Genus-level Identification
Genera Spironucleus Hexamita Octomitus
Flagellar pockets (cytostomal canals)a + + -
Central axis formed by recurrent axonemes, microtubular bands, endoplasmic reticulum - - +
Two terminal spikes - - +
Shape of nuclei S-shaped Spherical Reniform
Location of kinetosomes relative to nuclei Sub-apical External structure Between
Position of recurrent flagella relative to nuclei Medial Lateral Medial
Supra-nuclear microtubular band + + Reduced
Infra-nuclear microtubular band + + Reduced
Species-level Identification
Species S. barkhanus /S. salmonicida S. salmonis S. torosa S. vortens
Flagellar pocket ornamentation Crescent-shaped (barkhan) No ornamentation Ring-shaped swellings (torus) Counter-crossing ridges
Microtubular pattern around flagellar pocket 3 radiate bands - 3 concentric bands 3 staggard bands
Distinctive organelles - Electron-dense bodies / membrane-bound sac of free ribosomes - Hydrogenosomesb
aWhen present, the recurrent flagella are ensheathed
bMay also be present in other Spironucleus spp. but currently only documented in S. vortens
Table 2: Important ultrastructural aspects for genus and species level identification of diplomonad genera and Spironucleus spp. respectively. Adapted from Poynton and Sterud [1] with permission from John Wiley & Sons Ltd.
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