Description of six species of Psettarium (Digenea: Aporocotylidae) from pufferfishes of the genus Takifugu (Teleostei: Tetraodontiformes), including a redescription of the type-species Psettarium japonicum.

The type species of the aporocotylid blood flukes of the genus Psettarium Goto & Ozaki, 1930, P. japonicum (Goto & Ozaki, 1929) from Takifugu pardalis (Temminck & Schlegel) and Takifugu snyderi (Abe), is redescribed. In addition, three new and two unidentified species of the genus are described: Psettarium gotoi n. sp. from T. pardalis and Takifugu flavipterus Matsuura, Psettarium ozakii n. sp. from T. flavipterus, Psettarium shimazui n. sp. from Takifugu alboplumbeus (Richardson), Psettarium sp. 1 from T. pardalis and Psettarium sp. 2 from Takifugu porphyreus (Temminck & Schlegel). Each of the three new species can be variously differentiated from the congeners by the combination of the ovary position, testis distribution, the number of tegumental spine rows, the length of oesophagus, anterior caecum, metraterm and the confluent part of the vas deferens. The definition of the genus Psettarium has been emended. Psettarium pseudupenei (Lebedev & Parukhin, 1972) and Psettarium rachycentri (Lebedev & Paruchin, 1972) are treated as incertae sedis, as a conical projection at the male terminalia is lacking and the uterus turns clockwise in the former species, and, in the latter species, the male terminalia is posterior to oötype and the posterior caeca have thornlike projections. Psettarium now comprises 19 species including two Psettarium spp. from T. pardalis and T. porphyreus, nine of which infect Takifugu spp. Phylogenetic analyses of 28S rDNA and ITS2 rDNA sequences from five Psettarium species parasitizing Takifugu spp. revealed a monophyletic group. Each species formed a distinct clade based on 28S rDNA, with P. gotoi n. sp., P. sinense, and P. shimazui n. sp. closely related; this relationship was also supported by cox1 analysis.