To identify trematode diversity and life-cycles in the sub-Arctic Lake Takvatn, Norway, we characterised 120 trematode isolates from mollusc first intermediate hosts, metacercariae from second intermediate host fishes and invertebrates, and adults from fish and invertebrate definitive hosts, using molecular techniques. Phylogenies based on nuclear and/or mtDNA revealed high species richness (24 species or species-level genetic lineages) and uncovered trematode diversity (16 putative new species) from five families typical in lake ecosystems (Allocreadiidae, Diplostomidae, Plagiorchiidae, Schistosomatidae and Strigeidae). Sampling potential invertebrate hosts allowed matching of sequence data for different stages, thus achieving molecular elucidation of trematode life-cycles and exploration of host-parasite interactions. Phylogenetic analyses also helped identify three major mollusc intermediate hosts (Radix balthica, Pisidium casertanum and Sphaerium sp.) in the lake. Our findings increase the known trematode diversity at the sub-Arctic Lake Takvatn, showing that digenean diversity is high in this otherwise depauperate sub-Arctic freshwater ecosystem and indicating that sub-Arctic and Arctic ecosystems may be characterised by unique trematode assemblages.

Co-introduction and colonization of parasites with the introduction of new host species into aquatic habitats may depend on the host specificity and dispersal capabilities of the parasites. We compared the metazoan parasite community of an introduced three-spined stickleback (Gasterosteus aculeatus) population with that of the nearby source population in subarctic Norway. As expected from a small spatial scale (5 km), the parasite component communities in the two lakes were highly similar. All identifiable allogenic parasite taxa (Diphyllobothrium dendriticum, Diphyllobothrium ditremum, Diphyllobothrium spp., Schistocephalus solidus, Apatemon sp. and Diplostomum spp.) were also observed in both lakes while inter-lake differences were driven by autogenic parasite taxa (Eubothrium spp., Crepidostomum spp., Nematoda spp., Proteocephalus sp. and Gyrodactylus arcuatus). Contrary to expectation, the total number of parasite taxa was higher in the introduced stickleback population (12) compared to that found in the source population (9) with three parasite taxa (Eubothrium spp., Crepidostomum spp., Nematoda spp.) only occurring in the introduced population. These parasites were uncommon however and normally restricted to salmonids. Sticklebacks from both populations were heavily infected, particularly with eye-infecting metacercariae. Sequences from the DNA barcode region of cytochrome oxidase 1 indicated that these include Diplostomum lineage 6, a member of the Diplostomum baeri complex and a member of the Strigeinae. Despite high similarity between the two component communities, quantitative inter-lake differences were found at the infracommunity level. At this scale, parasite intensity was significantly higher in the source population for the two autogenic stickleback specialists: G. arcuatus and Proteocephalus sp., assumed to be the autogenic stickleback specialist Proteocephalus filicollis. Parasite infracommunities within each lake also resembled each other significantly more than infracommunities between lakes, primarily driven by the allogenic cestode D. ditremum, as well as G. arcuatus and Proteocephalus sp. Overall, quantitative dissimilarities between the two parasite communities were possibly explained by inter-lake differences in the density of sticklebacks and intermediate hosts.

Infections with the parasitic flagellate Ichthyobodo necator (Henneguy, 1883) cause severe skin and gill disease in rainbow trout Oncorhynchus mykiss (Walbaum, 1792) juveniles. The epidermal disturbances including hyperplasia and mucous cell exhaustion caused by parasitization are known, but no details on specific cellular and humoral reactions have been presented. By applying gene expression methods and immunohistochemical techniques, further details of immune processes in the affected skin can be presented. A population of I. necator was established in the laboratory and used to induce an experimental infection of juvenile rainbow trout. The course of infection was followed by sampling for parasite enumeration, immunohistochemistry (IHC) and quantitative PCR (qPCR) on days 0, 5, 9 and 14 post-infection. IHC showed a significant increase in the occurrence of IgM-positive cells in the skin of the infected fish, whereas IgT-positive cells were eliminated and the number of CD8-positive cells declined. qPCR studies supported the IHC findings showing a significant increase in IgM and a decrease in the CD8 gene expression. In addition, genes encoding innate immune genes such as lysozyme, SAA and cathelicidin 2 were up-regulated. Expression of cytokines (IL-1β, IL-4/13A, IL-6, IL-8, IL-10), the cell marker CD4 and the transcription factor GATA3 showed a significant increase after infection. Cytokine profiling including up-regulation of IL-4/13A and IL-10 genes and transcription factor GATA3 connected to the proliferation of IgM producing lymphocytes suggests a partial shift towards a Th2 response associated with the I. necator infection.

1. During the past two decades, understanding of the structure and function of glacier-fed stream ecosystems at temperate latitudes has increased substantially. In contrast, information on their tropical counterparts is very limited. We studied three neighbouring glacier-fed streams in the tropical Andes of Ecuador. Our main goals were (i) to determine overall longitudinal patterns in density, taxon richness and the composition of macroinvertebrate assemblages and driving factors in equatorial glacial streams and (ii) to examine variability among replicate streams in faunal metrics and assemblages, and stream-specific effects of supposed environmental key factors. 2. We measured four geographical and 17 environmental factors and collected five Surber samples (500 cm 2 ) of macroinvertebrates at each of nine sites, three sites along three streams. The streams were located 1-5 km apart. In each stream, the three sites were placed at comparable distances from the glacier and were grouped as 'upper' (50-200 m), 'middle' (1.5 km) and 'lower' sites (3.5-5.6 km). 3. In total, 2200 individuals (64% chironomids) were collected and 47 taxa (30 dipterans, 18 of these Chironomidae) identified. Density ranged from 176 to 372 ind. m )2 , and the number of taxa ranged from 2 to 6 at the upper sites and 868-3044 ind. m )2 and 21-27 taxa at the lower sites. Density, number of taxa, rarefied richness and axis-1 coordinates from a MDS ordination increased logarithmically with distance from the glacier. These faunal metrics were equally related to altitude and glacier per cent of catchment and correlated with maximum conductivity, mean temperature, mean daily maximum temperature and a channel stability index. As expected, the mean difference in distance decay in similarity was higher at the upper (47% km )1 ) than at the lower reaches (20% km )1 ) of the streams. 4. The number of taxa varied among sites within the upper and middle groups, but not among the lower sites. In contrast, but in accordance with our expectation, assemblage composition did not differ among upper sites but did so at middle and lower sites, following a supposed decrease in environmental harshness along the streams. Relationships between faunal metrics and the four environmental variables mean temperature, the Correspondence: Dean Jacobsen, Freshwater stability index, chlorophyll a and coarse particulate organic matter also varied among the three streams. Generalised linear model analyses revealed that temperature interacted with stream on macroinvertebrate density, while chlorophyll a had a significant effect on the number of taxa in interaction with stream and stability. 5. The basic predictions of the Milner et al. (2001a), model regarding longitudinal faunal patterns and temperature and stability as main driving factors were met by our three replicate equatorial glacial streams. Qualitative departures from the model were mainly because of zoogeographical differences. We demonstrated that variability in assemblages between comparable sites in closely situate...

The occurrence of trophically transmitted intestinal parasites in Arctic charr was analyzed from data material collected over two decades from Lake Takvatn, northern Norway. The main objectives were to investigate i) between-year variation in parasite infracommunity composition, ii) between-host variation in infracommunity composition and iii) possible interspecific interaction between parasites. The trematode Crepidostomum spp. and the cestode Eubothrium salvelini were most prevalent and had the highest mean intensities whereas the cestodes Proteocephalus sp. and Cyathocephalus truncatus were uncommon taxa. No clear indication of interspecific parasite interactions was evident. By far the most frequent infracommunity composition was E. salvelini co-existing with only Crepidostomum spp. Despite some indications of individual dietary specialization in Arctic charr, this strong species co-occurrence suggests that the majority of fish had been preying on the very different intermediate hosts of both parasites. Overall, even though some sporadic between-year variation was present, no long-term or cyclical variation in infracommunity composition and mean intensity was revealed, suggesting that the host-parasite system is in a steady state.Minor between-year variation was likely the result of changes in habitat and dietary utilization by Arctic charr, possibly linked to documented changes in the fish community structure of the lake.