'Thelohania montirivulorum' sp. nov. (Microspora: Thelohaniidae), a parasite of the Australian freshwater crayfish, 'Cherax destructor' (Decapoda: Parastacidae): fine ultrastructure, molecular characteristics and phylogenetic relationships

'Thelohania montirivulorum' sp. nov., a new species of microsporidian parasite, was found in a highland population of the Australian yabby, 'Cherax destructor'. Data are presented on fine ultrastructure, developmental morphology and DNA sequence of the small subunit ribosomal DNA (SSU rDNA) and internal transcribed spacer region. The phylogenetic relationships of 'T. montrivulorum' sp. nov. and other crayfish parasites in the genus 'Thelohania', based on the SSU rDNA sequence, are investigated. Fine ultrastructure, patterns of sporogony and SSU rDNA sequence similarities indicate 'T. montirivulorum' sp. nov. is congeneric with 'T. parastaci', a parasite of lowland populations of 'C. destructor', and with 'T. contejeani', a parasite of European freshwater crayfish. SSU rDNA data suggests 'Thelohania' species found in crustacean hosts are more closely related to the 'Vairimorpha/Nosema' clade of species from insect and crustacean hosts than to the fire ant parasites, 'T. solenopsae and Thelohania' sp.