Tor A. Bakke

The biology of gyrodactylid monogeneans: the "Russian-doll killers".

This article reviews the history of gyrodactylid research focussing on the unique anatomy, behaviour, ecology and evolution of the viviparous forms while identifying gaps in our knowledge and directions for future research. We provide the first summary of research on the oviparous gyrodactylids from South American catfish, and highlight the plesiomorphic characters shared by gyrodactylids and other primitive monogeneans. Of these, the most important are the crawling, unciliated larva and the spike sensilla of the cephalic lobes. These characters allow gyrodactylids to transfer between hosts at any stage of the life cycle, without a specific transmission stage. We emphasise the importance of progenesis in shaping the evolution of the viviparous genera and discuss the relative extent of progenesis in the different genera. The validity of the familial classification is discussed and we conclude that the most significant division within the family is between the oviparous and the viviparous genera. The older divisions into Isancistrinae and Polyclithrinae should be allowed to lapse. We discuss approaches to the taxonomy of gyrodactylids, and we emphasise the importance of adequate morphological and molecular data in new descriptions. Host specificity patterns in gyrodactylids are discussed extensively and we note the importance of host shifts, revealed by molecular data, in the evolution of gyrodactylids. To date, the most closely related gyrodactylids have not been found on closely related hosts, demonstrating the importance of host shifts in their evolution. The most closely related species pair is that of G. salaris and G. thymalli, and we provide an account of the patterns of evolution taking place in different mitochondrial clades of this species complex. The host specificity of these clades is reviewed, demonstrating that, although each clade has its preferred host, there is a range of specificity to different salmonids, providing opportunities for complex patterns of survival and interbreeding in Scandinavia. At the same time, we identify trends in systematics and phylogeny relevant to the G. salaris epidemics on Atlantic salmon in Norway, which can be applied more generally to parasite epidemiology and evolution. Although much of gyrodactylid research in the last 30 years has been directed towards salmonid parasites, there is great potential in using other experimental systems, such as the gyrodactylids of poeciliids and sticklebacks. We also highlight the role of glacial lakes and modified river systems during the ice ages in gyrodactylid speciation, and suggest that salmon infecting clades of G. salaris first arose from G. thymalli in such lakes, but failed to spread fully across Scandinavia before further dispersal was ended by rising sea levels. This dispersal has been continued by human activity, leading to the appearance of G. salaris as a pathogen in Norway. We review the history and current status of the epidemic, and current strategies for elimination of the parasite from Norway. Finally, we consider opportunities for further spread of the parasite within and beyond Europe.

Host specificity dynamics: observations on gyrodactylid monogeneans

The directly transmitted viviparous gyrodactylids have high species richness but low morphological and biological diversity, and many species are recorded from only a single host. They therefore constitute a guild of species ideal for studies of the evolutionary significance of host specificity. The group has the widest host range of any monogenean family, being found on 19 orders of bony fish. However, individual species range from narrowly specific (71% of 402 described species recorded from a single host) to extremely catholic (Gyrodactylus alviga recorded from 16 hosts). Gyrodactylid-host interactions extend from 60 mya (G. lotae, G. lucii) down to 150 years (G. derjavini on Oncorhynchus mykiss). Co-evolution with the host is comparatively rare within the gyrodactylids, but host switching or ecological transfer is common, and has been facilitated by the mixing of fish strains that followed glaciation. In this review, we consider the factors responsible for gyrodactylid specificity patterns, using examples from our work on salmonid gyrodactylids including G. salaris, responsible for major epidemics on wild Atlantic salmon (Salmo salar) in Norway since 1975, and G. thymalli from grayling and G. derjavini from trout.G. salaris has a wide host range with highest population growth rates on Norwegian salmon strains. However, growth rates are variable on both host strains and species, because of the multitude of micro- and macro-environmental factors influencing parasite mortality and fecundity. A better predictor of performance is the proportion of fishes of a strain which are innately resistant to the parasite, a measure which is negatively correlated with the time to peak infection in a host strain. Population growth rate is also negatively correlated with age of infection; the initial rate, therefore, predicts best the suitability of a fish as host for G. salaris. The host response to gyrodactylids appears to be the same mechanism in all salmonids with innate resistance as one end of a spectrum, but influenced by stress and probably under polygenic control. Hybrid experiments show that performance of G. salaris on a host is heritable, and usually intermediate between that of the parents. This host response mechanism, coupled with the initial parasite population growth on a fish, determines the host specificity, i.e. whether the fish will be susceptible, a responder or innately resistant. The use of population growth rate parameters allows comparison of different hosts as a resource for a gyrodactylid. In the case of G. salaris, East Atlantic and Baltic strains of Atlantic salmon are core hosts, but other salmonids can physiologically sustain infections for considerable periods, and may be important in parasite dispersal and transmission. A further group of non-salmonid fishes are unable to sustain G. salaris reproduction, but can act as transport hosts.Population growth parameters are very labile to stressors and environmental factors, particularly temperature and salinity, and also other aspects of host ecology and water quality. These factors may also influence the spectrum of hosts that can be infected under particular conditions, and probably favoured ecological transfer of gyrodactylids between host species in periglacial conditions. G. salaris may still be undergoing post-glacial range expansion (aided by anthropogenic spread) as shown by the increase in the species range over the last 25 years. The origin of G. salaris, G. teuchis and G. thymalli is discussed in relation to glacial refugiums during the last ice age.

Mitochondrial DNA variation of Gyrodactylus spp. (Monogenea, Gyrodactylidae) populations infecting Atlantic salmon, grayling, and rainbow trout in Norway and Sweden

Approximately 800 bp of the mitochondrial cytochrome oxidase I (COI) gene were sequenced from 76 Gyrodactylus specimens of 32 salmonid host populations, i.e. from Salmo salar, Thymallus thymallus, and Oncorhynchus mykiss in Norway, Sweden and Latvia. The COI sequences indicated a substantial intraspecific differentiation of Gyrodactylus salaris and Gyrodactylus thymalli. In total, 12 haplotypes were identified which group into five well supported clades, three clades with parasites from Atlantic salmon and two clades with parasites from grayling. The basal nodes linking the five clades together are only weakly supported. Thus, there is no support for the monophyly of all G. salaris haplotypes and the monophyly of all G. thymalli haplotypes. The lack of monophyly of the mitochondrial haplotypes of G. salaris and G. thymalli may indicate that G. salaris and G. thymalli represent (i). two polytypic species or (ii). one polytypic species, or (iii). refer to a complex of more than two sibling species. The mtDNA data indicate multiple introductions of G. salaris and G. thymalli into Norway. A minimum of three independent introductions of G. salaris and two independent introductions of G. thymalli are supported. This is congruent with earlier hypotheses on the introduction of G. salaris and G. thymalli into Norway.

Killing of Gyrodactylus salaris (Platyhelminthes, Monogenea) mediated by host complement

Gyrodactylus salaris, an important pathogen of Atlantic salmon Salmo salar, has been shown to be highly sensitive to factors in host serum and mucus, being killed rapidly (50% within 1 h) by serum at a dilution of 1:200. The time needed for killing was inversely proportional to serum concentration. Similar effects were noted using host mucus, which contained approximately 1/20th of the anti-Gyrodactylus activity of serum. Serum activity was abolished completely by heating at 45 degrees C for 30 min, and by addition of EDTA, but not by EGTA + 1 mM magnesium ions. Activity was not dependent on whether the serum was from infected or naive fishes, nor was it species specific. Attempts to pre-coat parasites in salmon anti-Gyrodactylus antibodies also failed to enhance the activity of fresh serum. These observations suggest that killing is due to the complement system of the host, acting via the alternate pathway. G. salaris appears to be exceptionally sensitive to complement, being killed at concentrations which could be experienced in vivo. The role of complement in the protection of fishes against gyrodactylid infection therefore deserves further investigation.

Temperature-dependent reproduction and survival of Gyrodactylus salaris Malmberg, 1957 (Platyhelminthes: Monogenea) on Atlantic salmon ( Salmo salar L.)

The relationship of survival and reproduction of Gyrodactylus salaris Malmberg on the Atlantic salmon (Salmo salar) to water temperature (2.5-19.0 degrees C), was studied on the basis of temporal sequence of births and age at death of individual parasites on isolated salmon, and of infrapopulation growth on isolated and grouped salmon. Mean life-span of the parasite was negatively correlated with water temperature: 33.7 days at 2.5 degrees C and 4.5 days at 19.0 degrees C. The average number of offspring per parasite peaked between 6.5 and 13.0 degrees C, and was approximately 2.4 at these two temperatures. Both the period between the successive births of the offspring (max 4) and the estimated generation time were negatively correlated with temperature. The innate capacity for increase (rm) was positively correlated with temperature: from 0.02 (/parasite/day) at 2.5 degrees C to 0.22 (/parasite/day) at 19.0 degrees C. Growth of the infrapopulations was positively correlated with water temperature and was higher on isolated fish than on grouped fish, though less than the potential parasite population growth estimated from rm. In the infrapopulations the mean intensity of parasites continued to increase throughout all the experiments on both isolated fish and on grouped fish.

Comparative susceptibility of native Scottish and Norwegian stocks of Atlantic salmon, Salmo salar L., to Gyrodactylus salaris Malmberg: Laboratory experiments

Abstract Salmon parr of each of three hatchery-reared stocks from the Rivers Shin and Conon in northeast Scotland, and from the River Lierelva in southeast Norway, were infected by exposing them concurrently to wild Norwegian salmon parr naturally infected with the ectoparasitic monogenean Gyrodactylus salaris . After exposure the fish were transferred to six holding tanks, two tanks to each stock. Each tank held 50 fish in a pooled group and another 12 fish individually isolated in small cages. At weekly intervals fish in each tank were anaesthetised and parasite numbers counted. No natural resistance was observed in the three stocks of salmon tested and all three were susceptible to G. salaris reproduction. However, 3–5 weeks post-infection in heterogeneity in the course of infection was observed in all three stocks, with highly susceptible, moderately susceptible, and apparently responding individuals in each tank. The results demonstrated an increasing ability to tolerate G. salaris with increasing size on the part of the salmon parr. In the pooled fish, parasite populations increased throughout the experiment, whereas in the isolated fish they tended to decrease 30–36 days post-infection. The experiments ended 50 days post-infection, mainly because of fish mortality. Possible explanations for the observed pattern of parasite population growth are discussed, together with the significance of these results for salmon populations in both countries.

Aqueous aluminium eliminates Gyrodactylus salaris (Platyhelminthes, Monogenea) infections in Atlantic salmon.

This study focuses on the effect of acidic water and aqueous aluminium on the monogenean ectoparasite Gyrodactylus salaris, infecting Atlantic salmon (Salmo salar) parr. G. salaris-infected salmon were exposed to various combinations of acidity and aluminium concentrations. The most pronounced effect was the elimination of parasites after 4 days when 202 micrograms Al/l was added to the water. The effect of aluminium was concentration dependent, but was relatively independent of pH (5.2, 5.6 and 5.9). At the lowest pH of 5.0 the effect of aluminium was enhanced. Acidic aluminium-poor water had no or minor effects on the G. salaris infections except at pH 5.0 where all parasites were eliminated within 9 days. The G. salaris populations increased exponentially in untreated control water. The results show for the first time that aqueous aluminium can, to a limited extent, have a positive effect on fish health. This study emphasizes that basic knowledge about abiotic environmental factors is of importance in order to understand the population dynamics, range extension and dispersal of ectoparasites such as G. salaris. Finally, our results suggest that aluminium treatment could form an effective disinfection method against ectoparasites in hatcheries and laboratories, as well as complementing the controversial rotenone treatments used against natural populations of G. salaris.

Increased susceptibility of salmonids to the monogenean Gyrodactylus salaris following administration of hydrocortisone acetate.

Gyrodactylus salaris infects numerous salmonid species, ranging from the fully susceptible (Norwegian strains of Salmo salar), through species which, though initially susceptible, eventually eliminate their infections (Salvelinus alpinus and S. fontinalis) to entirely resistant (Salmo trutta) species. Here we describe experiments in which Salvelinus alpinus, S. fontinalis and Salmo trutta, implanted with hydrocortisone acetate to simulate stress-induced immunosuppression, were challenged with G. salaris. With previously uninfected Salvelinus fontinalis, G. salaris infections on fish treated with hydrocortisone acetate grew larger, and for longer, than on sham-treated controls. A similar result was obtained with S. trutta. Patterns of infection on Arctic charr, Salvelinus alpinus, were more complex, because individual fish varied from susceptible to highly resistant. Fish were therefore initially infected with G. salaris, and the most highly resistant group of individuals identified and disinfected. After 6 months recovery from this primary infection, hydrocortisone acetate was administered to half the fish, and all were challenged with G. salaris. Parasite populations on the hydrocortisone-treated individuals were consistently larger than those on the sham-treated controls, exceeding 30 parasites per fish after 5 weeks, in comparison with less than 10 parasites per fish on controls. These results indicate that hydrocortisone administration can lead to enhanced gyrodactylid populations on a range of salmonids. This suggests that the response to G. salaris is mediated by the immune system, and that the spectrum of responses observed in different species are, at least in part, due to the same mechanism. At a practical level, stress-induced immunosuppression during handling and transport of cultured salmonids may prove an important factor in the dissemination of G. salaris between watersheds.

The susceptibility of Atlantic salmon (Salmo salar L.) x brown trout (Salmo trutta L.) hybrids to Gyrodactylus salaris Malmberg and Gyrodactylus derjavini Mikailov.

Salmo salar and Salmo trutta co-exist in coastal river systems in Europe and produce hybrids with little loss of viability or growth. This report describes the susceptibility of pure full-sibs of S. salar and S. trutta and their reciprocal half-sib hybrids to their respective gyrodactylids, Gyrodactylus salaris and Gyrodactylus derjavini. The pure-bred salmon and trout, and half-sib hybrids, were produced using eggs and sperm from wild anadromous S. salar (River Alta stock, North Norway) and wild anadromous S. trutta (River Fossbekk stock, Southwest Norway). Infections were initiated by exposing experimental fishes (0+) to S. salar naturally infected with G. salaris (River Lierelva strain) or S. trutta naturally infected with G. derjavini (River Sandvikselva strain). Fishes were then kept individually isolated under standardized conditions at 12 degrees C. Pure-bred S. salar were susceptible but frequently mounted a response to G. salaris without eliminating the infection, whereas pure-bred S. trutta were innately resistant to this species. Pure-bred S. trutta ranged from innately resistant to susceptible to G. derjavini but later most of the susceptible trout mounted a host response to G. derjavini. Pure-bred S. salar were also susceptible to this species, although parasite population growth rates were reduced and a host response frequently appeared eliminating G. derjavini. The abundance of both gyrodactylids was lower on the hybrids than on their respective pure-bred natural hosts, and a parental sire- and dam-influence on the resistance of hybrids was observed. When the sire was S. salar, the susceptibility of hybrids to G. salaris was similar to that of pure S. trutta; when the dam was S. salar both innately resistant, intermediately susceptible and responding individuals were present. In the case of G. derjavini, when the sire was S. trutta, infections on hybrids were similar to those on pure S. salar; when the dam was S. trutta, an increased level of susceptibility was observed. The present results provide evidence that: (1) Norwegian salmon stocks are variable in their susceptibility/resistance, with some fish able to control S. salaris infections; (2) trout stocks are innately resistant to G. salaris; (3) individual trout show a spectrum in susceptibility/resistance to G. derjavini, ranging from innate resistance through slightly susceptible to highly susceptible but with acquired resistance controlling infection; (4) although G. derjavini infections grow poorly on salmon, this host stock is susceptible to the parasite, but can limit infection by a host reaction; (5) susceptibility/resistance traits to gyrodactylids are genetically controlled and resistance can be transferred as a dominant trait through interspecific crosses between different salmonids; (6) interspecific hybrids between susceptible and resistant salmonids have a pattern of susceptibility to gyrodactylids intermediate to that of the parents; (7) resistance to gyrodactylids may be controlled by relatively few genes in salmonids; (8) epidemiologically, hybrids may act as a reservoir for gyrodactylids, may support a wider diversity of species than either parent and may disseminate gyrodactylids of both host species.

The effect of various metals on Gyrodactylus salaris (Platyhelminthes, Monogenea) infections in Atlantic salmon ( Salmo salar )

Atlantic salmon (Salmo salar) parr (age 0+), infected by the ectoparasite Gyrodactylus salaris, were exposed to aqueous aluminium (Al), copper (Cu), zinc (Zn), iron (Fe) and manganese (Mn), at 4 different concentrations. There was a negative correlation between G. salaris infections and metal concentrations in both Zn- and Al-exposed salmon. In the Zn-experiment, all 4 concentrations tested caused a decrease in the G. salaris infections, while in the Al-experiment the G. salaris infection did not decline at the lowest concentration. The number of G. salaris increased continuously during the experiments in all control groups, and in all groups exposed to Cu, Fe and Mn. At the highest concentration, however, copper seemed to impair the growth of G. salaris infection. The results show that aqueous Al and Zn are environmental factors of importance controlling the distribution and abundance of the pathogen G. salaris. Other pollutants might also have an influence on the occurrence of G. salaris. Finally, the results demonstrate that aqueous Al and Zn have a stronger effect on the parasite than on the salmonid host, suggesting that both metals may be used as a pesticide to control ectoparasites such as G. salaris.