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Large freshwater mussels (Unionida) are long-lived, have large bodies, and produce thousands to millions of larvae (glochidia) that usually must attach to host fish tissue to complete their life cycle. This is an obligate parasitic stage of mussel larvae. However, less than one in onemillion find a suitable host and survive. The degree of host specificity varies among unionid species, from specialists that can successfully parasitize only one or a few closely related fish species to generalists that can complete development on a taxonomically broad range of fish species. In addition, freshwater mussels are among the most threatened groups of animals. This is due to habitat destruction, the introduction of non-native species, and the loss of host fish on which their larvae (glochidia) are obligate parasites. Glochidiosis harms fish by affecting their growth; on the other hand, freshwater mussels play an important role in freshwaters by improving water quality and ridding the water of bacteria, algae, and pollutants, they are an indicator species of water quality. During our parasitological survey of fish from the Macedonian part of Lake Prespa in April 2022, many glochidia were found on the gills, skin, and fins of two endemic fishes, Prespa roach (Rutilus prespensis) and Prespa nase (Chondrostoma prespense), in the range of tens to thousands on one host. We thus recorded these two endemic species as new hosts of A. cygnea.

Genetic variation in resistance against parasite infections is a predominant feature in host–parasite systems. However, mechanisms maintaining genetic polymorphism in resistance in natural host populations are generally poorly known. We explored whether differences in natural infection pressure between resource‐based morphs of Arctic charr (Salvelinus alpinus) have resulted in differentiation in resistance profiles. We experimentally exposed offspring of two morphs from Lake Þingvallavatn (Iceland), the pelagic planktivorous charr (“murta”) and the large benthivorous charr (“kuðungableikja”), to their common parasite, eye fluke Diplostomum baeri, infecting the eye humor. We found that there were no differences in resistance between the morphs, but clear differences among families within each morph. Moreover, we found suggestive evidence of resistance of offspring within families being positively correlated with the parasite load of the father, but not with that of the mother. Our results suggest that the inherited basis of parasite resistance in this system is likely to be related to variation among host individuals within each morph rather than ecological factors driving divergent resistance profiles at morph level. Overall, this may have implications for evolution of resistance through processes such as sexual selection., Mechanisms maintaining genetic polymorphism in resistance in natural host populations are generally poorly known. Here, we show no differences in resistance between sympatric morphs of Arctic charr, but clear differences among families within each morph. Our results suggest that the inherited basis of parasite resistance in this system is related to variation among host individuals within each morph rather than ecological factors driving divergent resistance profiles at morph level.

Hosts can defend themselves against parasites either by preventing or limiting infections (resistance) or by limiting parasite-induced damage (tolerance). However, it remains underexplored how these defense types vary over host development with shifting patterns of resource allocation priorities. Here, we studied the role played by developmental stage in resistance and tolerance in Atlantic salmon (Salmo salar). This anadromous fish has distinct life stages related to living in freshwater and seawater. We experimentally exposed 1-year-old salmon, either at the freshwater stage or at the stage transitioning to the marine phase, to the trematode Diplostomum pseudospathaceum. Using 56 pedigreed families and multivariate animal models, we show that developmental transition is associated with reduced resistance but does not affect tolerance. Furthermore, by comparing tolerance slopes (host fitness against parasite load) based on additive genetic effects among infected and unexposed control relatives, we observed that the slopes can be largely independent of the infection, that is, they may not reflect tolerance. Together, our results suggest that the relative importance of different defense types may vary with host development and emphasize the importance of including control treatments for more confident interpretations of tolerance estimates. peerReviewed

Co-infections are common in host-parasite interactions, but studies about their impact on the virulence of parasites/diseases are still scarce. The present study compared mortality induced by a fatal bacterial pathogen, Flavobacterium columnare between brown trout infected with glochidia from the endangered freshwater pearl mussel, Margaritifera margaritifera, and uninfected control fish during the parasitic period and after the parasitic period (i.e. glochidia detached) in a laboratory experiment. We hypothesised that glochidial infection would increase host susceptibility to and/or pathogenicity of the bacterial infection. We found that the highly virulent strain of F. columnare caused an intense disease outbreak, with mortality reaching 100% within 29 h. Opposite to the study hypothesis, both fresh ongoing and past infection (14 months post-infection) with glochidia prolonged the fish host’s survival statistically significantly by 1 h compared to the control fish (two-way ANOVA: fresh-infection, F1, 82 = 7.144, p = 0.009 and post-infection, F1, 51 = 4.227, p = 0.044). Furthermore, fish survival time increased with glochidia abundance (MLR: post-infection, t = 2.103, p = 0.045). The mechanism could be connected to an enhanced non-specific immunity or changed gill structure of the fish, as F. columnare enters the fish body mainly via the gills, which is also the glochidia’s attachment site. The results increase current knowledge about the interactions between freshwater mussels and their (commercially important) fish hosts and fish pathogens and also emphasise the importance of (unknown) ecosystem services (e.g., protection against pathogens) potentially associated with imperilled freshwater mussels.

It is unknown what circumstances promote particular bacterial defenses against bacterial viruses (phages). Almeida & Hoikkala et al. show that mucin, derived from mucus, greatly accelerates CRISPR-Cas defenses against phage in an opportunistic pathogen. Parasitism by bacteriophages has led to the evolution of a variety of defense mechanisms in their host bacteria. However, it is unclear what factors lead to specific defenses being deployed upon phage infection. To explore this question, we co-evolved the bacterial fish pathogen Flavobacterium columnare and its virulent phage V156 in presence and absence of a eukaryotic host signal (mucin) for sixteen weeks. The presence of mucin leads to a dramatic increase in CRISPR spacer acquisition, especially in low nutrient conditions where over 60% of colonies obtain at least one new spacer. Additionally, we show that the presence of a competitor bacterium further increases CRISPR spacer acquisition in F. columnare. These results suggest that ecological factors are important in determining defense strategies against phages, and that the phage-bacterium interactions on mucosal surfaces may select for the diversification of bacterial immune systems.

Coinfections of two or more parasites within one host are more of a rule than an exception in nature. Interactions between coinfecting parasites can greatly affect their abundance and prevalence. Characteristics of the host, such as genetic diversity, can also affect the infection dynamics of coinfecting parasites. Here we investigate for the first time the association of coinfection patterns of two marine apicomplexans, Rhytidocystis sp. and Selenidium pygospionis, with the genetic diversity of their host, the polychaete Pygospio elegans, from natural populations. Host genetic diversity was determined with seven microsatellite loci and summarized as allelic richness, inbreeding coefficient, and individual heterozygosity. We detected nonsignificant correlations between infection loads and both individual host heterozygosity and population genetic diversity. Prevalence and infection load of Rhytidocystis sp. were higher than that of S. pygospionis and both varied spatially. Coinfections were common and almost all hosts infected by S. pygospionis were also infected by Rhytidocystis sp.. Rhytidocystis sp. infection load was significantly higher in dual infections. Our results suggest that factors other than host genetic diversity might be more important in marine apicomplexan infection patterns and experimental approaches would be needed to further determine how interactions between the apicomplexans and their host influence infection. peerReviewed

1. The freshwater pearl mussel Margaritifera margaritifera (FPM) is an endangered unionid which has a glochidium larva that attaches to the gills of Atlantic salmon Salmo salar or brown trout S. trutta, although some FPM populations have been shown to exclusively attach to only one of these species. The origin of host fish populations may be crucial for conservation actions for this mussel species, but the relative suitability of local (sympatric) and non- local (allopatric) salmonid populations as the hosts for FPM has been studied only rarely. We hypothesised that FPM glochidia would show adaptation to local salmonid strains and, there-fore, that they would be more successful (abundant, larger) attached to sympat-ric than to allopatric fish. 2. Here, we investigated the infection success (abundance and growth of encysted larvae in fish) of FPM in local versus non- local fish by caging different strains of brown trout and Atlantic salmon in rivers where FPM populations are present. 3. Higher abundances of glochidia in local fish were observed in three brown trout streams, and larger glochidia were found in sympatric hosts in one brown trout stream and in one salmon river. Furthermore, non- local allopatric fish were not better hosts than local fish in any of the FPM populations tested, neither in brown trout or salmon rivers and neither in abundance nor size of larvae. Therefore, the results supported the hypothesis that glochidia show local adap-tation by being more successful when attached to local fish strains. 4. Thus, the local, sympatric fish strain should be preferred in FPM conservation programmes that involve captive breeding of juvenile mussels and introduction of host fish, but the regional assessment of local host dependency of FPM also would be important outside the current study area. 5. The results also indicate the importance of restoration of original salmonid pop-ulations in FPM rivers to enable the natural, effective reproduction cycle of FPM in their original, sympatric hosts, and thus to promote the recovery of endan-gered FPM populations. peerReviewed

Several northern freshwater fishes have gone through rapid adaptive radiation after the last glacial period, resulting in new species or intraspecific morphs with distinct life histories. Parasite infections can promote adaptive radiations and spatiotemporal differences in patterns of infections can potentially reveal incipient or ongoing speciation processes. We investigated intraspecific differentiation in a freshwater gadoid fish, burbot (Lota lota), by exploring differences in parasite infections between two potential life-history morphs in Lake Konnevesi, Central-Finland, one reproducing species characteristically in shallow littoral waters in February and the other possibly in deep profundal zone roughly a month later. By conducting a sampling campaign on reproducing fish over two consecutive years, we found significant differences in infections between the fish captured from littoral and profundal sites. More specifically, larval trematode and cestode infections were consistently less abundant in profundal fish, tracking long-term exposure differences in shallow waters. In contrast, trophically transmitted metazoan infections in the fish gut, reflecting shorter-term differences in feeding, showed higher variation between sampling years rather than depths. We also found suggestive evidence of higher trematode-inflicted tissue damage per parasite in the profundal fish, implying lower tolerance to the infection. These results offer further evidence that burbot captured from littoral and profundal sites represent differentiated life-history morphs. We propose that ecological and evolutionary differentiation within burbot populations across its circumpolar distribution may be more widespread than previously acknowledged. peerReviewed

Effect of freshwater mussels’ (Unionoida) glochidia on the growth of fish host has remained poorly studied. We compared the specific growth rate of the juvenile, PIT-marked brown trout (Salmo trutta) between uninfected controls to those experimentally infected (average initial intensity of infection 8000 fish−1) with Margaritifera margaritifera glochidia, kept in high and low feeding. Growth and mortality of fish were monitored for 10 months. Our hypothesis was that glochidiosis would impair the growth of fish. According to our hypothesis, infected fish gained statistically significantly less weight than the control fish throughout the experiment. A proportional increase in weight of control individuals was 11% higher than that of the infected fish. However, neither the feeding regime (high, low) nor the period (September–November, November–March, March–May), had a significant effect on the growth difference between control and infected fish. As the effect of infection on the growth of fish was subtle and no effect on host mortality was detected either, this may turn public opinion favorable for M. margaritifera conservation even if the salmonid host population is important for commercial or recreational fishing.

As a consequence of climate change and open net-pen salmon farming, wild Atlantic salmon Salmo salar are increasingly likely to encounter elevated temperatures and parasite abundances during their early marine migration. Such stressors can compromise fitness by diminishing liver energy stores and impairing cardiac muscle. To assess whether temperature and infestation by salmon lice Lepeophtheirus salmonis are important correlates of liver energy stores and cardiac muscle performance in juvenile salmon, we experimentally infested fish at 3 abundances of louse infestation (zero, low, and high) and 5 temperatures (10, 13, 16, 19, and 22°C). At the end of the experiment (i.e. when sea lice reached adulthood), we calculated the percent dry weight of the liver (%DWL; a proxy for liver energy stores) and cardiosomatic index (CSI; a proxy for cardiac muscle performance) of each fish and fitted 5 linear mixed-effects models to both of these responses. For both %DWL and CSI, the best-supported model included additive fixed effects for both infestation level and temperature. Our top models predicted that, relative to zero infestation, high infestation reduces %DWL by 5.7% (95% CI: 5.3-6.2%) and increases CSI by 15.9% (14.4-18.0%), and low infestation reduces %DWL by 2.6% (2.2-3.0%) and increases CSI by 7.8% (6.7-10.0%). Our work suggests that stressors associated with ocean warming and coastal salmon aquaculture can compromise wild salmon fitness through the impairment of vital organs. peerReviewed

An increasing threat to local, native freshwater mussels (Unionida)—an ecologically important but globally alarmingly declining group— is the invasion by exotic bivalves. The Enemy Release Hypothesis predicts that introduced species should benefit from enemy-mediated competition because they are less likely to be harmed by natural enemies, such as parasites, than their native competitors. We investigated within-site differences in parasitism between sympatric native (tot. five spp.) and invasive (tot. three spp.) bivalves in eight northern European waterbodies, which harboured totally 15 parasite taxa. In paired comparisons using within-site averages, the mean number of parasite species in the native bivalves was 2.3 times higher, and the sum of parasite prevalences 2.4 times higher, than in the invasive bivalves. This may lead to enemy-mediated compet- itive release of invaders and contribute to the success of invasive freshwater bivalves, in general. However, while the invasive clam Corbicula fluminea was completely free from parasites, parasite parameters of the other invader, Sinanodonta woodiana, were relatively high, indicating that the role of parasites can be invader-specific and urges further research. Under- standing the factors affecting success of freshwater bivalve invasions, such as parasitism, can aid invasion control and conservation of local, native (endangered) bivalves. Open access funding provided by University of Jyväskylä (JYU). Co-authors’ cooperation was initiated with the support of the Poznan ́ City Council under the ‘‘Academic and Scientific Poznan ́’’ Program. The study was funded by Estonian Ministry of Education and Research (institutional research funding project IUT 21-2 to Tiina Nõges) and by Estonian Research Council, Mobilitas Pluss research project MOBJD29. This study was also supported by the Polish Minister of Science and Higher Education, under the program ‘‘Regional Initiative of Excellence’’ in 2019–2022 (Grant No. 008/RID/2018/19), by Poznan ́ University of Life Sciences, University of Jyva ̈skyla ̈, and by CNR Short Term Mobility Project 2016. We thank Henryk Gierszal and Henn Timm for valuable assistance in field and laboratory. Open access funding provided by University of Jyväskylä (JYU). Co-authors’ cooperation was initiated with the support of the Poznan ́ City Council under the ‘‘Academic and Scientific Poznan ́’’ Program. The study was funded by Estonian Ministry of Education and Research (institutional research funding project IUT 21-2 to Tiina Nõges) and by Estonian Research Council, Mobilitas Pluss research project MOBJD29. This study was also supported by the Polish Minister of Science and Higher Education, under the program ‘‘Regional Initiative of Excellence’’ in 2019–2022 (Grant No. 008/RID/2018/19), by Poznan ́ University of Life Sciences, University of Jyva ̈skyla ̈, and by CNR Short Term Mobility Project 2016. We thank Henryk Gierszal and Henn Timm for valuable assistance in field and laboratory.

Preinfection by one parasitic species may facilitate or by contrast hamper the subsequent penetration and/or establishment of other parasites in a host. The biology of interacting species, timing of preinfection, and dosage of subsequent parasite exposure are likely important variables in this multiparasite dynamic infection process. The increased vulnerability to subsequent infection can be an important and often overlooked factor influencing parasite virulence. We investigated how the preinfection by freshwater pearl mussel Margaritifera margaritifera glochidia could influence the success of subsequent infection by the common trematode Diplostomum pseudospathaceum in brown trout Salmo trutta and vice versa whether preinfection by the trematode made fish more susceptible to glochidia infection. The first experiment was repeated twice with different (low and high) exposure doses to initiate the subsequent trematode infection, while in the second experiment we varied the timing of the preinfection with trematodes. The preinfection with glochidia made fish more vulnerable to subsequent infection with trematodes. Since the trematodes penetrate through the gills, we suggest that increased host vulnerability was most likely the result of increased respiration caused by the freshwater pearl mussel glochidia encysted on gills. In turn, brown trout preinfected with trematodes were more vulnerable to the subsequent glochidial infection, but only if they were preinfected shortly before the subsequent infection (20 hr). Fish preinfected with trematodes earlier (2 weeks before the subsequent infection) did not differ in their vulnerability to glochidia. These effects were observed at moderate intensities of infections similar to those that occur in nature. Our study demonstrates how the timing and sequence of exposure to parasitic species can influence infection success in a host–multiparasite system. It indicates that the negative influence of glochidia on host fitness is likely to be underestimated and that this should be taken into consideration when organizing freshwater pearl mussel restoration procedures. peerReviewed

Parasitism has been proposed as a factor in host speciation, as an agent affecting coexistence of host species in species-rich communities and as a driver of post-speciation diversification. Young adaptive radiations of closely related host species of varying ecological and genomic differentiation provide interesting opportunities to explore interactions between patterns of parasitism, divergence and coexistence of sympatric host species. Here, we explored patterns in ectoparasitism in a community of 16 fully sympatric cichlid species at Makobe Island in Lake Victoria, a model system of vertebrate adaptive radiation. We asked whether host niche, host abundance or host genetic differentiation explains variation in infection patterns. We found significant differences in infections, the magnitude of which was weakly correlated with the extent of genomic divergence between the host species, but more strongly with the main ecological gradient, water depth. These effects were most evident with infections of Cichlidogyrus monogeneans, whereas the only host species with a strictly crevice-dwelling niche, Pundamilia pundamilia, deviated from the general negative relationship between depth and parasitism. In accordance with the Janzen-Connell hypothesis, we also found that host abundance tended to be positively associated with infections in some parasite taxa. Data on the Pundamilia sister species pairs from three other islands with variable degrees of habitat (crevice) specialization suggested that the lower parasite abundance of P. pundamilia at Makobe could result from both habitat specialization and the evolution of specific resistance. Our results support influences of host genetic differentiation and host ecology in determining infections in this diverse community of sympatric cichlid species.

Genetic variation in defence against parasite infections is fundamental for host–parasite evolution. The overall level of defence of a host individual or population includes mechanisms that reduce parasite exposure (avoidance), establishment (resistance) or pathogenicity (tolerance). However, how these traits operate and evolve in concert is not well understood. Here, we investigated genetic variation in and associations between avoidance, resistance and tolerance in a natural host–parasite system. Replicated populations of Atlantic salmon ( Salmo salar ) and sea trout (an anadromous form of brown trout, Salmo trutta ) were raised under common garden conditions and infected with the eye fluke Diplostomum pseudospathaceum . We demonstrate significant genetic variation in the defence traits across host populations and negative associations between the traits, with the most resistant populations showing the weakest avoidance and the lowest infection tolerance. These results are suggestive of trade-offs between different components of defence and possibly underlie the genetic variation in defence traits observed in the wild. Because the three defence mechanisms affect host–parasite evolution in profoundly different ways, we emphasize the importance of studying these traits in concert.

SUMMARYFactors 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, theAustralapatemonsp. trematode infecting two sympatric species of freshwater leeches,Erpobdella octoculataandHelobdella 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 ofE. octoculata, but not that ofH. stagnalis, was impaired by the infection irrespective of the parasite abundance. This may increase susceptibility ofE. octoculatato 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.

Influence of fish personality on infection rate is almost not studied. In the experiments on the young-of-the-year Oncorhynchus mykiss and cercariae of a trematode Diplostomum pseudospathaceum we tested the hypothesis that infection rate differs between more and less active (“bold” and “shy”) fish. Will individual differences in infection persist upon re-infection? Fish serve as a second intermediate host for this trematode. A positive correlation was found between the results of consecutive infections. Accumulation of parasites with successive infections leads to an aggregated distribution of D. pseudospathaceum among the hosts, affecting individual fitness and polymorphism in fish populations. Persistent individual differences in parasite burden among fish and, as a result, vulnerability for predators confirms the role of parasites as an important factor of natural selection. peerReviewed

Parasitism is considered a major selective force in natural host populations. Infections can decrease host condition and vigour, and potentially influence, for example, host population dynamics and behavior such as mate choice. We studied parasite infections of two common marine fish species, the sand goby (Pomatoschistus minutus) and the common goby (Pomatoschistus microps), in the brackish water Northern Baltic Sea. We were particularly interested in the occurrence of parasite taxa located in central sensory organs, such as eyes, potentially affecting fish behavior and mate choice. We found that both fish species harbored parasite communities dominated by taxa transmitted to fish through aquatic invertebrates. Infections also showed significant spatiotemporal variation. Trematodes in the eyes were very few in some locations, but infection levels were higher among females than males, suggesting differences in exposure or resistance between the sexes. To test between these hypotheses, we experimentally exposed male and female sand gobies to infection with the eye fluke Diplostomum pseudospathaceum. These trials showed that the fish became readily infected and females had higher parasite numbers, supporting higher susceptibility of females. Eye fluke infections also caused high cataract intensities among the fish in the wild. Our results demonstrate the potential of these parasites to influence host condition and visual abilities, which may have significant implications for survival and mate choice in goby populations.

Even for parasitoids with a wide host range, not all host species are equally suitable, and host qualityoften depends on the plant the host feeds on. We compared oviposition choice and offspring perfor-mance of a generalist pupal parasitoid,Pteromalus apum(Retzius) (Hymenoptera: Pteromalidae), ontwo congeneric hosts reared on two plant species under field and laboratory conditions. The plantscontain defensive iridoid glycosides that are sequestered by the hosts. Sequestration at the pupal stagediffered little between host species and, although the concentrations of iridoid glycosides in the twoplant species differ, there was no effect of diet on the sequestration by host pupae. The rate of success-ful parasitism differed between host species, depending on the conditions they were presented in. Inthe field, where plant-associated cues are present, the parasitoid usedMelitaea cinxia(L.) overMeli-taea athalia(Rottemburg) (Lepidoptera: Nymphalidae), whereas moreM. athaliawere parasitisedin simplified laboratory conditions. In the field, brood size, which is partially determined by rate ofsuperparasitism, depended on both host and plant species. There was little variation in other aspectsof offspring performance related to host or plant species, indicating that the two host plants are ofequal quality for the hosts, and the hosts are of equal quality for the parasitoids. Corresponding tothis, we found no evidence for associative learning by the parasitoid based on their natal host, so withrespect to these host species they are truly generalist in their foraging behaviour. peerReviewed

Parasitism has been proposed as a factor in host speciation, as an agent affecting coexistence of host species in species‐rich communities and as a driver of post‐speciation diversification. Young adaptive radiations of closely related host species of varying ecological and genomic differentiation provide interesting opportunities to explore interactions between patterns of parasitism, divergence and coexistence of sympatric host species. Here, we explored patterns in ectoparasitism in a community of 16 fully sympatric cichlid species at Makobe Island in Lake Victoria, a model system of vertebrate adaptive radiation. We asked whether host niche, host abundance or host genetic differentiation explains variation in infection patterns. We found significant differences in infections, the magnitude of which was weakly correlated with the extent of genomic divergence between the host species, but more strongly with the main ecological gradient, water depth. These effects were most evident with infections of Cichlidogyrus monogeneans, whereas the only host species with a strictly crevice‐dwelling niche, Pundamilia pundamilia, deviated from the general negative relationship between depth and parasitism. In accordance with the Janzen–Connell hypothesis, we also found that host abundance tended to be positively associated with infections in some parasite taxa. Data on the Pundamilia sister species pairs from three other islands with variable degrees of habitat (crevice) specialization suggested that the lower parasite abundance of P. pundamilia at Makobe could result from both habitat specialization and the evolution of specific resistance. Our results support influences of host genetic differentiation and host ecology in determining infections in this diverse community of sympatric cichlid species. peerReviewed

SUMMARYParasites manipulate their hosts’ phenotype to increase their own fitness. Like any evolutionary adaptation, parasitic manipulations should be costly. Though it is difficult to measure costs of the manipulation directly, they can be evaluated using an indirect approach. For instance, theory suggests that as the parasite infrapopulation grows, the investment of individual parasites in host manipulation decreases, because of cost sharing. Another assumption is that in environments where manipulation does not pay off for the parasite, it can decrease its investment in the manipulation to save resources. We experimentally infected rainbow trout Oncorhynchus mykiss with the immature larvae of the trematode Diplostomum pseudospathaceum, to test these assumptions. Immature D. pseudospathaceum metacercariae are known for their ability to manipulate the behaviour of their host enhancing its anti-predator defenses to avoid concomitant predation. We found that the growth rate of individual parasites in rainbow trout increased with the infrapopulation size (positive density-dependence) suggesting cost sharing. Moreover, parasites adjusted their growth to the intensity of infection within the eye lens where they were localized suggesting population density sensing. Results of this study support the hypothesis that macroparasites can adjust their growth rate and manipulation investment according to cost sharing level and infrapopulation size.

Fish parasites may potentially harm fisheries and aquaculture. Infected fish are unusable, even though they would not pose a direct risk to human health. The aim of this thesis was to investigate the ecology and epidemiology of three previously unknown or poorly known spore-forming parasites infecting muscles of economically important fish species. The first novel microsporidian species, here described as Microsporidium luciopercae, was found from pike-perch (Sander lucioperca) and European perch (Perca fluviatilis) and formed opaque-looking patches in the muscles. The second novel microsporidian species, Myosporidium spraguei, was found from pike-perch and burbot (Lota lota) and occurred within xenomas in the musculature. The third muscle-dwelling parasite, Henneguya zschokkei (Cnidaria), formed white plasmodia in the muscle of whitefish (Coregonus lavaretus). Among the six study lakes, the prevalence of M. luciopercae in wild pike-perch and perch varied from 0 to 1 %, while M. spraguei was more common (prevalence of 5–26 % in pike-perch and 65 % in burbot). M. spraguei was strongly aggregated and most abundant in the middle-sized (37–45 cm) pike-perch and in larger burbot (> 35 cm), but the infection was not related to host sex, sampling season or host condition. In the case of H. zschokkei, in two inland fish farms, one year-old whitefish were virtually uninfected, but the prevalence of infection was higher in 2 and 3 year old fish (up to 36 % of 3 year old fish were infected). New infections appeared seasonally in July-August. Neither the size of plasmodia nor the proportion of different plasmodium types changed from 2 to 3 year old fish, suggesting slow development of H. zschokkei in whitefish. However, the proportion of infected fish with melanised plasmodia increased with fish age, suggesting an age-dependent host response against this strongly aggregated parasite.

The abundance of freshwater pearl mussels (Margaritifera margaritifera) has declined widely during the past century, and new conservation initiatives are needed. This thesis focused on the relationship between M. margaritifera and its salmonid host required for reproduction of this species. First, by exposing fish experimentally to glochidium larvae of M. margaritifera, different M. margaritifera populations were shown to demonstrate strong differences in their ability to parasitize different salmonid species. Atlantic salmon was clearly a better host for mussels in large river channels, whereas in small headwater tributaries brown trout was the best, or the only suitable, host. These findings provide a previously unrecognised explanation for the collapse and the lack of recruitment especially of the salmon-specific M. margaritifera populations; a high proportion of large salmon rivers were dammed for hydropower production in the 1960s, which prevented the migration of salmon and thus left M. margaritifera without the appropriate host in these rivers. Furthermore, an invasive salmonid, brook trout, was widely introduced to small tributaries above the dams in the past, but in this study was shown to be an unsuitable host for M. margaritifera. Thus, and due to the tendency of brook trout to replace native brown trout, the spread of brook trout is an additional threat to M. margaritifera. An indication of local adaptation of M. margaritifera, i.e. higher infectivity in sympatric salmonid host strain than in allopatric populations of the same species, was also detected. Finally, a new, non- destructive approach to search for M. margaritifera populations, involving electrofishing and quick visual examination of the gills of captured salmonids, revealed the occurrence of 3 previously unknown populations. The results of this thesis highlight the importance of taking into account the roles of salmonid fish in future efforts to search, protect and restore freshwater pearl mussel populations.

In this thesis, two bucephalid trematode parasites Rhipidocotyle campanula and R. fennica, which use the same first (Anodonta anatina) and second intermediate (Rutilus rutilus) host were studied. The aim was to investigate the effect of temperature on one of the key processes in the transmission of these parasites: 1) the emergence of cercarial larvae from A. anatina over short (1 h) and 2) long (throughout the annual cercarial shedding period, from May to October) time periods as well, as on 3) mussel survival and 4) the seasonal timing of cercarial release. In addition, the aim was to study how the cercarial shedding traits are linked to the 5) geographical occurrence and abundance of the Rhipidocotyle species. In the experimental studies, the cercarial emergence by R. fennica increased significantly with increasing temperature over short and long time periods, while that by R. campanula was unaffected by temperature. R. campanula clearly started seasonal cercarial release earlier and at a lower temperature than R. fennica. Survival of mussels, especially cercariae-shedding mussels, was lower at higher temperature, and the shedding of R. campanula cercariae was associated with higher mussel mortality than the shedding of R. fennica. The average duration of the seasonal cercarial release period of both species was unaffected by temperature at the individual host level, but at the host population level the cercarial shedding period of R. fennica (but not of R. campanula) was longer at higher temperature. The field study showed that the occurrence, mean prevalence and abundance of R. fennica – in accordance with the experimentally observed association of cercarial release with high temperature – decreased from the south (61–64 °N) to the low north (65–66 °N), but this pattern was not detected in R. campanula.

Two bucephalid trematodes, Rhipidocotyle campanula and R. fennica are known to infect the duck mussel, Anodonta anatina. The infection will lead to decrease growth, reproduction and survival of A. anatina. Given the important role of temperature on cercariae production, a vital component of the parasite’s transmission and life cycle success, the effect of temperature on cercariae production of two Rhipidocotyle sp. in their molluscan host, Anodonta anatina, was investigated. Mussels were collected from the Rivers, Haajaistenjoki and Kuusaankoski, marked and allocated to three temperature treatments—high, intermediate and low. Between May 31-October 28, 2011, clams were individually monitored every third week for cercarial emergence in the laboratory. Cercariae production was temperature-dependent but differential. R. campanula emergence started in late May and R. fennica in July. In the River Haajaistenjoki, in high temperature, average annual R. fennica production was higher (17000 larvae per mussel) than R. campanula (3000 larvae per mussel), but the opposite in low temperature (600 vs. 1300 larvae). Similar results were found from the River Kuusaankoski. Results suggest that R. campanula would be better adapted to low temperatures while R. fennica should have an advantage in high temperature habitats.

FM Matthias Bandilla osoittaa väitöstutkimuksessaan, että kirjolohen kalatäitartunnalla on selkeä yhteys Flavobacterium columnare -infektioon, joka on yksi ongelmallisimmista bakteeritaudeista suomalaisessa kalanviljelyssä tällä hetkellä. Vaikka heikko kalatäitartunta ei aiheuta kalan kuolemaa, yhdessä bakteerialtistuksen kanssa se voi olla tappava. Vastaavien vuorovaikutusten ymmärtäminen olisi tärkeää vesiviljelyn kannalta, koska erilaisten mikrobien aiheuttamien tautien ja loisten yhteisesiintyminen kalanviljelylaitoksella on ennemminkin sääntö kuin poikkeus.Kalatäi (Argulus coregoni) on pääasiassa lohikaloilla esiintyvä äyriäispintaloinen, jonka levinneisyys ulottuu koko pohjoiselle vyöhykkeelle. Täin toukat kiinnittyvät kalaisännän pintaan ja syövät isännästään verta ja kudoksia. Kalatäikoiraat etsivät yleensä kalan pinnalta naaraan pariutuakseen, jonka jälkeen naaraat irrottautuvat kalasta muniakseen keskimäärin 300 munaa. – Kalatäit kuolevat avovesikauden loppuun mennessä ja talvehtivat ainoastaan munina. Suomessa lajilla on yksi sukupolvi vuoden aikana, mutta ilmaston lämpeneminen voisi mahdollistaa myös toisen sukupolven, joka toisi väistämättä lisää ongelmia kalankasvatukseen, Bandilla toteaa.Tutkimuksen kohteena myös kalatäin ekologiaVäitöskirjatutkimuksessaan Bandilla tutki paitsi kalatäiloisinnan yhteyttä bakteeritautiin, myös kalatäin siirtymistä kalaisäntään ja pariutumista. Tutkimuksessa selvisi, että pieni osa kalayksilöistä kantoi suurimman osan loistaakasta. Näköaisti osoittautui hajuaistia tärkeämmäksi, kun täit etsivät isäntää. Vain koiraat reagoivat vastakkaisen sukupuolen tuottamaan hajuun. Kalatäikoirailla ja -naarailla oli erilaiset lisääntymisstrategiat: koiraat toimivat aktiivisina parittelukumppanin etsijänä ja naaraat passiivisempina pysytellen kalan pinnalla. – Torjuntastrategiat, joissa yhdistetään tehokkaiden kemiallisten torjunta-aineiden käyttö ja loiseliöiden biologian tunteminen, voivat vähentää tappioita ja ympäristövaikutuksia. Argulus coregoni -kalatäin biologian tutkimus on johtanut lois-isäntäpopulaatiomallin kehittämiseen, joka on ensimmäinen askel kestävän kalatäitorjunnan kehittämiseksi kalanviljelylaitoksilla, Bandilla kertoo. Argulus coregoni is an ectoparasite preferentially infecting salmonid fish. In this thesis I focussed on aspects of transmission, host and mate location of A. coregoni and its link with secondary diseases in fish. Under fish farming conditions, metanauplii and juvenile stages of the parasite were aggregated among the host population; i.e. a relatively small proportion of the host population carried the majority of the parasite population. In laboratory experiments I showed that such aggregated distribution was likely to be driven by differences in exposure to the parasite and not by differences in susceptibility or by acquired immunity. The transmission of A. coregoni metanauplii in the laboratory was highly successful, with 80% attaching to the host within 1-2 h. Extended transmission periods due to host absence did not decrease the infectivity of the parasite, but some consequences of a prolonged transmission process became apparent during the early development; parasites that experienced long transmission periods were smaller compared to argulids with short transmission time. However, in late phases of development the smaller parasites compensated their decreased size by an increased growth rate. Since A. coregoni is a sexually reproducing species, such “catch-up” growth might be beneficial in terms of reaching the optimal mating size, higher fecundity and sexual selection. In Y-maze experiments it was shown that visual cues are most important for host finding, however the parasite could also detect fish odour which might act as an auxiliary cue to find a host. Males responded to female odour but not vice versa, indicating that two different reproductive strategies exist, where males are the active mate searchers while females are rather stationary on the host and wait for partners. Host switching experiments support this view since adult males were more active in switching between hosts than were adult females. Knowledge gained in this study elucidates the sophisticated behavioural patterns of A. coregoni and can be used for developing sustainable control strategies at fish farms, which is important since this thesis showed that argulids can enhance the susceptibility of fish to Flavobacterium columnare, the causative agent of columnaris disease

Only abstract. Paper copies of master’s theses are listed in the Helka database (http://www.helsinki.fi/helka). Electronic copies of master’s theses are either available as open access or only on thesis terminals in the Helsinki University Library. Vain tiivistelmä. Sidottujen gradujen saatavuuden voit tarkistaa Helka-tietokannasta (http://www.helsinki.fi/helka). Digitaaliset gradut voivat olla luettavissa avoimesti verkossa tai rajoitetusti kirjaston opinnäytekioskeilla. Endast sammandrag. Inbundna avhandlingar kan sökas i Helka-databasen (http://www.helsinki.fi/helka). Elektroniska kopior av avhandlingar finns antingen öppet på nätet eller endast tillgängliga i bibliotekets avhandlingsterminaler. This study investigates the effect of law enforcement in predatory economies. A predatory economy is an economy where long-term growth performance is lower than optimal, due to a high share of predation. The focus is purely economical and strictly on the private economy. A predation trap occurs, which depends on the initial allocations of entrepreneurship in the economy. The economy can find its way out of the trap, if the flow of new entrepreneurs is sufficient. This study amends an existing model of predation by introducing different law enforcement regimes. It shows that it is possible to regulate the parasitic activities through increased law enforcement. The law enforcement changes the initial conditions required for the predation trap to exist, and thus improve long-term growth performance. However, the law enforcement needs to be cost-effective in order to be worth introducing. On low initial levels of entrepreneurship, introducing law enforcement does not necessarily lift the country out from the predation trap. The country still needs a sufficient enough flow of new entrepreneurs into the economy to free it from predation. Hence, this study explains partly why poor countries stay poor.

Otto Seppälä tutki väitöskirjassaan loisten kykyä manipuloida isäntiään mahdollisena strategiana tehostaa loisten siirtymistä uusiin isäntäyksilöihin. Monien loisten on havaittu vaikuttavan mm. isäntiensä käyttäytymiseen ja ulkonäköön. Erityisesti ravintoketjuissa saaliseläimistä saalistajiin kulkeutuvilla loisilla isännän manipulointia on yleisesti pidetty loisten evolutiivisena sopeumana altistaa isännät saalistajille ja täten tehostaa loisten siirtymistä elinkierrossa eteenpäin. Manipulaatioteoriaa on kuitenkin vain harvoin testattu kriittisesti selvittämällä saalistuskokein loisten aiheuttamien käyttäytymismuutosten vaikutusta isäntien saalistusalttiuteen.Seppälä selvitti useiden eri kalalajien silmissä yleisen Diplostomum spathaceum -loisen vaikutusta kalojen käyttäytymiseen, värinsäätelyyn ja saalistusalttiuteen. Trophically-transmitted parasites may predispose infected hosts to predation by altering their phenotype. This can be either an adaptation of the parasites to enhance their transmission to the next hosts in the life cycle or a non-adaptive side-effect of infection. In this thesis, I examined whether the Diplostomum spathaceum (Trematoda) eye fluke can manipulate the phenotype of its fish intermediate hosts to increase their susceptibility to predation, and if this could be an adaptation of the parasite to enhance its onward transmission to the bird definitive hosts. In laboratory experiments, I found that anti-predator behaviour of infected fish was reduced compared to uninfected fish. Parasitized fish did not prefer the surface layers of the water column more than control fish, but did show a weaker reaction to an approaching simulated avian predator. Furthermore, their ability to adjust to the environment using cryptic coloration and cryptic behaviour was reduced. These changes led to an increase in the susceptibility of fish to simulated avian predation (capture by dip-net). This result was not reproduced when fish were exposed to predation by wild birds in a field experiment, possibly because the experimental set-up allowed birds to feed on fish in an easy, unnatural manner. Catchability of fish in the laboratory increased with the coverage of parasite-induced cataracts, which suggests that impaired vision may be the definitive mechanism leading to manipulation. Moreover, cataract formation was most intensive after parasites had completed their development, resulting in host manipulation only after parasites had reached infectivity to bird hosts. Furthermore, manipulation was not observed to be costly for the parasite, because it did not predispose fish to predation by non-host piscivorous fish. These findings suggest that manipulation of the fish host may increase the probability of parasite transmission to bird hosts, and thus be a parasite strategy evolved to enhance transmission.

FM Mari Saarinen on tutkinut väitöskirjassaan Paraergasilus rylovi -äyriäisloisen biologiaa sen isännässä, Anodonta piscinalis -pikkujärvisimpukassa. Työ on perustavaa laatua oleva, sillä tästä loisesta ei lajinkuvauksen lisäksi ole tiedetty aiemmin mitään.Saarisen tutkimuksissa kartoitetaan äyriäisloisen esiintymistä ja loisinnan voimakkuutta eri simpukkalajeilla eri osissa Suomea. Tulokset osoittivat, että loinen on erikoistunut käyttämään pikkujärvisimpukkaa isäntänään ja että loinen esiintyy Suomessa vain maan eteläosissa. Isännän elinympäristöllä ja koolla havaittiin olevan vaikutusta loisen runsauteen; loinen esiintyi järvissä runsaampana kuin virtaavissa vesissä ja infektion intensiteetti oli suurempi pienemmissä simpukoissa. Isäntäsimpukan käyttäytyminen ja lisääntyminen vaikuttivat myös P. rylovi –loisen runsauteen. Loisia oli enemmän syvemmälle järven pohjasedimenttiin kaivautuneissa simpukoissa, joiden kasvu oli heikompaa. Lisääntyvillä naarassimpukoilla oli enemmän loisia kuin lisääntymättömillä naarailla. Lisääntymisestä aiheutuvan stressin on tiedetty lisäävän loisiintumisalttiutta, mutta tämä vuorovaikutus osoitettiin nyt ensimmäisen kerran selkärangattomalla isännällä Among Unionidae clams, the parasite Paraergasilus rylovi was specific to Anodonta piscinalis. The parasite occurred in southern Finland. The mean intensity of infection of P. rylovi was higher in lake populations compared to rivers. Within a lake, P. rylovi was more abundant at moderate depth compared to shallow or deep habitats. Paraergasilus rylovi produced eggs between June and August and new infections were established in clams in August. The abundance of P. rylovi increased with proportionate burrowing depth of A. piscinalis. There was an inverse relationship between P. rylovi intensity and age/size of the host, but no dependence between the intensity of infection and host sex. Female A. piscinalis, which brooded glochidia larvae were more abundantly infected by P. rylovi than non-reproducing females. The parasites were possible to expel from the gills of A. piscinalis by using high water temperature or low oxygen. Paraergasilus rylovi were expelled before an experiment conducted to study local adaptation of P. rylovi. In the reciprocal transplant experiment between two locations, both infectivity and reproductive performance of P. rylovi were higher in home-clams in both transplant sites, indicating that P. rylovi may be genetically specialized to its local host populations. In other a field experiment, anoxia-stressed clams were more susceptible to infection by P. rylovi than unstressed individuals when exposed to parasites about 11 months after the stress treatment. The stressed clams also showed significantly lower growth, lower reproduction and lower survival than the unstressed clams. The result indicates that stress may have long-lasting effect on host susceptibility to parasitism in natural populations.