An alloherpesvirus infection of European perch Perca fluviatilis in Finland.

The order Herpesvirales includes viruses that infect aquatic and terrestrial vertebrates and several aquatic invertebrates (i.e. mollusks), and share the commonality of possessing a double-stranded DNA core surrounded by an icosahedral capsid. Herpesviruses of the family Alloherpesviridae that infect fish and amphibians, including channel catfish virus and koi herpesvirus, negatively impact aquaculture. Here, we describe a novel herpesvirus infection of wild European perch from lakes in Finland. Infected fish exhibited white nodules on the skin and fins, typically in the spring when prevalence reached nearly 40% in one of the sampled lakes. Transmission electron microscopic examination of affected tissues revealed abundant nuclear and cytoplasmic virus particles displaying herpesvirus morphology. Degenerate PCR targeting a conserved region of the DNA polymerase gene of large DNA viruses amplified a 520 bp product in 5 of 5 affected perch skin samples tested. Phylogenetic analysis of concatenated partial DNA polymerase and terminase (exon 2) gene sequences produced a well-supported tree grouping the European perch herpesvirus with alloherpesviruses infecting acipenserid, esocid, ictalurid, and salmonid fishes. The phenetic analysis of the European perch herpesvirus partial DNA polymerase and terminase nucleotide gene sequences ranged from 34.6 to 63.9% and 39.6 to 59.6% to other alloherpesviruses, respectively. These data support the European perch herpesvirus as a new alloherpesvirus, and we propose the formal species designation of Percid herpesvirus 2 (PeHV2) to be considered for approval by the International Committee on Taxonomy of Viruses.

Infection, specificity and host manipulation of Australapatemon sp. (Trematoda, Strigeidae) in two sympatric species of leeches (Hirudinea).

Factors that drive parasite specificity and differences in infection dynamics among alternative host species are important for ecology and evolution of host-parasite interactions, but still often poorly known in natural systems. Here, we investigated spatiotemporal dynamics of infection, host susceptibility and parasite-induced changes in host phenotype in a rarely explored host-parasite system, the Australapatemon sp. trematode infecting two sympatric species of freshwater leeches, Erpobdella octoculata and Helobdella stagnalis. We show significant variation in infection abundance between the host species in both space and time. Using experimental infections, we also show that most of this variation likely comes from interspecific differences in exposure rather than susceptibility. Moreover, we demonstrate that the hiding behaviour of E. octoculata, but not that of H. stagnalis, was impaired by the infection irrespective of the parasite abundance. This may increase susceptibility of E. octoculata to predation by the final avian host. We conclude that differences in patterns of infection and in behavioural alterations among alternative sympatric host species may arise in narrow spatial scales, which emphasises the importance of local infection and transmission dynamics for parasite life cycles.

Sexual segregation of Echinorhynchus borealis von Linstow, 1901 (Acanthocephala) in the gut of burbot (Lota lota Linnaeus).

Helminths often occupy defined niches in the gut of their definitive hosts. In the dioecious acanthocephalans, adult males and females usually have similar gut distributions, but sexual site segregation has been reported in at least some species. We studied the intestinal distribution of the acanthocephalan Echinorhynchus borealis von Linstow, 1901 (syn. of E. cinctulus Porta, 1905) in its definitive host, burbot (Lota lota Linnaeus). Over 80% of female worms were found in the pyloric caeca, whereas the majority of males were in the anterior two-thirds of the intestine. This difference was relatively consistent between individual fish hosts. Worms from different parts of the gut did not differ in length, so site segregation was not obviously related to worm growth or age. We found proportionally more males in the caeca when a larger fraction of the females were found there, suggesting mating opportunities influence gut distribution. However, this result relied on a single parasite infrapopulation and is thus tentative. We discuss how mating strategies and/or sexual differences in life history might explain why males and females occupy different parts of the burbot gut.

The natural history of Echinorhynchus bothniensis Zdzitowiecki and Valtonen, 1987 (Acanthocephala) in a high Arctic lake.

The acanthocephalan Echinorhynchus bothniensis Zdzitowiecki and Valtonen, 1987 differs from most other species in the genus Echinorhynchus Zoega in Müller, 1776 by infecting mysids (order Mysida) instead of amphipods (order Amphipoda) as intermediate hosts. Here we report on the occurrence of E. bothniensis in mysids (Mysis segerstralei Audzijonyte et Väinölä) and in its fish definitive hosts in a high Arctic lake. Out of 15 907 sampled mysids, 4.8% were infected with a mean intensity of 1.05 worms (range 1-5), although there was notable variation between samples taken in different years and sites. Larger mysids appear more likely to be infected. Of five fish species sampled, charr,Salvelinus alpinus (Linnaeus), and a benthic-feeding whitefish morph, Coregonus lavaretus (Linnaeus), were the most heavily infected (mean abundances of 80 and 15, respectively). The adult parasite population in fish exhibited a female-biased sex ratio (1.78 : 1). Although E. bothniensis is rather unique in infecting mysids, many aspects of its natural history mirror that of other acanthocephalan species.

The occurrence of Echinorhynchus salmonis Müller, 1784 in benthic amphipods in the Baltic Sea.

The acanthocephalan Echinorhynchus salmonis Müller, 1784 is a common parasite of salmonid fish, but it has rarely been reported from an intermediate host. Samples of benthic amphipods, Monoporeia affinis (Lindström), were taken from multiple, deep sites (usually below 70 m) in the Gulf of Bothnia over the course of more than a decade and examined for acanthocephalans. Overall, only 0.44% of 23 296 amphipods were infected, all with just a single worm. This prevalence is consistent with several previous reports of acanthocephalans in deep-water, benthic amphipods, but it appears low compared to that often reported for acanthocephalan species infecting littoral amphipods. Parasite occurrence did not exhibit a clear regional pattern (i.e. northern vs southern sites) nor did it have any relationship with site depth. At sites sampled over multiple years, parasite abundance was consistently low (mostly < 0.01), though two spikes in abundance (over 0.06) were also observed, indicating that infection can be substantially higher at particular times or in particular places. The median density of E. salmonis in samples containing the parasite was estimated as 8.4 cystacanths per m(2).

Behavioural adaptations of argulid parasites (Crustacea: Branchiura) to major challenges in their life cycle.

Fish lice (Argulus spp.) are obligate ectoparasites, which contrary to most aquatic parasites, retain the ability to swim freely throughout the whole of their life. In fish farms, they can quickly increase in numbers and without effective control cause argulosis, which results in the reduced growth and survival of their fish hosts. The morphology of Argulus spp, including their sensory organs, is suitable for both parasitism and free-swimming. By spending a considerable amount of time away from their host, these parasites risk being excessively dispersed, which could endanger mating success. Here we present a review of recent studies on the behaviour of Argulus spp, especially the aggregative behaviour that mitigates the dilution of the parasite population. Aggregation of parasites, which is especially important during the period of reproduction, occurs on different scales and involves both the aggregation of the host and the aggregation of the parasites on the host. The main behavioural adaptations of Argulus spp, including searches for hosts and mates, host manipulation and host choice, are all focused on the fish. As these ectoparasites repeatedly change hosts and inflict skin damage, they can act as vectors for fish pathogens. The development of environmentally friendly measures for the control and prevention of argulosis needs to take into account the behaviour of the parasites.

Fauna europaea: helminths (animal parasitic).

Fauna Europaea provides a public web-service with an index of scientific names (including important synonyms) of all living European land and freshwater animals, their geographical distribution at country level (up to the Urals, excluding the Caucasus region), and some additional information. The Fauna Europaea project covers about 230,000 taxonomic names, including 130,000 accepted species and 14,000 accepted subspecies, which is much more than the originally projected number of 100,000 species. This represents a huge effort by more than 400 contributing specialists throughout Europe and is a unique (standard) reference suitable for many users in science, government, industry, nature conservation and education. Helminths parasitic in animals represent a large assemblage of worms, representing three phyla, with more than 200 families and almost 4,000 species of parasites from all major vertebrate and many invertebrate groups. A general introduction is given for each of the major groups of parasitic worms, i.e. the Acanthocephala, Monogenea, Trematoda (Aspidogastrea and Digenea), Cestoda and Nematoda. Basic information for each group includes its size, host-range, distribution, morphological features, life-cycle, classification, identification and recent key-works. Tabulations include a complete list of families dealt with, the number of species in each and the name of the specialist responsible for data acquisition, a list of additional specialists who helped with particular groups, and a list of higher taxa dealt with down to the family level. A compilation of useful references is appended.

Increased ventilation by fish leads to a higher risk of parasitism.

BACKGROUND: Fish are common intermediate hosts of trematode cercariae and their gills can potentially serve as important sites of penetration by these larval stages. We experimentally tested the hypothesis that volume of ventilation flow across the gills contributes to acquisition of these parasites by fish. We manipulated the intensity of ventilation by using different oxygen concentrations. METHODS: Juvenile Oncorhynchus mykiss were individually exposed for 10 minutes to a standard dose of Diplostomum pseudospathaceum cercariae at three levels of oxygen concentration, 30, 60 and 90%. Ventilation amplitude (measured as a distance between left and right operculum), operculum beat rate, and the number of cercariae established in the eyes of fish were recorded. RESULTS: Fish reacted to low oxygen concentration with wider expansion of opercula (but not with increasing beat rate), leading to an increase in ventilation volume. As expected, the intensity of infection increased with decreasing oxygen saturation-probably due to a higher exposure to cercariae caused by increased ventilation under low oxygen concentrations. The number of cercariae acquired by an individual fish was positively correlated with ventilation amplitude and with ventilation volume, but not with operculum beat rate. However, even though the infection rate increased under these circumstances, the proportion of larval trematodes successfully establishing in fish eyes decreased with increasing ventilation volume, suggesting that the high flow velocity, although increasing host exposure to cercarial parasites, may interfere with the ability of these parasites to penetrate their hosts. There was no difference in the behaviour of trematode cercariae exposed to low and high oxygen concentrations. CONCLUSION: A reduction in oxygen saturation resulted in an increase in ventilation volume across the gills and in doing so an increase in the exposure of fish to cercariae. A significant correlation between ventilation volume and parasitism represents the first experimental evidence that this physiological mechanism generates variation in transmission of parasites to fish hosts. Other factors that modify ventilation flow, e.g. physiological or social stressors, are expected to produce similar effects on the transmission success of the parasites penetrating fish hosts using the gills.

Starvation can diversify the population structure and virulence strategies of an environmentally transmitting fish pathogen.

BACKGROUND: Generalist bacterial pathogens, with the ability for environmental survival and growth, often face variable conditions during their outside-host period. Abiotic factors (such as nutrient deprivation) act as selection pressures for bacterial characteristics, but their effect on virulence is not entirely understood. "Sit and wait" hypothesis expects that long outside-host survival selects for increased virulence, but maintaining virulence in the absence of hosts is generally expected to be costly if active investments are needed. We analysed how long term starvation influences bacterial population structure and virulence of an environmentally transmitting fish pathogen Flavobacterium columnare. RESULTS: F. columnare populations in distilled water and in lake water were monitored for 5 months. During the experiment, the population structure of F. columnare diversified by rough and soft colony morphotypes appearing among the ancestral rhizoid ones. After 5 months starvation in lake water, the virulence of the starved and ancestral bacterial isolates was tested. The starved rhizoid isolates had significantly higher virulence than the ancestral rhizoid, whereas the virulence of the rough isolates was low. CONCLUSIONS: We suggest that F. columnare population diversification is an adaptation to tolerate unpredictable environment, but may also have other biological significance. Maintaining and increasing virulence ensures efficient invasion into the host especially under circumstances when the host density is low or the outside-host period is long. Changing from rhizoid into a rough morphotype has trade-offs in making bacteria less virulent and unable to exploit the host, but may ensure bacterial survival under unpredictable conditions. Our study gives an example how abiotic selection can diversify virulence of environmentally transmitting bacterial pathogen.

Vulnerability and diet breadth predict larval and adult parasite diversity in fish of the Bothnian Bay.

Recent studies of aquatic food webs show that parasite diversity is concentrated in nodes that likely favour transmission. Various aspects of parasite diversity have been observed to be correlated with the trophic level, size, diet breadth, and vulnerability to predation of hosts. However, no study has attempted to distinguish among all four correlates, which may have differential importance for trophically transmitted parasites occurring as larvae or adults. We searched for factors that best predict the diversity of larval and adult endoparasites in 4105 fish in 25 species studied over a three-year period in the Bothnian Bay, Finland. Local predator-prey relationships were determined from stomach contents, parasites, and published data in 8,229 fish in 31 species and in seals and piscivorous birds. Fish that consumed more species of prey had more diverse trophically transmitted adult parasites. Larval parasite diversity increased with the diversity of both prey and predators, but increases in predator diversity had a greater effect. Prey diversity was more strongly associated with the diversity of adult parasites than with that of larvae. The proportion of parasite species present as larvae in a host species was correlated with the diversity of its predators. There was a notable lack of association with the diversity of any parasite guild and fish length, trophic level, or trophic category. Thus, diversity is associated with different nodal properties in larval and adult parasites, and association strengths also differ, strongly reflecting the life cycles of parasites and the food chains they follow to complete transmission.

Environment may be the source of Flavobacterium columnare outbreaks at fish farms.

Flavobacterium columnare, causing columnaris disease, was isolated for the first time from free water and biofilms in the environment outside fish farms. Fourteen isolates were found from Central Finland from a river by a water intake of a salmonid farm and 400 m upstream of the farm. One isolate was from a lake not under the influence of any fish farming. The bacterium could not be isolated from five other lakes in Central Finland or from three lakes in Eastern Finland, none of them in use for fish farming. Among the environmental isolates there was both genetic variability and difference in virulence, but the isolates were less virulent than the isolates originating from a disease outbreak at a fish farm. The isolates were able to survive for months outside the fish host and also to change their colony morphology, a phenomenon probably used as a survival strategy for F. columnare. This indicates that waters upstream of fish farms are a potential source of columnaris outbreaks at the farms during the summer. The results support the hypothesis that fish farms and farming practices may select for the virulent strains of F. columnare occurring in environmental waters to cause the infections at the farms.

Trichodinid (Ciliophora: Trichodinidae) Infections in Perch (Perca fluviatilis) experimentally exposed to pulp and paper mill effluents.

Wild-caught European perch (Perca fluviatilis) were exposed in the laboratory to untreated bleached pulp and paper mill effluent in two separate experiments. The first experiment was conducted at 7-8°C using effluent concentrations of 5 and 10%, and the second experiment was conducted at ambient river temperature of 4-20°C using an effluent concentration of 1%. Trichodinid ciliates were identified and enumerated at the end of the exposure using a mucus subsampling technique from gill and skin as well as a formalin immersion technique, which provided total counts on each fish. Four different trichodinid species were identified on the fish. Prevalence of infection, mean number, and mean density of Trichodina spp. decreased on fish exposed to effluents compared with controls. Prevalence of infection, mean number, and mean density of Trichodinella epizootica decreased on fish exposed to 5% and 10% effluents but increased on fish exposed to 1% effluents compared with controls. These results demonstrate that trichodinid ciliates vary in their susceptibility to at least certain types of contaminants and cautions against using trichodinids as environmental indicators without delineating species.

Virulent and nonvirulent Flavobacterium columnare colony morphologies: characterization of chondroitin AC lyase activity and adhesion to polystyrene.

AIMS: Colony morphology variants of fish pathogenic Flavobacterium columnare were studied to clarify the role of colony morphology change in the virulence of the bacterium. Typical rhizoid colony (Rz) variants are virulent and moderately adherent, nonrhizoid rough (R) colony variants are nonvirulent and highly adherent, and soft colony (S) variants are nonvirulent and poorly adherent. METHODS AND RESULTS: Chondroitin AC lyase activity, adhesion to polystyrene at different temperatures and after modification of bacterial surface, and lipopolysaccharide (LPS) profiles of the variants were studied. The chondroitinase activity was significantly higher in the virulent, rhizoid variants than in the rough variants of the same strain. Temperature significantly increased the adhesion of rhizoid variants up to 20°C. Modification of bacterial surface suggested that adhesion molecules contain both carbohydrates and proteins. LPS did not differ between the variants of the same strain. CONCLUSIONS: The results suggest that in Fl. columnare both rhizoid colony morphology and high chondroitinase activity are needed for virulence and that temperature may promote the adhesion of the virulent variants to surfaces at fish farms. SIGNIFICANCE AND IMPACT OF THE STUDY: New information is produced on the virulence mechanisms of Fl. columnare and the reasons behind the survival of the bacterium at fish farms.

Establishment and interspecific associations in two species of Ichthyocotylurus (Trematoda) parasites in perch (Perca fluviatilis).

BACKGROUND: Co-infections of multiple parasite species in hosts may lead to interspecific associations and subsequently shape the structure of a parasite community. However, few studies have focused on these associations in highly abundant parasite species or, in particular, investigated how the associations develop with time in hosts exposed to co-infecting parasite species for the first time. We investigated metacercarial establishment and interspecific associations in the trematodes Ichthyocotylurus variegatus and I. pileatus co-infecting three age cohorts of young perch (Perca fluviatilis). RESULTS: We found that the timing of transmission of the two Ichthyocotylurus species was very similar, but they showed differences in metacercarial development essentially so that the metacercariae of I. pileatus became encapsulated faster. Correlations between the abundances of the species were significantly positive after the first summer of host life and also within the main site of infection, the swim bladder. High or low abundances of both parasite species were also more frequent in the same host individuals than expected by chance, independently of host age or size. However, the highest abundances of the species were nevertheless observed in different host individuals and this pattern was consistent in all age cohorts. CONCLUSIONS: The results suggest similar temporal patterns of transmission, non-random establishment, and facilitative rather than competitive associations between the parasite species independently of the age of the infracommunities. However, we suggest that spatial differences in exposure are most likely responsible for the segregation of the parasite species observed in the few most heavily infected hosts. Regardless of the underlying mechanism, the result suggests that between-species associations should be interpreted with caution along with detailed examination of the parasite distribution among host individuals.

Eye fluke-induced cataracts in natural fish populations: is there potential for host manipulation?

Manipulation of host phenotype (e.g. behaviour, appearance) is suggested to be a common strategy to enhance transmission in trophically transmitted parasites. However, in many systems, evidence of manipulation comes exclusively from laboratory studies and its occurrence in natural host populations is poorly understood. Here, we examined the potential for host manipulation by Diplostomum eye flukes indirectly by quantifying the physiological effects of parasites on fish. Earlier laboratory studies have shown that Diplostomum infection predisposes fish to predation by birds (definitive hosts of the parasites) by reducing fish vision through cataract formation. However, occurrence of cataracts and the subsequent potential for host manipulation in natural fish populations has remained poorly explored. We studied the occurrence of eye fluke-induced cataracts from 7 common fish species (Gymnocephalus cernuus, Rutilus rutilus, Leuciscus leuciscus, Alburnus alburnus, Osmerus eperlanus, Coregonus lavaretus and Gasterosteus aculeatus) from the Bothnian Bay in the Baltic Sea. We found that the parasite-induced cataracts were common in fish and they also reached high levels which are likely to predispose fish to predation. However, we observed such cataracts only in species with the highest parasite abundances, which suggests that only certain hosts may be strongly affected by the infection.

Increasing water temperature and disease risks in aquatic systems: climate change increases the risk of some, but not all, diseases.

Global warming may impose severe risks for aquatic animal health if increasing water temperature leads to an increase in the incidence of parasitic diseases. Essentially, this could take place through a temperature-driven effect on the epidemiology of the disease. For example, higher temperature may boost the rate of disease spread through positive effects on parasite fitness in a weakened host. Increased temperature may also lengthen the transmission season leading to higher total prevalence of infection and more widespread epidemics. However, to date, general understanding of these relationships is limited due to scarcity of long-term empirical data. Here, we present one of the first long-term multi-pathogen data sets on the occurrence of pathogenic bacterial and parasitic infections in relation to increasing temperatures in aquatic systems. We analyse a time-series of disease dynamics on two fish farms in northern Finland from 1986 to 2006. We first demonstrate that the annual mean water temperature increased significantly on both farms over the study period and that the increase was most pronounced in the late summer (July-September). Second, we show that the prevalence of infection (i.e. proportion of fish tanks infected each year) increased with temperature. Interestingly, this pattern was observed in some of the diseases (Ichthyophthirius multifiliis, Flavobacterium columnare), whereas in the other diseases, the pattern was the opposite (Ichthyobodo necator) or absent (Chilodonella spp.). These results demonstrate the effect of increasing water temperature on aquatic disease dynamics, but also emphasise the importance of the biology of each disease, as well as the role of local conditions, in determining the direction and magnitude of these effects.

Parasite-induced aggression and impaired contest ability in a fish host.

BACKGROUND: Success of trophically transmitted parasites depends to a great extent on their ability to manipulate their intermediate hosts in a way that makes them easier prey for target hosts. Parasite-induced behavioural changes are the most spectacular and diverse examples of manipulation. Most of the studies have been focused on individual behaviour of hosts including fish. We suggest that agonistic interactions and territoriality in fish hosts may affect their vulnerability to predators and thus the transmission efficiency of trophically transmitted parasites. The parasite Diplostomum spathaceum (Trematoda) and juvenile rainbow trout, Oncorhynchus mykiss, were used to study whether infection can alter aggression rates and territorial behaviour of intermediate fish hosts. RESULTS: The changes in behaviour of rainbow trout, Oncorhynchus mykiss, infected with an eye fluke Diplostomum spathaceum (Trematoda), was monitored over the course of an experimental infection for 1.5 months. At the beginning of their development, not yet infective D. spathaceum metacercariae decreased the aggressiveness of rainbow trout. By the time that metacercariae were fully infective to their definitive hosts, the aggressiveness increased and exceeded that of control fish. Despite the increased aggressiveness, the experimentally infected fish lost contests for a territory (dark parts of the bottom) against the control fish. CONCLUSIONS: The results obtained indicate that the parasitized fish pay the cost of aggressiveness without the benefit of acquiring a territory that would provide them with better protection against predators. This behaviour should increase transmission of the parasite as expected by the parasite manipulation hypothesis.

Vertebrate diets derived from trophically transmitted fish parasites in the Bothnian Bay.

Parasites that are transmitted through predator-prey interactions may be used as indicators of trophic relationships between organisms. Yet, they are rarely used as such in the construction of topological (predator-prey) food webs. We constructed food webs of vertebrate trophic interactions using observed diet alone, trophically transmitted parasites alone, and the combination of the two based on data from 31 species of fish from the Bothnian Bay, Finland. The fish food web contained 530 links derived from observed diet, 724 links inferred from parasitism, and 1,058 links calculated from a combination of both stomach contents and parasites. This sub-web constructed from stomach contents had a mean of 17.1 links per fish species, while that using parasites had 23.4 links per fish. Combining the two diet indicators yielded 34.1 links per fish species, illustrating the complementarity of the two methods. Mean number of prey species per fish species was 12.5 using observed diet items, 15.8 using parasites, and 24.5 using both measures together. Mean number of predators per fish species was 7.4 using observed diet, 11.7 using parasites and 15.0 using both. A positive correlation was found between the mean number of parasites and the number of prey taxa in the diet among the fishes. Omnivorous fish had the highest diversity of both parasite species and prey items, while benthophagous fish had among the lowest. Mean total abundance and mean total prevalence of parasites correlated positively with fish size, with piscivores being the largest with the highest abundance and prevalence, while planktivores and benthivores had the lowest. Trophically transmitted parasites may be used to help construct vertebrate sub-webs and derive information about food web processes. Parasites alone provided equivalent if not more information than observed diet. However, resolution is improved by using parasites and observed diet together.

Intensive fish farming and the evolution of pathogen virulence: the case of columnaris disease in Finland.

Ecological changes affect pathogen epidemiology and evolution and may trigger the emergence of novel diseases. Aquaculture radically alters the ecology of fish and their pathogens. Here we show an increase in the occurrence of the bacterial fish disease Flavobacterium columnare in salmon fingerlings at a fish farm in northern Finland over 23 years. We hypothesize that this emergence was owing to evolutionary changes in bacterial virulence. We base this argument on several observations. First, the emergence was associated with increased severity of symptoms. Second, F. columnare strains vary in virulence, with more lethal strains inducing more severe symptoms prior to death. Third, more virulent strains have greater infectivity, higher tissue-degrading capacity and higher growth rates. Fourth, pathogen strains co-occur, so that strains compete. Fifth, F. columnare can transmit efficiently from dead fish, and maintain infectivity in sterilized water for months, strongly reducing the fitness cost of host death likely experienced by the pathogen in nature. Moreover, this saprophytic infectiousness means that chemotherapy strongly select for strains that rapidly kill their hosts: dead fish remain infectious; treated fish do not. Finally, high stocking densities of homogeneous subsets of fish greatly enhance transmission opportunities. We suggest that fish farms provide an environment that promotes the circulation of more virulent strains of F. columnare. This effect is intensified by the recent increases in summer water temperature. More generally, we predict that intensive fish farming will lead to the evolution of more virulent pathogens.

Being successful in the world of narrow opportunities: transmission patterns of the trematode Ichthyocotylurus pileatus.

Parasites with complex life cycles face 2 major challenges for transmission in northern latitudes. They have to cope with the general unpredictability associated with the series of transmission events required for completion of the cycle, and transmission has to be completed within a narrow temporal window because of strong seasonality. Despite this, some parasites show high transmission success, suggesting the operation of effective transmission mechanisms. We explored the transmission of Ichthyocotylurus pileatus (Trematoda) from its snail (Valvata macrostoma) to fish (Perca fluviatilis) hosts by examining some key characteristics in the dynamics of the cercarial emergence from snails. Transmission took place within a few weeks mainly in July, thus verifying the narrow temporal window for transmission. The output of the short-lived cercariae from the snails was low and variable in magnitude, but nevertheless resulted in a rapid and high rate of infection in newly hatched fish. The cercarial emergence showed a strong circadian rhythm with most of the cercariae emerging in early evening and night, which might represent the most likely mechanism underlying the high rate of transmission in this species. We emphasize the importance of holistic approaches combining aspects of multiple host species in studies on transmission of complex life-cycle parasites.