Variations of opisthaptoral hard parts of Gyrodactylus salaris Malmberg, 1957 (Monogenea: Gyrodactylidae) on rainbow trout Oncorhynchus mykiss (Walbaum, 1792) in a fish farm,with comments on the spreading of the parasite in south-eastern Norway

Studies were carried out on Gyrodactylus salaris Malmberg, 1957 on rainbow trout Oncorhynchus mykiss (Walbaum, 1792) from a fish farm in Lake Tyrifjorden, south-eastern Norway. The anchors were larger, and the shape of the anchors and the ventral bar differed slightly, as compared with the same parts of G. salaris on parr of Atlantic salmon Salmo salar L. in northern and north-western Norwegian rivers. At different water temperatures (0.8°C, 10.0°C, 18.0°C), the opisthaptoral hard parts of G. salaris on rainbow trout showed considerable variation in size, but varied only slightly in shape. It was found that the total length of the anchors of G. salaris on rainbow trout may considerably exceed the previously reported maximum of 80 m for the species. The spread of G. salaris to south-eastern Norway is described and discussed.     

Seasonal variations of the opisthaptoral hard parts of Gyrodactylus derjavini Mikailov, 1975 (Monogenea: Gyrodactylidae) on brown trout Salmo trutta L. parr and Atlantic salmon S. salar L. parr in the River Sandvikselva,Norway

Seasonal variations in the size and shape of the marginal hooks, anchors and ventral bar of the opisthaptor of Gyrodactylus derjavini Mikailov, 1975 were studied. The G. derjavini specimens were collected from brown trout Salmo trutta L. parr and Atlantic salmon S. salar L. parr in the River Sandvikselva, southeastern Norway. Samples were taken at roughly monthly intervals during a 13-month period. The marginal hooks, anchors and ventral bars showed considerable seasonal variation in size, but varied very little in shape. The size increased when the water temperature decreased and vice versa.     

Gyrodactylus gvozdevi n. sp. (Gyrodactylidae: Monogenea) from Noemacheilus dorsalis (Kessler) in Kazakhstan

Gyrodactylus gvozdevi n. sp. (Gyrodactylidae: Monogenea) is described from the skin of the freshwater fish Noemacheilus dorsalis (Kessler) (Cobitidae: Cypriniformes) from Kazakhstan. This species is most closely related to G. pseudonemachili Ergens & Bykhovsky, 1967 in the shape and size of the anchors and both the ventral and dorsal bars, but can be distinguished from it by the shape and size of the hookproper of the marginal hooks.     

A special technique for studying haptoral sclerites of monogeneans

The marginal hooks and anchors have been digested from the haptors of the monogeneans Gyrodactylus salaris and Discocotyle sagittata. These structures then can be measured and drawn more accurately than those surrounded by haptoral tissue. The technique also facilitates the study of isolated hooks with SEM. The results of studies using this digestion technique are presented.     

Gyrodactylus sommervillae n. sp. (Monogenea) from Abramis brama (L.) and Rutilus rutilus (L.) (Cyprinidae) in Oxfordshire,UK

Gyrodactylus sommervillae n. sp. (Monogenea) is described from the skin, fins and gills of Abramis brama (L.) and Rutilus rutilus (L.) from Blenheim Palace Lake, Oxfordshire, UK. G. sommervillae n. sp. closely resembles G. aphyae Malmberg, 1957, G. kearni Ergens, 1990 and G. lamberti Ergens, 1990 in the approximate shape of the marginal hook sickle and the anchors, but can be separated on the precise shape of the marginal hooks and the ventral bar.     

On the ontogeny of the haptor and the evolution of the Monogenea

Data on the ontogeny of the posterior haptor of monogeneans were obtained from more than 150 publications and summarised. These data were plotted into diagrams showing evolutionary capacity levels based on the theory of a progressive evolution of marginal hooks, anchors and other attachment components of the posterior haptor in the Monogenea (Malmberg, 1986). 5 + 5 unhinged marginal hooks are assumed to be the most primitive monogenean haptoral condition. Thus the diagrams were founded on a 5 + 5 unhinged marginal hook evolutionary capacity level, and the evolutionary capacity levels of anchors and other haptoral attachement components were arranged according to haptoral ontogenetical sequences. In the final plotting diagram data on hosts, type of spermatozoa, oncomiracidial ciliation, sensilla pattern and protonephridial systems were also included. In this way a number of correlations were revealed. Thus, for example, the number of 5 + 5 marginal hooks correlates with the most primitive monogenean type of spermatozoon and with few sensillae, many ciliated cells and a simple protonephridial system in the oncomiracidium. On the basis of the reviewed data it is concluded that the ancient monogeneans with 5 + 5 unhinged marginal hooks were divided into two main lines, one retaining unhinged marginal hooks and the other evolving hinged marginal hooks. Both main lines have recent representatives at different marginal hook evolutionary capacity levels, i.e. monogeneans retaining a haptor with only marginal hooks. For the main line with hinged marginal hooks the name Articulon-choinea n. subclass is proposed. Members with 8 + 8 hinged marginal hooks only are here called Proanchorea n. superord. Monogeneans with unhinged marginal hooks only are here called Ananchorea n. superord. and three new families are erected for its recent members: Anonchohapteridae n. fam., Acolpentronidae n. fam. and Anacanthoridae n. fam. (with 7 + 7, 8 + 8 and 9 + 9 unhinged marginal hooks, respectively). Except for the families of Articulonchoinea (e.g. Acanthocotylidae, Gyrodactylidae, Tetraonchoididae) Bychowsky's (1957) division of the Monogenea into the Oligonchoinea and Polyonchoinea fits the proposed scheme, i.e. monogeneans with unhinged marginal hooks form one old group, the Oligonchoinea, which have 5 + 5 unhinged marginal hooks, and the other group form the Polyonchoinea, which (with the exception of the Hexabothriidae) has a greater number (7 + 7, 8 + 8 or 9 + 9) of unhinged marginal hooks. It is proposed that both these names, Oligonchoinea (sensu mihi) and Polyonchoinea (sensu mihi), will be retained on one side and Articulonchoinea placed on the other side, which reflects the early monogenean evolution. Except for the members of Ananchorea [Polyonchoinea], all members of the Oligonchoinea and Polyonchoinea have anchors, which imply that they are further evolved, i.e. have passed the 5 + 5 marginal hook evolutionary capacity level (Malmberg, 1986). There are two main types of anchors in the Monogenea: haptoral anchors, with anlages appearing in the haptor, and peduncular anchors, with anlages in the peduncle. There are two types of haptoral anchors: peripheral haptoral anchors, ontogenetically the oldest, and central haptoral anchors. Peduncular anchors, in turn, are ontogenetically younger than peripheral haptoral anchors. There may be two pairs of peduncular anchors: medial peduncular anchors, ontogentically the oldest, and lateral peduncular anchors. Only peduncular (not haptoral) anchors have anchor bars. Monogeneans with haptoral anchors are here called Mediohaptanchorea n. superord. and Laterohaptanchorea n. superord. or haptanchoreans. All oligonchoineans and the oldest polyonchoineans are haptanchoreans. Certain members of Calceostomatidae [Polyonchoinea] are the only monogeneans with both (peripheral) haptoral and peduncular anchors (one pair). These monogeneans are here called Mixanchorea n. superord. Polyonchoineans with peduncular anchors and unhinged marginal hooks are here called the Pedunculanchorea n. superord. The most primitive pedunculanchoreans have only one pair of peduncular anchors with an anchor bar, while the most advanced have both medial and lateral peduncular anchors; each pair having an anchor bar. Certain families of the Articulonchoinea, the Anchorea n. superord., also have peduncular anchors (parallel evolution): only one family, the Sundanonchidae n. fam., has both medial and lateral peduncular anchors, each anchor pair with an anchor bar. Evolutionary lines from different monogenean evolutionary capacity levels are discussed and a new system of classification for the Monogenea is proposed.In agreeing to publish this article, I recognise that its contents are controversial and contrary to generally accepted views on monogenean systematics and evolution. I have anticipated a reaction to the article by inviting senior workers in the field to comment upon it: their views will be reported in a future issue of this journal. EditorIn agreeing to publish this article, I recognise that its contents are controversial and contrary to generally accepted views on monogenean systematics and evolution. I have anticipated a reaction to the article by inviting senior workers in the field to comment upon it: their views will be reported in a future issue of this journal. Editor     

A description of a new species Gyrodactylus moldovicus sp. n. (Monogenea: Gyrodactylidae) from the European mudminnow Umbra krameri Walbaum, 1792 from the lower Dnester basin

Gyrodactylus moldovicus sp. n. found on gills, body and in nasal cavities of the European mudminnow (Umbra krameri) differs from G. slovacicus Ergens, 1963 also living on the this host by bigger size of the body, anchors and marginal hooks; from G. cylindriformes Mueller et Van Cleave, 1932 living on the American mudminnow Umbra limi--by bigger size of the body; from G. limi Wood et Mizelle, 1957 also from U. limi--by the form of ventral and dorsal bars and form of marginal hooks. It differs from other freshwater gyrodactylids by special type of marginal hooks which have a hook-like end of the blade. Gyrodactylus moldovicus, G. slovacicus and G. limi have marginal hooks of quite different morphological types. By the morphology of anchors, ventral and sometimes dorsal bars and also morphology of cirrus, G. moldovicus is most related to three species from Cyprininae: G. stankovichi Ergens, 1970, G. longoacuminatus Zitnan, 1964 f. typica and G. shulmani Ling, 1962.     

The description of Gyrodactylus mulli sp. n. (Monogenea: Gyrodactylidae) from the Black Sea blunt-snouted mullet Mullus barbatus ponticus

A new species Gyrodactylus mulli sp. n. is described from the thorax fins of blunt-snouted mullet Mullus barbatus ponticus by the collections of B. E. Bychowsky made in 1947 near Karadag on the Black Sea. Gyrodactylus mulli sp. n. differs from G. alviga Dmitrieva et Gerasev, 2000 (described from blunt-snouted mullet too) in having another morphotype of the marginal hooks. The new species differs from G. proterorhini Ergens, 1967, which has the identical type of the marginal hooks, in having longer membrane of the ventral connective bar, longer point of the anchors, shorter marginal hooks, and lesser size of the cirrus.     

Variations of opisthaptoral hard parts of Gyrodactylus salaris Malmberg, 1957 (Monogenea: Gyrodactylidae) on parr of Atlantic Salmon Salmo salar L. in laboratory experiments

On parr of Atlantic salmon Salmo salar L. in laboratory experiments at different water temperatures, opisthaptoral hard parts of Gyrodactylus salaris Malmberg, 1957 differed in size. Their shape, on the other hand, varied only slightly. The opisthaptoral hard parts were largest at the lowest water temperatures and decreased in size with increasing water temperatures (1.5°C to 20.0°C). At both 18.0°C and 20.0°C the opisthaptoral hard parts of G. salaris were smaller than has been found under natural conditions. In the experiments, the mean size of the opisthaptor in the G. salaris micropopulation at each water temperature gradually increased or decreased compared to the mean size at the start of the experiments. At the end of the experiments the mean size of the opisthaptoral hard parts in the G. salaris micropopulations showed a significant regression to the different water temperatures.     

Gyrodactylus pictae n. sp. (Monogenea: Gyrodactylidae) from the Trinidadian swamp guppy Poecilia picta Regan, with a discussion on species of Gyrodactylus von Nordmann, 1832 and their poeciliid hosts

Gyrodactylus pictae n. sp. is recorded from Poecilia picta in heterospecific shoals with the guppy P.reticulata in Northern Trinidad. G. pictae is morphologically similar to G. turnbulli Harris, 1986, but the hamuli and marginal hooks are slightly smaller and more gracile. The toe and the point of the marginal hook have a distinctly different shape, providing the best morphological characters for distinguishing the two species. The rDNA ITS1 and ITS2 sequences differ from those of G. turnbulli (the closest relative) by >5, suggesting that these two taxa are not sibling species. The origin of the two species on poeciliids of the subgenus Micropoecilia is discussed, and it is suggested that this may represent a case of host–parasite co-evolution.     

<Emphasis Type="Italic">Gyrodactylus thlapi</Emphasis>n. sp. (Monogenea) from <Emphasis Type="Italic">Pseudocrenilabrus philander philander</Emphasis> (Weber) (Cichlidae) in the Okavango Delta,Botswana

A new species of Gyrodactylus von Nordmann, 1832 and the first monogenean reported from Botswana is described from the cichlid Pseudocrenilabrus philander philander,bringing the number of gyrodactylids described from the African Continent to 18. Gyrodactylus thlapi n. sp. can be readily discriminated from the other described species almost exclusively by the shape of its hamuli, which have short roots and large, elongate ventral bar attachment points with the dorsal bar attachment point, demarcating the junction between the hamulus shaft and the root, positioned at the anterior terminus of the attachment plane. The marginal hooks of G. thlapi n. sp., which most closely resemble those of G. groschafti Ergens, 1973, are compared and discussed.     

Gyrodactylus longidactylus n. sp., a monogenean from Pomatoschistus lozanoi (de Buen) (Gobiidae) from the North Sea

Gyrodactylus longidactylus n. sp., a gyrodactylid monogenean parasitising the gills of Pomatoschistus lozanoi (de Buen, 1923) in the North Sea is described. It is a species with rather small anchors which are only connected to a ventral bar which lacks a ventral bar membrane. The marginal hooks have long handles, which are always longer than the total length of the anchor. The pharynx has long pharyngeal processes. Measurements, drawings of the opisthaptoral hard parts, penis and pharynx, and SEM micrographs of this monogenean are presented. G. longidactylus n. sp. is the first monogenean species described from P. lozanoi.     

Gyrodactylus neili n. sp. (Monogenea: Gyrodactylidae), a parasite of chain pickerel Esox niger Lesueur (Esocidae) from freshwaters of New Brunswick, Canada

Gyrodactylus neili n. sp. (Monogenea: Gyrodactylidae) is described from the fins and body surface of Esox niger Lesueur (chain pickerel) (Esocidae) from the St. Croix River drainage, New Brunswick, Canada. G.␣neili n. sp. resembles most closely G. fryi Cone & Dechtiar, 1984, a parasite of E. masquinongy in North America, in having relatively large thin hamuli, well-developed marginal hook sickles with a relatively long, wide blade and short handle, a ventral bar with small antero-lateral processes and tongue-shaped membrane, and a cirrus with many small spines in two rows. The new species is easily separated from G. fryi by the length of the hamuli (70-76 m versus 92 m, respectively), by the distal width of the sickle (7–9 versus 14-16 m, respectively) and by subtle differences in the shape of the toe and heel of the marginal hook sickle. Sequence data (922 bp) of rDNA (internal transcribed spacers 1 and 2 and 5.8S) of G. neili n. sp. returned no identical matches in GenBank. The 5.8 sequence alone, however, was identical to morphologically similar gyrodactylids of the subgenus Gyrodactylus from cyprinid fishes in Eurasia. The discovery of G. neili n. sp. and features of its genetic makeup support the idea that this lineage parasitised ancestral cyprinids and that it radiated, possibly through predator/prey interactions, to an ancestor of contemporary Esox. It is concluded that DNA comparison of monogeneans on Holarctic freshwater hosts, such as E.␣lucius, may shed light on the nature of speciation of these parasites.     

<Emphasis Type="Italic">Gyrodactylus salaris</Emphasis> (Monogenea,Gyrodactylidae) infections on resident Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) in southern Norway

This study surveys the distribution of Gyrodactylus salaris on resident Arctic charr, Salvelinus alpinus, in lakes connected to three south-Norwegian watercourses: Numedalsvassdraget, Skiensvassdraget and Hallingdalsvassdraget. Gyrodactylus salaris infected charr was only recorded in Numedalsvassdraget. The parasites had the same mitochondrial haplotype as those previously reported on charr in Lake Pålsbufjorden, which is part of Numedalsvassdraget. Since the G. salaris-charr association is persistent in Pålsbufjorden and has a wide distribution above the stretches of the watercourse inhabited by anadromous salmonids, this is considered a stable, although perhaps relatively young, host-parasite system. More detailed analyses of these interactions revealed seasonal variations in the parasite population dynamics between late summer and late autumn, with heavier infections occurring in males and older fish in October. This is explained by the combined action of seasonal differences in temperature and physiology and ecology of host cohorts. It is assumed that the occurrence of G. salaris on charr in Pålsbufjorden resulted from a host switch to charr from rainbow trout, Onchorynchus mykiss. Host switches may cause significant expansions of the geographical range of pathogenic variants of G. salaris. Therefore, observations of frequently occurring G. salaris on charr have implications for the diagnosis, management and control of salmonid gyrodactylosis.     

Philureter trigoniopsis, a new genus and species (Dactylogyridae, Ancyrocephalinae) from the ureters and urinary bladder of Galaxias maculatus (Osmeriformes: Galaxiidae) in Patagonia (Argentina)

The monotypic Philureter n. gen. (Ancyrocephalinae; Dactylogyridae) is proposed to accommodate Philureter trigoniopsis n. sp. with the following features: presence of a cuplike ventral haptor armed with 14 hooks and 2 anchor/bar complexes; dorsal pair of anchors poorly defined and variable in shape, 1 frequently absent; tandem, intercecal gonads, testis bilaterally lobulated. Philureter trigoniopsis n. sp. is described from the ureters and urinary bladder of Galaxias maculatus (Jenyns, 1842) (Osmeriformes) in Patagonian Andean lakes, Argentina.     

Craniomandibular Variation in the Taxonomically Problematic Gerbil Genus <Emphasis Type="Italic">Gerbillus</Emphasis> (Gerbillinae,Rodentia): Assessing the Influence of Climate,Geography, Phylogeny,and Size

The taxonomy of Gerbillus, the most speciose gerbil genus, is highly debated. Of particular contention is the relationship of Dipodillus to Gerbillus; some consider it to be a closely related genus, while others synonymize it with Gerbillus—either with or without recognizing it as a subgenus. The main objective of this study is to test the validity of common taxonomic groupings within the Gerbillus-Dipodillus species complex, which was achieved by using geometric morphometrics to examine cranial and mandibular variation in 34 out of the 52 Gerbillus-Dipodillus species. Craniomandibular size and shape were highly correlated, indicating strong allometric patterns in shape variation. The common taxonomic groups were significantly different in craniomandibular size and shape, yet they did overlap considerably in morphospace. A notable exception was the extreme divergence of Monodia (G. mauritaniae) from all other species in the occlusal view of the mandible. Morphospace overlap is likely a consequence of both phylogenetic history and environmental adaptation. Only the ventral cranium was associated with climate, particularly in areas related to resource acquisition. Geographic distance was not significantly associated with craniomandibular morphometric distance, and the groups overlapped greatly in their geographic range. Cranial and mandibular regions differed in discrimination power—the ventral cranium had among the highest, while the dorsal cranium and the occlusal mandible had the lowest. Craniomandibular regions varied in association with climate, phylogeny, and size—previous studies suggest this difference may be a consequence of different genetic controls for shape variation.     

Phenotypic Buffering in a Monogenean: Canalization and Developmental Stability in Shape and Size of the Haptoral Anchors of Ligophorus cephali (Monogenea: Dactylogyridae)

Phenotypic variation results from the balance between sources of variation and counteracting regulatory mechanisms. Canalization and developmental stability are two such mechanisms, acting at two different levels of regulation. The issue of whether or not they act concurrently as a common developmental buffering capacity has been subject to debate. We used geometric morphometrics to quantify the mechanisms that guarantee phenotypic constancy in the haptoral anchors of Ligophorus cephali. Canalization and developmental stability were appraised by estimating inter- and intra-individual variation, respectively, in size and shape of dorsal and ventral anchors. The latter variation was estimated as fluctuating asymmetry (FA) between anchor pairs. The general-buffering-capacity hypothesis was tested by two different methods based on correlations and Principal Components Analyses of the different components of size and shape variation. Evidence for FA in the dorsal and ventral anchors in both shape and size was found. Our analyses supported the hypothesis of a general developmental buffering capacity. The evidence was more compelling for shape than for size and, particularly, for the ventral anchors than for the dorsal ones. These results are in line with previous studies of dactylogyrids suggesting that ventral anchors secure a firmer, more permanent attachment, whereas dorsal anchors are more mobile. Because fixation to the host is crucial for survival in ectoparasites, we suggest that homeostatic development of the ventral anchors has been promoted to ensure the morphological constancy required for efficient attachment. Geometric morphometrics can be readily applied to other host-monogenean models, affording not only to disentangle the effects of canalization and developmental stability, as shown herein, but to further partition the environmental and genetic components of the former.     

Morphology and molecular taxonomy of Gyrodactylus jennyae n. sp. (Monogenea) from tadpoles of captive Rana catesbeiana Shaw (Anura), with a review of the species of Gyrodactylus Nordmann, 1832 parasitising amphibians

Gyrodactylus jennyae n. sp. is described from the body surface and mouthparts of tadpoles of the bullfrog Rana catesbeiana Shaw imported presumably from Missouri, USA, into a federal government facility in Moncton, New Brunswick, Canada. Its morphology resembles most closely that of G. chologastris Mizelle, Whittaker & McDougal, 1969 described from two amblyopsids (blind cave fishes) in Kentucky and North Carolina. Both species have long slender hamuli, a ventral bar with a relatively long membrane and small anterolateral processes, a cirrus with two rows of small spines and marginal hooks with a well-developed sickle heel and short handle. The two species differ morphologically; G. jennyae has a marginal hook sickle with a more pronounced heel than that found in G. chologastris. A BLAST search using a 945 base pair sequence that included the nuclear ribosomal DNA internal transcribed spacers 1 and 2 and the 5.8S rRNA gene from G. jennyae n. sp. showed that the overall similarity with other Gyrodactylus sequences on GenBank was relatively low. The ITS1 region was similar to that of G. misgurni Ling, 1962; however, no ITS2 and 5.8S rRNA sequences are available for that species. A separate search using 5.8S sequences revealed that G. markakulensis Gvosdev, 1950 and G. laevis Malmberg, 1957 were the closest to G. jennyae (1 and 2 bp differences, respectively). These species are parasites of cyprinids (or their predators) and are similar to G. jennyae and G. chologastris in having a double row of small hooks on the cirrus and overall similar morphologies of the haptoral hard parts. There are now five species of Gyrodactylus described exclusively from amphibians and this appears to have involved at least three separate host-switches from fishes.     

Policlithrum ponticum sp. n. (Monogenea: Gyrodactylidae: Polyclithrinae) from Mugil cephalus from the Black Sea and problems of suprageneric systematics of the gydrodactylids

Polyclithrum ponticum sp. n. is described and P. mugilini Rogers, 1967 is redescribed. Both monogenean species are parasites of Mugil cephalus in the Black Sea. The new species differs from P. mugilini, P. alberti and P. boegeri by the lesser size of anchors, while it is distinguished from P. corallense by the larger size of these structures. P. ponticum sp. n. differs from all formerly described species by the greater length of dorsal connective bar. In both species from the Black Sea, "ear-like" structures situated near the external roots of anchors are described for the first time. It is suggested, that these structures take part in longitudinal, two-lobe folding of the haptor. The process of opening the haptor is probably performed by the additional bars of the haptor (bars 2 and 3 after: Rogers, 1967), joined to each other and with the anchors. The fifth pair of additional bars (Ernst e. a., 2000) derives from the "beard" of ventral connective bar and is united with its basal part. The sixth pair of additional bars (Ernst e. a., 2000) is considered as a typical "ribs" of the haptor, and therefore the "ribs" are represented by three pairs. Differences between marginal hooks of P. ponticum sp. n. and P. mugilini are insignificant, that probably depends on the presence of "ribs" of the haptor. Based on the subdivision of marginal hooks into two groups, the presence of additional supporting structure in the haptor, and the presence of the seminal receptacle, it is suggested that the subfamily Polyclithrinae Rogers, 1967 should include the genera Polyclithrum Rogers, 1967, Swingleus Rogers, 1969, Macrogyrodactylus Mamlberg, 1959, and probably Fundulotrema Hargis, 1955. Based on such characters as the lack of the anchors, the presence of suckers in the haptor, and ovipositing of eggs, it seems to be expedient to use the following taxa in systematics of gyrodactylids: Isancistrinae Fuhrmann, 1928 (genera Isancistrum, Anacanthocotyle); Gyrdicotylinae Vercammen-Grandjean, 1960 (Gyrdicotyle) and Ooegyrodactylinae Harris, 1983 (genera Phanerothecium, Ooegyrodactylus, Nothogyrodactylus, Hyperopletes).     

Seasonal dynamics and persistence of Gyrodactylus salaris in two riverine anadromous Arctic charr populations

The seasonal occurrence of the monogenean ectoparasite Gyrodactylus salaris Malmberg infecting Arctic charr, Salvelinus alpinus (L.) in the two rivers Skibotnelva and Signaldalselva in northern Norway was studied in the period from autumn 2003 to autumn 2005. Skibotnelva has been infected with the parasite since 1979, and treated with rotenone twice. Most likely resident Arctic charr avoided the rotenone treatment in small tributary streams, and thus was the source of the repeated re-infection of this river. G. salaris was first recorded in Signaldalselva in the year 2000 and it is still untreated. Unlike Atlantic salmon, which is highly susceptible to G. salaris, Arctic charr can display a wide range of host-responses to G. salaris infections. Arctic charr were sampled by electro fishing with a total sample of 681 Arctic charr. The results from this study demonstrate an evident seasonal dynamic in G. salaris infection in charr in both rivers. Parasite intensities fluctuated with the rise and fall in temperature through the year, with an autumn high and spring low. There was a significantly lower prevalence and mean intensity of G. salaris in Skibotnelva than in Signaldalselva. There were also a lower prevalence and intensity of G. salaris in the older than in the youngest charr. The different history of infection and treatment in the two rivers might be the underlying cause of these observed dissimilarities. The current study indicates that Arctic charr is a good natural host for G. salaris.