Isorchis cannoni n. sp. (Digenea: Atractotrematidae) from Great Barrier Reef rabbitfishes and the molecular elucidation of its life cycle.

We describe Isorchis cannoni n. sp. from the rabbitfishes Siganus fuscescens (Houttuyn) and Siganus lineatus (Valenciennes) (Siganidae) collected off Heron Island, southern Great Barrier Reef, Australia and, using molecular data, demonstrate that 'Cercariae queenslandae II' of Cannon (1978) from the gastropod Clypeomorus batillariaeformis Habe & Kosuge (Cerithiidae) is the larval form of this new species. The cercariae of I. cannoni n. sp. develop in rediae, encyst in the environment after emergence, and are inferred to then be consumed by grazing rabbitfish. Additionally, we provide a new report of Isorchis currani Andres, Pulis & Overstreet, 2016 from the type host, Selenotoca multifasciata (Richardson) (Scatophagidae) collected in Moreton Bay, south-east Queensland, Australia, greatly expanding the known geographical range of this species. Molecular sequence data (ITS1, ITS2 and 28S rDNA) generated for I. cannoni n. sp. and the new specimens of I. currani, confirm the identification of I. currani and demonstrate a distinct genotype for I. cannoni n. sp. relative to other species of Isorchis Durio & Manter, 1969, for which molecular data are available. Isorchis cannoni n. sp. is morphologically distinct from all other species in the genus, and is further distinguished by utilizing species of Siganidae as definitive hosts, rather than species of Chanidae or Scatophagidae. Because haploporid and atractotrematid cercariae have well-developed reproductive organs, we find cercariae of these closely related families morphologically distinguishable in the same way as adult trematodes: atractotrematids have two symmetrical testes and haploporids have a single testis or, rarely, two tandem or oblique testes.