DAO 60:49-64 (2004)  -  doi:10.3354/dao060049

Ultrastructure of Spironucleus salmonis n. comb. (formerly Octomitus salmonis sensu Moore 1922, Davis 1926, and Hexamita salmonis sensu Ferguson 1979), with a guide to Spironucleus species

Sarah L. Poynton1,2,*, M. Reza Saghari Fard2,3, John Jenkins4, Hugh W. Ferguson5

1Department of Comparative Medicine, Johns Hopkins University School of Medicine, Broadway Research Building, 733 North Broadway, Room 807, Baltimore, Maryland 21205, USA
2Department of Inland Fisheries, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 310, 12587 Berlin, Germany
3MSc in International Agricultural Sciences, College of Agriculture and Horticulture, Humboldt University Berlin, Invalidenstrasse 42, 10115 Berlin, Germany
4Department of Veterinary Pathology, Armed Forces Institute of Pathology, Washington, DC 20306, USA
5Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, UK

ABSTRACT: Diplomonad flagellates can be associated with significant morbidity and mortality in fishes, particularly in farmed salmonids. Diagnosis using only light microscopy is limited, and species cannot be confirmed. We therefore undertook a comprehensive transmission electron microscopy study of 20 trophozoites from the intestine of farmed juvenile rainbow trout Oncorhynchus mykiss from Northern Ireland (reported as Hexamita salmonis by Ferguson in 1979). Re-assignment to the genus Spironucleus was determined based on the anteriorly tapering and intertwined elongate nuclei, anterior-medial kinetosomes, and the presence of a flagellar pocket. At the species level we observed a tri-radiate pattern of microtubules in the opening of the asymetrical striated lamina, comprising 3 to 5 microtubules following the edge of the flagellar pocket, 3 radiating away from the opening of the striated lamina, and 4 to 5 curving over the striated lamina. We observed electron-dense plaques adjacent to the kinetosomes, electron-dense bodies, numerous free ribosomes, aggregations of glycogen, bowl-shaped membranous structures, rough endoplasmic reticulum, and a novel distinctive pyriform sac of densely packed free ribosomes at the posterior of the cell (8-shaped in transverse section, and delineated by furrowed endoplasmic reticulum). We now propose to rename H. salmonis sensu Ferguson (1979) Spironucleus salmonis. Our review of the morphology of Octomitus salmonis illustrated by Moore (1922a,b) and Davis (1926), has shown that this organism also belongs to the genus Spironucleus. We synonymise H. salmonis sensu Ferguson (1979) with O. salmonis sensu Moore (1922) and Davis (1926), and rename them S. salmonis. An expanded diagnostic guide, including new cytoplasmic elements, is presented for the 4 species of Spironucleus from fishes (S. barkhanus, S. salmonis, S. torosa and S. vortens). We recommend that type descriptions of diplomonads be based on organisms taken directly from the host rather than from in vitro culture in order to reliably include the cytoplasmic organelles.

KEY WORDS: Diplomonad · Flagellate · Hexamita salmonis · Oncorhynchus mykiss · Rainbow trout · Spironucleus salmonis · Ultrastructure

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