The use of host specificity, pathogenicity, and molecular markers to differentiate between Gyrodactylus salaris Malmberg, 1957 and G. thymalli Zitnan, 1960 (Monogenea: Gyrodactylidae).

The validity of Gyrodactylus thymalli has been questioned, based on its morphological and genetic resemblance to G. salaris. This taxonomic problem has practical implications regarding correct diagnosis of G. salaris, which has proved to be highly pathogenic to stocks of wild Norwegian Atlantic salmon. The host specificity and pathogenicity of G. salaris and G. thymalli were experimentally tested on salmon and grayling. Both parasite species were able to infect, live and reproduce on both fish species. G. salaris was highly pathogenic for the experimental stock of salmon, while grayling mounted an effective response against this parasite. Both fish species responded to an infection with G. thymalli. The results did not support the hypothesis of conspecificity between G. thymalli and G. salaris. The ribosomal RNA gene intergenic spacer of both species was sequenced. Variation in sequence was lower than expected for different species. Variation in the sequences of tandemly repeated elements was found and may prove useful in distinguishing G. salaris and G. thymalli.

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.

In vitro cultivation and temperature-dependent growth of two strains of Spironucleus barkhanus (Diplomonadida: Hexamitidae) from Atlantic salmon Salmo salar and grayling Thymallus thymallus.

Spironucleus barkhanus from muscle abscesses of farmed Atlantic salmon Salmo salar L., and from the gall bladder of grayling Thymallus thymallus (L.) was cultivated axenically in a medium routinely used for cultivation of the human pathogen Giardia duodenalis. Trophozoites multiplied by binary division, but multinucleated cells (< 10 nuclei) were frequently observed. Complete cell divisions were never observed in these cells. Both strains grew at all temperatures tested (5, 10, 15 and 20 degrees C). However, continuous growth of the salmon strain at 20 degrees C was not possible. Growth potential, calculated as minimum generation time, showed relatively small differences both between strains and among temperatures, but the results indicate that the grayling strain had both a slightly higher optimum temperature and a higher upper temperature limit than the salmon strain.

Systemic spironucleosis in sea-farmed Atlantic salmon Salmo salar, caused by Spironucleus barkhanus transmitted from feral Arctic char Salvelinus alpinus?

A hexamitid flagellate was found in the gall bladder and intestine of Arctic char Salvelinus alpinus in northern Norway. Scanning and transmission electron microscopy showed this flagellate to be identical to Spironucleus barkhanus from grayling Thymallus thymallus and farmed Atlantic salmon Salmo salar. It is hypothesized that systemic spironucleosis in sea-caged Atlantic salmon was due to transmission of flagellates from feral char to the salmon.