Description of three species of Isorchis (Digenea: Atractotrematidae) from Australia.

Three species of Isorchis Durio and Manter, 1969 are described from Australian waters. Isorchis megas sp. nov. is described from the spotbanded scat, Selenotoca multifasciata (Richardson), off Western Australia (WA) and Northern Territory (NT); Isorchis currani sp. nov. is described from S. multifasciata off NT; and Isorchis anomalus sp. nov. is described from the milkfish, Chanos chanos Forsskål, off WA. Isorchis megas sp. nov. can be differentiated from the other species of Isorchis by possessing a single, large egg that is greater than 20% of the body length; having a shorter body (the largest specimen is less than 500 μm); and utilizing a scatophagid rather than a chanid host. Isorchis currani sp. nov. can be differentiated from species of Isorchis other than I. megas sp. nov. by utilizing a scatophagid rather than a chanid host; it is differentiated from I. megas sp. nov. in having eggs that are 11-15% of the body length. Isorchis anomalus sp. nov. can be differentiated from all other species of Isorchis in possessing an irregular shaped genital pore rather than one that is circular to oblong. A Bayesian inference analysis of partial 28S rDNA sequences of the three new species of Isorchis and 30 other haploporoids revealed 1) the monophyly of the Atractotrematidae Yamaguti, 1939, 2) the two species of Isorchis infecting S. multifasciata were each other's closest relative, and 3) that Isorchis was most closely related to Pseudomegasolena Machida and Komiya, 1976 rather than Atractotrema Goto and Ozaki, 1929 although sequence data are not yet available for a member of Pseudisorchis Ahmad, 1985.