SEMs of diplomonad fish parasite Spironucleus vortens. cr - compound lateral ridge.lpr + rpr - left and right peripheral ridge, respectively. Note their rope-like form. pp - posterior papillum. Note flagellar pockets in (4). r - recurrent flagellum. 5 - an atypical specimen with transposed posterior structures. 6 - laterla view of posterior end. A fairly complicated flagellate! (Sterud & Poynton 2002 JEM)
To clarify the complicated morphology:
Drawings of the 'double-celled' Spironucleus vortens. The two recurrent flagella pass inside the cell and emerge at the posterior end. (Poynton et al. 1995 JEM)
Spironucleus may strike you as being particularly symmetrical. In fact, it very well is a 'double cell', containing two nuclei (slightly wrapping around each other helically, hence Spironucleus), and two sets of flagella. The path of the recurrent flagella makes sense when considering that the single cells of the group have three flagella pointing one way, and the fourth pointing another. These double cells are case of the two cells being 'stuck together' at the side of the recurrent flagellum. Here's a TEM to show the elongate nuclei and the flagella passing between them:
Top: longitudinal section at the anterior end of the flagellate. n - nuclei; k - kinetosomes. Bottom: cross-section of the anterior end. Note the recurrent flagella (r; circled in red) passing between the nuclei. (Poynton et al. 1995 JEM)
Here's another species of Spironucleus, S.berkhanaus, in arctic char blood:
Parasitic Spironucleus barkhanus in the blood of arctic char, as well as in isolation. Note that it's a different species from the one above, which may explain the lack of lateral ridges.(Sterud et al. 2003 Dis Aquatic Organisms)
Note how the two species seem a bit different from each other. This shows the morphological diversity in the group. In fact, Spironucleus seems to be a bit polyphyletic or paraphyletic at best (Jørgensen & Sterud 2007 Protist; Kolisko et al. 2008 BMC Evol Biol).
Phylogeny of Spironucleus. This poor genus seem to be ruthlessly strewn all over Diplomonadida. Note position of S.vortens and S.berkhanus. (Jørgensen & Sterud 2007 Protist)
Diplomonads have an interesting tale involving cell cycle defects and duplications of the nucleus and flagella, but I'll leave you in suspense for a while. That's a bigger topic, and I should probably introduce our cute friend Giardia first (cute friend in SEM, and horrible foe in the intestine...). Giardia is a independent case of cell 'doubling', and is organised quite differently. Further discussion of diplomonads should happen...eventually. Feel free to nag me about it if you're really interested!
JORGENSEN, A., & STERUD, E. (2007). Phylogeny of Spironucleus (Eopharyngia: Diplomonadida: Hexamitinae) Protist, 158 (2), 247-254 DOI: 10.1016/j.protis.2006.12.003
Kolisko, M., Cepicka, I., Hampl, V., Leigh, J., Roger, A., Kulda, J., Simpson, A., & Flegr, J. (2008). Molecular phylogeny of diplomonads and enteromonads based on SSU rRNA, alpha-tubulin and HSP90 genes: Implications for the evolutionary history of the double karyomastigont of diplomonads BMC Evolutionary Biology, 8 (1) DOI: 10.1186/1471-2148-8-205
POYNTON, S., FRASER, W., FRANCIS-FLOYD, R., RUTLEDGE, P., REED, P., & NERAD, T. (1995). Spironucleus vortens N. Sp. from the Freshwater Angelfish Pterophyllum scalare: Morphology and Culture The Journal of Eukaryotic Microbiology, 42 (6), 731-742 DOI: 10.1111/j.1550-7408.1995.tb01625.x
Sterud E, Poppe T, & Bornø G (2003). Intracellular infection with Spironucleus barkhanus (Diplomonadida: Hexamitidae) in farmed Arctic char Salvelinus alpinus. Diseases of aquatic organisms, 56 (2), 155-61 PMID: 14598991
STERUD, E., & POYNTON, S. (2002). Spironucleus vortens (Diplomonadida) in the Ide, Leuciscus idus (L.) (Cyprinidae): a Warm Water Hexamitid Flagellate Found in Northern Europe The Journal of Eukaryotic Microbiology, 49 (2), 137-145 DOI: 10.1111/j.1550-7408.2002.tb00357.x