Gynichthys diakidnus n. g., n. sp. (Digenea: Cryptogonimidae) from the grunt Plectorhinchus gibbosus (Lace ´pe `de, 1802) (Perciformes: Haemulidae) off the Great Barrier Reef, Australia Terrence L. Miller Æ Thomas H. Cribb Received: 17 April 2009 / Accepted: 19 May 2009 Ó Springer Science+Business Media B.V. 2009 Abstract Gynichthys diakidnus n. g., n. sp. (Dige- nea: Cryptogonimidae) is described from the fish Plectorhinchus gibbosus (Lace ´pe `de) (Perciformes: Haemulidae) off Heron and Lizard Islands on the Great Barrier Reef, Australia. The monotypic Gynichthys n. g. is distinguished from all other cryptogonimid genera by the combination of a fusiform body, the lack of oral spines, a forebody that occupies approximately half or more of the body length, a deeply lobed ovary, opposite to slightly oblique testes, a seminal vesicle that is confined mainly in the forebody and the presence of multiple gonotyls in the form of two small slightly muscular pores or pseudosucker-like structures in the mid-line well anterior to the ventral sucker. Bayesian inference analysis of LSU rDNA data revealed that G. diakidnus n. sp. grouped relatively distant to species of the cryptogonimid genus Oligogonotylus Watson, 1976, which also have multiple gonotyls, suggesting that the presence of multiple gonotyls is homoplasious and has thus at least evolved twice in the family. The secondary structure of the internal transcribed spacer 2 (ITS2) rDNA region was inferred for G. diakidnus using minimum free energy and homology modelling algorithms. A four helix model was inferred with helices I and IV being relatively short ( \ 30 nucleo- tides) and helix three being the longest; this structure is homologous with that observed for other digeneans and eukaryotes in general. Introduction Plectorhinchus gibbosus (Lace ´pe `de) is a relatively large haemulid that is widely distributed throughout the Indo-West Pacific from the Red Sea to South Africa in the west, north to the Ryukyu Islands, south to Australia and east to the Caroline and Mariana Islands of Micronesia (Froese & Pauly, 2009). Its diet consists mainly of benthic macroinvertebrates (Kulbicki et al., 2005), but, like other haemulids, individuals prey opportunistically on teleosts, making them susceptible to infection by cryptogonimid trematodes. This species has recently been reported harbouring four species of the Cryptogonimidae Ward, 1917 (Platyhelminthes: Digenea) from three genera, Beluesca littlewoodi Miller & Cribb, 2007, B. longicolla Miller & Cribb, 2007, Chelediadema marjoriae Miller & Cribb, 2007 and Siphoderina grunnitus Miller & Cribb, 2008 (Miller & Cribb, 2007c, 2008). The only other digenetic trematode reported from this host is the lecithasterid Weketrema hawaiiense (Yamaguti, 1970) by Bray & Cribb (2001) off the Great Barrier Reef, Australia. Here T. L. Miller (&) Á T. H. Cribb School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia e-mail: t.miller5@uq.edu.au T. H. Cribb Centre for Marine Studies, The University of Queensland, Brisbane, QLD 4072, Australia 123 Syst Parasitol (2009) 74:103–112 DOI 10.1007/s11230-009-9194-6