Revision of Isoparorchis Southwell, 1913 (Digenea, Hemiuroidea, Isoparorchiidae), parasites of the air bladder of freshwater catfishes: a molecular and morphological study

Isoparorchis Southwell, 1913 (Digenea, Hemiuroidea, Isoparorchiidae), parasites of the air bladder of freshwater catfishes (Osteichthyes, Siluriformes) from East, Southeast and South Asia and Australia, is revised based on a molecular and morphological study. Materials, including syntypes where available, were examined from Russia (Primorskiy Kray), Japan, Vietnam, Cambodia, Bangladesh, India and Australia. The entire second internal transcribed spacer region of the ribosomal DNA (ITS2 rDNA) was sequenced for 18 distinct samples. Four of the five previously described species are considered to be valid: Isoparorchis trisimilitubis Southwell, 1913 (type species) from India, I. hypselobagri (Billet, 1898) from Vietnam, I. eurytremum (Kobayashi, 1915) from Japan and Russia and I. tandani Johnston, 1927 from Australia. Isoparorchis pakistani Bilqees and Khatoon, 1972 from Pakistan is regarded as a species inquirenda. Not only syntypes of I. trisimilitubis but also other materials from India and Bangladesh consisted of specimens of two morphologically distinct species. One is identified as I. trisimilitubis, and the other remains an undescribed species, Isoparorchis sp. 3. Distinctions in the ITS2 sequences obtained for samples from India support the interpretation that there are two species there. One sequence is related to I. trisimilitubis, but it remains undetermined whether the other sequence (Isoparorchis sp. 1) relates to Isoparorchis sp. 3. Some materials from East, Southeast and South Asia require further critical studying for definitive species identification. Each species is described and figured with a summarized life cycle where known. Molecular data are given. A key to the species is presented