Complexity in the recognition of species of Hexangium Goto & Ozaki, 1929 (Trematoda: Microscaphidiidae) from Indo-Pacific Siganidae.

Species of Hexangium (Microscaphidiidae) were collected from 12 species of rabbitfishes (Siganidae) from multiple Indo-West Pacific locations: three on the Great Barrier Reef in Queensland, two in south-east Queensland, Ningaloo Reef in Western Australia, New Caledonia and Japan; no specimens were found from Palau or French Polynesia. Parallel molecular (cox1 mtDNA, ITS2 and 28S rDNA) and morphological analyses, together with consideration of host and geographical distribution, lead us to recognise six species. Specimens from Siganus fuscescens (Houttuyn) from Japan are identified as the type species, H. sigani Goto & Ozaki, 1929, which was originally described from S. fuscescens from off Japan. Hexangium affinum Tubangui & Masiluñgan, 1944, originally described from Siganus javus (Linnaeus) from the Philippines but previously considered a synonym of H. sigani, is reinstated, and H. secundum Annereaux, 1947 is considered its junior synonym. Four new species are described: H. urabeae n. sp., H. kuramochii n. sp., H. robustum n. sp. and H. reimeri n. sp. Based on morphology alone, individual specimens of several combinations of these species are difficult to distinguish. Although none of the combinations of species are entirely mutually cryptic, we consider taxonomy of this genus to be dependent on molecular analysis; morphological distinction of species requires substantial numbers of well-preserved specimens. Most species of Siganus examined harbour two or three species of Hexangium; sometimes one species is dominant, but in several two species were both common. All six Hexangium species recognised here have been found in multiple locations; four species were found at each of two Great Barrier Reef sites.