Revision of Metahaliotrema Yamaguti, 1953 (Monogenoidea: Dactylogyridae), with new and previously described species from the spotted scat Scatophagus argus (Linnaeus) (Perciformes: Scatophagidae) in Vietnam.

An emended diagnosis of Metahaliotrema Yamaguti, 1953 (Monogenoidea: Dactylogyridae) is provided based on specimens of six species collected from the spotted scat Scatophagus argus (Linnaeus) (Scatophagidae) in Vietnam: M. scatophagi Yamaguti, 1953 (type-species); M. cf. yamagutii Mizelle & Price, 1964; M. mizellei Venkatanarasaiah, 1981; M. kulkarnii Venkatanarasaiah, 1981; M. ypsilocleithrum n. sp.; and M. similis n. sp. Methaliotrema filamentosum Venkatanarasaiah, 1981 from the whipfin silver-biddy Gerres filamentosus Cuvier (Gerreidae) is included as the only other valid member of the genus. Metahaliotrema arii Yamaguti, 1953 from an ariid catfish is considered incertae sedis within the Dactylogyridae; and Metahaliotrema srivastavai Singh & Agarwal, 1994 from a bagrid catfish is transferred to Chauhanellus Bychowsky & Nagibina, 1969 as Chauhanellus srivastavai (Singh & Agarwal, 1994) n. comb. Metahaliotrema geminatohamula Pan, Ding & Zhang, 1995 from spotted scat in China is determined to be a junior subjective synonym of M. scatophagi. The two new species and M. scatophagi, M. mizellei, and M. kulkarnii are described or redescribed based on specimens from Vietnam.

Two new axinid species (Monogenea: Axinidae) from the Pharao flyingfish Cypselurus naresii (Günther) (Beloniformes: Exocoetidae) in the Gulf of Tonkin off Vietnam.

A total of 21 Pharao flyingfish Cypselurus naresii (Günther) from the Gulf of Tonkin off Vietnam was examined for monogeneans. Ten individuals were parasitised by 72 specimens of two new axinid species of two rare and little known genera, Unnithanaxine Price, 1962 containing only one species, U. parawa (Unnithan, 1957), and Loxuroides Price, 1962 containing two species, L. sasikala (Unnithan, 1957) and L. fungilliformis Zhang, Ding, Liu & Wang, 1999. Unnithanaxine naresii n. sp. and Loxuroides pricei n. sp. are described and differentiated from the related species. Unnithanaxine naresii n. sp. is morphologically similar to U. parawa but is distinguished by the size of the clamps and reproduction organs, the number of spines in the lateral groups of the genital atrium, and in parasitism in a host fish species of a different genus. Loxuroides pricei n. sp. differs from L. fungilliformis in the greater size of the body, the number of clamps, testes, spines on cirrus and genital atrium, and in parasitism in a different host family. Similarly, L. pricei can be separated from L. sasikala in having a shorter distance from the anterior extremity to genital atrium or vaginal region, fewer testes, and a slightly greater number of spines on cirrus and genital atrium.