Dramatic phenotypic plasticity within species of Siphomutabilus n. g. (Digenea: Cryptogonimidae) from Indo-Pacific caesionines (Perciformes: Lutjanidae) Terrence L. Miller • Thomas H. Cribb Received: 28 January 2013 / Accepted: 26 July 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract A survey of Indo-Pacific lutjanids of the subfamily Caesioninae revealed the presence of Sipho- dera gurukun Machida, 1910 and two new cryptogon- imid taxa from off Heron and Lizard Islands on the Great Barrier Reef, Australia, Ningaloo Reef, Western Australia and Rasdhoo Atoll, Maldives. A combined morphological and genetic characterisation of these species shows that they form a clade distinct from the type-species of Siphodera Linton, 1910, S. vina- ledwardsii (Linton, 1901), and warrants the proposal of a new genus. Here we propose Siphomutabilus n. g. and transfer Siphodera gurukun Machida, 1986 as the type-species, Siphomutabilus gurukun (Machida, 1986) n. comb. Siphodera aegyptensis Hassanine & Gibson, 2005 is transferred to Siphomutabilus as S. aegyptensis (Hassanine & Gibson, 2005) n. comb. based on morphological and ecological similarities. Siphomuta- bilus raritas n. sp. is described from Caesio cuning (Bloch) off Lizard Island and S. bitesticulatus n. sp. is described from Pterocaesio marri Schultz off Heron Island. The two new species are unique in that they have two testes, making their morphology broadly consistent with that of Metadena Linton, 1910, yet the molecular analyses conducted here indicates that they are unequiv- ocally united with Siphomutabilus gurukun (which has multiple testes) to the exclusion of Metadena lutiani (Yamaguti, 1942), which was sequenced here. The dramatic phenotypic plasticity observed among such closely related species of Siphomutabilus suggests a secondary modification of what is generally considered a robust generic diagnostic character within this and other digenean families, highlighting the need for a combined morphological and molecular diagnostic approach when characterising these taxa. Siphodera Linton, 1910 is amended to include just two species, the type-species S. vinaledwardsii (Linton, 1901) Linton, 1910 and S. cirrhiti Yamaguti, 1970, which are distinguished by their lack of oral spines and multiple testes that are primarily extracaecal. Siphodera ghan- ensis Fischthal & Thomas, 1968 is considered a species incertae sedis here based on significant morphological and ecological differences compared with species of Siphodera and Siphomutabilus n. g. Introduction Siphodera Linton, 1910 (Digenea: Cryptogonimidae) currently contains five species, which have been reported from a wide range of hosts, mainly fishes of the Lutjanidae (Hassanine & Gibson, 2005). Species in this genus are generally characterised by lacking oral spines and having more than two testes. Two of the T. L. Miller (&) School of Marine and Tropical Biology, James Cook University, PO Box 6811, Cairns, QLD 4870, Australia e-mail: terrence.miller@uqconnect.edu.au T. H. Cribb School of Biological Sciences, The University of Queensland, Brisbane, QLD 4072, Australia 123 Syst Parasitol (2013) 86:101–112 DOI 10.1007/s11230-013-9436-5