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Electron microscopical identification of the flagellate Spironucleus torosa (Hexamitidae) from burbot Lota lota (Gadidae) with comments upon its probable introduction to this freshwater host.

Hexamitid flagellates from the rectum of the freshwater gadid burbot, from the river Glomma in southeastern Norway, were studied by scanning and transmission electron microscopy. The surface morphology of the flagellates was consistent with that of Spironucleus torosa, previously only found in the cod, haddock, and saithe, all marine gadids. Posteriolateral depressions with central protrusions were present in the flagellates, a feature reported from S. torosa only. Further, the microtubular cytoskeleton of the present species had the same general arrangement as in S. torosa, clearly different from what is described for other Spironucleus spp. It is therefore concluded that the present flagellate from burbot is this species. Any recent exchange of parasites between the marine hosts and burbot is believed to be only theoretically possible. The burbot became established in Norwegian rivers and lakes after the last ice age, some 7,000-8,000 yr ago, by following water courses across Sweden from the Baltic Sea. In the Baltic Sea there were, and still are, sympatric populations of burbot and cod, and the burbot is believed to have been infected before migrating westward.