Guidelines for species descriptions of diplomonad flagellates from fish

Diplomonad flagellates are common commensals of the digestive tract, and less common pathogenic parasites occurring in the digestive tract and systemically in numerous fish species. Many aspects of infections are poorly understood, including host-flagellate specificity, geographic ranges, and pathogenicity of different species. Much confusion is attributable to inadequate determination of genus and species. Although older literature reports Hexamita, Octomitus and Spironucleus from fish, recent studies confirm only Spironucleus. To address this problem, we describe ultrastructural features of trophozoites permitting reliable identification to genus and species, and techniques for their elucidation. Pioneering work by Brugerolle and colleagues established that genera can be distinguished by transmission electron microscopy (TEM). We now demonstrate that at the species level, surface ornamentations (especially at the posterior end of the body), and the pattern of bands of microtubules accompanying the flagellar pocket (in transverse section through the middle of the body), are of particular taxonomic value. Both scanning and TEM are essential for robust species descriptions and type material must be deposited in a recognized reference collection. Taxonomic studies are enhanced by in vitro culture, with tolerance and optimum for different conditions providing important supplementary information. Molecular characterization of fish diplomonads is in its infancy.