrydis (Yamaguti, 1937) n. comb. from Inegocia japonica (Cuvier), Inegocia ochiaii Imamura, and Cociella crocodilus (Cuvier) (Japan, China). Other species requiring further study but potentially members of Platycephalotrema include Ancyrocephalus vesiculosus Murray, 1931, Haliotrema indicum Tripathi, 1957, Haliotrema swatowensis Yao, Wang, Xia, & Chen, 1998, and Haliotrema pteroisi Paperna, 1972. The primary features differentiating Platycephalotrema include species having: (1) tandem gonads (testis postgermarial); (2) two prostatic reservoirs, each emptying independently into the base of the male copulatory organ; (3) a dextral vaginal pore and large vaginal vestibule; (4) dorsal and ventral pairs of morphologically similar anchors; (5) a ventral bar with spatulate ends; (6) a dorsal bar with bifurcated ends, and (7) absence of an accessory piece. The new species are described, and P. thysanophrydis is redescribed based on newly collected and museum specimens.

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lected from the gills of vermiculated sailfin catfish, Pterygoplichthys disjunctivus (Weber, 1991) (Siluriformes: Loricariidae), in inland waters of Okinawa-jima island, Okinawa Prefecture, Japan. Unilatus unilatus and U. brittani represent new country records for Japan. Trinigyrus peregrinus n. sp. is characterized by a coiled male copulatory organ that forms a circle and does not articulate with its accessory piece. Heteropriapulus heterotylus was newly collected from two rivers in south-central Okinawa-jima island. These monogeneans are all considered to be native to South America and to have been co-introduced with the host fish into the inland waters of the island by release of ornamental pet fish.

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The siphonostomatoid copepod Caligus undulatus Shen & Li, 1959 has been widely reported from plankton samples obtained from neritic and oceanic waters off coasts of the Indo-West Pacific and Atlantic Oceans. Until now, its fish host has remained unknown. This copepod belongs to an intriguing group of congeners that, despite being part of a chiefly parasitic group, are consistently found as zooplankters. Quite unexpectedly, in October 2019, a fish host of C. undulatus was discovered in the Seto Inland Sea, Japan—namely, the Japanese sardinella Sardinella zunasi (Bleeker, 1854). Both juvenile (chalimus) and adult individuals of this caligid were observed as parasites of the fish host. The discovery suggests that the species has an alternative life cycle as previously proposed for other purportedly ‘planktonic’ congeners and might frequently switch hosts during the adult stage. Thus, the C. undulatus group is newly proposed as a species group in the genus, in which five species are known as planktonic. Some hypotheses on the modified life cycle of caligids also briefly discussed.