Molecular and morphological variation within swim bladder nematodes, Cystidicola spp.

We sequenced four rDNA regions (ITS-1, ITS-2, 5.8S, and D3 region of 28S) from the parasitic nematode Cystidicola spp. from seven species of fish host and 11 locations in Canada and Finland to elucidate suspected unresolved genetic variation within the genus. A holarctic species, Cystidicola farionis from the swim bladder of Salmonidae and Osmeridae, and a nearctic species, Cystidicola stigmatura from trout and char (Salvelinus spp.), differ in life history, host and geographic range, reproductive strategy, and adult and egg morphology. These nematodes were identical at three rDNA regions (ITS-1, 5.8S, and D3); however, two ITS-2 variants were found that differed at four nucleotide positions: variant I (366 bp) was found in British Columbia populations of C. farionis and in C. stigmatura and variant II (369 bp) was found in Ontario and Finnish populations of C. farionis. These results demonstrate that two species with distinct morphology and biology can have identical rDNA sequences, while two morphologically identical isolates have different ITS-2 sequences. Thus, rDNA spacer regions may not be useful for distinguishing biologically valid species or subspecies in some nematode groups. Although phenotypic variation suggested a third undescribed species of Cystidicola from lake whitefish (Coregonus clupeaformis), rDNA analysis did not provide meaningful evidence of its uniqueness.

Parasite communities in two sympatric morphs of Arctic charr, Salvelinus alpinus (L.), in northern Norway

In this study from Fjellfrøsvatn, an oligotrophic lake in northern Norway, the parasite communities in two sympatric Arctic charr populations were compared. The dwarf morph, which inhabits the profundal zone, exhibited the lowest parasite diversity, seven species, and 72% of these charr harboured only one or two parasite species. In contrast, 10 parasite species were encountered in the larger normal charr, and between 5 and 8 species were present in 73% of these fish, which also utilised a broader food and habitat niche. Proteocephalus sp. was by far the most abundant species in the dwarf charr, probably because this morph fed intensively upon the benthic copepod Acanthocyclops gigas. On the other hand, parasites that are transmitted with littoral benthic prey (i.e., Phyllodistomum umblae, Cyathocephalus truncatus, Cystidicola farionis, and Crepidostomum spp.) were almost absent in the dwarf charr, though they were common in the normal morph. Also, Diphyllobothrium spp. were more prevalent in the normal charr, and this was attributed to their feeding upon limnetic copepods in the pelagic zone. The only recorded parasite with a direct life cycle, the copepod Salmincola edwardsii, had relatively similar abundances in the two morphs. The considerable differences in parasite community structure and abundance between the two charr populations were closely related to differences in the width and composition of the habitat and food niches between the morphs.

Infections of Diphyllobothrium dendriticum, D. ditremum (Cestoda), and Cystidicola farionis (Nematoda) in a north Norwegian population of Arctic charr (Salvelinus alpinus) during winter

Infections of the parasites Diphyllobothrium dendriticum, Diphyllobothrium ditremum, and Cystidicola farionis in Arctic charr (Salvelinus alpinus) from Takvatn, northern Norway, were followed throughout a 9-month overwintering period. The relative density of all three parasite species increased with the age of the host in response to ontogenetic shifts in habitat and food of Arctic charr in the lake. Our results indicate that some recruitment of the three parasite species, particularly D. ditremum, takes place during the winter. However, winter parasite recruitment was lower than in the ice-free season. In spite of high parasite infection in some individuals, no strong indications of parasite-induced host winter mortality were found for any of the three species. Low overwintering mortality of heavily parasitized charr may be connected with a stock-reduction program in the lake that has improved the conditions of the fish, and the residual stock is probably less affected by parasites.