A total of 52 specimens of the Pacific seabream Acanthopagrus pacificus Iwatsuki, Kume et Yoshino from the Gulf of Tonkin off Vietnam were examined for monogeneans. Twenty fish were parasitised by 101 individuals of five monogenean species, including two known species Allodiscocotyla diacanthi Unnithan, 1962 and Heterapta chorinemi (Tripathi, 1956), as well as three new species, Polylabroides tienyenensis sp. n., Polylabroides tonkinensis sp. n. and Metacamopia lebedevi sp. n. Polylabroides tienyenensis and P. tonkinensis are morphologically more similar to Polylabroides guangdongensis Zhang et Yang, 2001 in comparison with other species within the genus, based on the absence of small spines on the cirrus. However, P. tonkinensis is distinguished from P. guangdongensis by fewer clamps on the haptor and by the different shapes of the large spines on the cirrus. Similarly, P. tienyenensis differs from P. guangdongensis by vaginal ducts with fewer branches, fewer clamps and smaller egg size. Metacamopia lebedevi is distinguished from Metacamopia chorinemi (Yamaguti, 1953) by the arrangement of testes (one row vs two rows), diverticula absent in the oesophagus, and the number of anchor pairs (one vs two); it differs from Metacamopia oligoplites Takemoto, Amato et Luque, 1996 by the smaller haptor, shape and absence of small sclerotised hooks, the number of ribs in their clamps, and the position of the testes; it can be separated from Metacamopia indica (Unnithan, 1962) by having fewer testes and lacking sclerotised structures in the vagina. The present study also provides the measurements for A. diacanthi, H. chorinemi, and proposes a new key to all species of Polylabroides.

Federal Scientific Center of the East Asia Terrestrial Biodiversity Far Eastern Branch Russian Academy of Sciences Vladivostok Russia

Georgia Southern University Statesboro United States of America

Graduate University of Science and Technology Vietnam Academy of Science and Technology Cau Giay Hanoi Vietnam

Institute of Ecology and Biological Resources Vietnam Academy of Science and Technology Cau Giay Hanoi Vietnam

Institute of Marine Environment and Resources Ngo Quyen Hai Phong Vietnam

Research Institute for Aquaculture No 1 Dinh Bang Tu Son Bac Ninh province Vietnam

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