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.

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.

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.

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.

The efficacy of two immunostimulants against Flavobacterium columnare infection in juvenile rainbow trout (Oncorhynchus mykiss).

Bacterium Flavobacterium columnare is the causative agent of columnaris disease in many wild and farmed fish species. Immunostimulants are used with success in aquaculture against many pathogens, but the ability to improve innate resistance to columnaris disease has not been studied. Fingerling rainbow trout were treated with two immunostimulants, yeast beta-glucan and beta-hydroxy-beta-methylbutyrate (HMB). Selected innate immune function parameters, the production of reactive oxygen species (ROS) by whole blood and by isolated head kidney leukocytes, plasma lysozyme activity and complement bacteriolytic activity, were determined to assess the immune status of fish. The fish were then bath challenged with virulent F. columnare bacteria, and the mortality of fish was recorded. Given orally both stimulants raised the levels of immune function parameters, but did not improve survival in challenge at any concentration of the stimulants used. Intra peritoneal injection of beta-glucan increased parameter values several fold, but no beneficial effect of injected glucan on survival was noted. As a control, antibiotic medication administered prior to and during the challenge infection prevented the mortality. Innate immune mechanisms, even when induced to high levels with immunostimulants, as evidenced here, were not able to increase resistance against F. columnare. This may be connected to the external character of the infection. The results from the treatments with beta-glucan and HMB suggest that there is little prospect of preventing columnaris disease by means of immunostimulants in early life stage of rainbow trout. However, the efficacy of other immune stimulants remains open.

Saprophytism of a fish pathogen as a transmission strategy.

Fish farming creates conditions where disease transmission is enhanced and antibiotic treatments are commonly used to cure bacterial diseases to prevent severe losses due to infections. Ability to persist in such an environment has been suggested to lead to the evolution of high virulence. Columnaris disease caused by Flavobacterium columnare is a growing problem in freshwater fish farming. Transmission of the disease is poorly known, and survival of F. columnare in the rearing environment has not been studied. This paper addresses both transmission of columnaris disease and survival strategy of F. columnare. Saprophytic activity of F. columnare was studied by infecting rainbow trout fingerlings before and immediately after death and by following bacterial shedding from the fish carcasses. From fish killed immediately after infection, bacteria were shed at high rates for 5 days, and from fish exposed to F. columnare post mortem for 8 days. In another experiment, rainbow trout fingerlings were experimentally infected with F. columnare and monitored for transmission of the bacteria post infection until and after the death of the fish. The transmission of columnaris disease to living rainbow trout was the most efficient from dead fish, from which bacteria were shed into water at higher rates than from living fish. We also found that F. columnare can survive at least for 5 months in both sterilized distilled and lake water. These results show that death of the host causes no cost for F. columnare; it thrives in alive and dead fish, and in water. Saprophytism may have been a transition stage to pathogenicity of this originally harmless water bacterium, and maintained as an effective transmission and survival strategy of F. columnare. Our findings also suggest that F. columnare may be able to persist in the rearing environment during antibiotic treatments of the living fish.

Interactions among co-infecting parasite species: a mechanism maintaining genetic variation in parasites?

Individuals of free-living organisms are often infected simultaneously by a community of parasites. If the co-infecting parasites interact, then this can add significantly to the diversity of host genotypexparasite genotype interactions. However, interactions between parasite species are usually not examined considering potential variation in interactions between different strain combinations of co-infecting parasites. Here, we examined the importance of interactions between strains of fish eye flukes Diplostomum spathaceum and Diplostomum gasterostei on their infectivity in naive fish hosts. We assessed the infection success of strains of both species in single-strain exposures and in co-exposures with a random strain of the other species. Parasite infection success did not consistently increase or decrease in the co-exposure treatment, but depended on the combinations of co-infecting parasite strains. This disrupted the relative infectivity of D. spathaceum strains observed in single-strain exposures. The infection success of D. gasterostei strains was independent of exposure type. These results suggest that interactions among parasite species may be strain specific and potentially promote maintenance of genetic polymorphism in parasite populations.

Between-population similarity in intestinal parasite community structure of pike (Esox lucius)--effects of distance and historical connections.

The effect of geographical distance on similarity in parasite communities of freshwater fish has received considerable attention in recent years, and it has become evident that these apparently simple relationships are influenced by, among other things, colonization ability of parasites and degree of connectivity between the populations. In the present paper, we explored qualitative and quantitative similarity in the intestinal parasite communities of pike (Esox lucius) in a particular system where previously interconnected groups of lakes became isolated ca. 8,400 yr ago. Contrary to our expectations, we did not find differences in similarity between the lake groups or a negative effect of distance among the populations. This supports the role of common ancestral colonization events and shows that no significant loss of species has occurred during the past 8,000 yr. However, the communities were dominated by a single parasite species, the cestode Triaenophorus nodulosus. The exclusion of this species from the data had a significant negative impact on the community similarities and also revealed a negative relationship between distance and quantitative similarity. This suggests that patterns of community organization may be obscured by a single dominant species. We also highlight the need for further studies in different systems and host species, as well as detailed reanalysis of existing data sets, to unravel the controversy in the relationship between distance and similarity in parasite communities.

Flavobacterium columnare colony types: connection to adhesion and virulence?

Four different colony morphologies were produced by Flavobacterium columnare strains on Shieh agar plate cultures: rhizoid and flat (type 1), non-rhizoid and hard (type 2), round and soft (type 3), and irregularly shaped and soft (type 4). Colonies produced on AO agar differed from these to some extent. The colony types formed on Shieh agar were studied according to molecular characteristics [Amplified Fragment Length Polymorphism (AFLP), Automated Ribosomal Intergenic Spacer Analysis (ARISA), and whole cell protein SDS-PAGE profiles], virulence on rainbow trout fingerlings, and adhesion on polystyrene and fish gills. There were no molecular differences between colony types within one strain. Type 2 was the most adherent on polystyrene, but type 1 was the most virulent. Adhesion of F. columnare strains used in this study was not connected to virulence. From fish infected with colony type 1, three colony types (types 1, 2 and 4) were isolated. Contrary to previous studies, our results suggest that strong adhesion capacity may not be the main virulence factor of F. columnare. Colony morphology change might be caused by phase variation, and different colony types isolated from infected fish may indicate different roles of the colony morphologies in the infection process of columnaris disease.

Control of freshwater fish louse Argulus coregoni: a step towards an integrated management strategy.

Harmful infections by ectoparasites of the genus Argulus occur repeatedly in freshwater fish farming operations where the management has largely been ineffective. Preventative methods and regular monitoring are rarely applied, so that chemical interventions become necessary. According to the Integrated Pest Management (IPM) approach, a sustainable management or control program for a parasite should be based on knowledge of the ecology of the parasite along with adoption of several prevention and control methods, the application of which is dependent upon the prevailing infection level. The application of multiple management tactics is especially important because parasites can develop resistance to chemical treatments. We took a step towards sustainable management of Argulus populations and tested the effect of several types of treatments on survival of A. coregoni at different stages in its life cycle. Parasite juveniles and adults were highly sensitive to potassium permanganate treatments (0.01 g l(-1)), which lead to 100% mortality, whereas treatments with formalin (0.6 ml l(-1)), sodium chloride (20 g l(-1)) or malachite-green/formalin were not effective. Mechanical treatment by shaking infected fish in a hand net was an effective means of detaching parasites from the fish, and resulted in > 80% decreases in parasite numbers. Compared to eggs in control treatments, both drying over a minimum period of 24 h and formalin treatments (120 ml l(-1)) led to significantly higher mortality of A. coregoni eggs. Other treatments, i.e. drying over a period of 15 h, baths in potassium permanganate (1 g l(-1)) or sodium chloride (50 g l(-1)), did not significantly affect the viability of eggs. Based on the present results and previously published papers, we present an initial framework showing how A. coregoni populations could be managed effectively.

Host manipulation by parasites in the world of dead-end predators: adaptation to enhance transmission?

Trophically transmitted parasites often alter their intermediate host's phenotype, thereby predisposing the hosts to increased predation. This is generally considered a parasite strategy evolved to enhance transmission to the next hosts. However, the adaptive value of host manipulation is not clear as it may be associated with costs, such as increased susceptibility to predators that are unsuitable next hosts for the parasites. We examined the ratio between the benefits and costs of host manipulation for transmission success of Acanthocephalus lucii (Acanthocephala), a parasite that alters the hiding behaviour and pigmentation of its isopod hosts. We experimentally compared the susceptibility of infected and uninfected isopods to predation by perch (Perca fluvialis; definitive host of the parasite) and dragonfly larvae (dead end). We found that the parasite predisposed the isopods to predation by both predators. However, the increased predation vulnerability of the infected isopods was higher towards perch. This suggests that, despite the costs due to non-host predation, host manipulation may still be advantageous for the parasite.

Effects of Acanthocephalus lucii (Acanthocephala) on intermediate host survival and growth: implications for exploitation strategies.

Intermediate host exploitation by parasites is presumably constrained by the need to maintain host viability until transmission occurs. The relationship between parasitism and host survival, though, likely varies as the energetic requirements of parasites change during ontogeny. An experimental infection of an acanthocephalan (Acanthocephalus lucii) in its isopod intermediate host (Asellus aquaticus) was conducted to investigate host survival and growth throughout the course of parasite development. Individual isopods were infected by exposure to fish feces containing parasite eggs. Isopods exposed to A. lucii had reduced survival, but only early in the infection. Mean infection intensity was high relative to natural levels, but host mortality was not intensity dependent. Similarly, a group of naturally infected isopods harboring multiple cystacanths did not have lower survival than singly infected isopods. Isopods that were not exposed to the parasite exhibited sexual differences in survival and molting, but these patterns were reversed or absent in exposed isopods, possibly as a consequence of castration. Further, exposed isopods seemed to have accelerated molting relative to unexposed controls. Infection had no apparent effect on isopod growth. The effects of A. lucii on isopod survival and growth undermine common assumptions concerning parasite-induced host mortality and the resource constraints experienced by developing parasites.

Proximate factors affecting the larval life history of Acanthocephalus lucii (Acanthocephala).

The growth and eventual size of larval helminths in their intermediate hosts presumably has a variety of fitness consequences. Therefore, elucidating the proximate factors affecting parasite development within intermediate hosts should provide insight into the evolution of parasite life histories. An experimental infection that resulted in heavy intensities of an acanthocephalan (Acanthocephalus lucii) in its isopod intermediate host (Asellus aquaticus) permitted the examination of parasite developmental responses to variable levels of resource availability and intraspecific competition. Isopods were infected by exposure to egg-containing fish feces, and larval infrapopulations were monitored throughout the course of A. lucii development. The relative rate of parasite growth slowed over time, and indications of resource constraints on developing parasites, e.g., crowding effects, were only observed in late infections. Consequently, the factors likely representative of resource availability to larval parasites (host size and molting rate) primarily affected parasite size in late infections. Moreover, at this stage of infection, competitive interactions, gauged by variation in worm size, seemed to be alleviated by greater resources, i.e., larger hosts that molted more frequently. The relatively rapid, unconstrained growth of young parasites may be worse for host viability than the slower, resource-limited growth of larger parasites.

Morphology, development and probable systematic position of Cercariaeum crassum Wesenberg-Lund, 1934 (Digenea), a parasite of Pisidium amnicum in eastern Finland.

Cercariaeum crassum Wesenberg-Lund, 1934 is redescribed at the cercariaeum stage and the daughter-rediae and cercaria are also described on the basis of new material from Pisidium amnicum collected in the Liikasepuro River (eastern Finland). The species is allocated to the family Allocreadiidae, although its generic affiliation remains unknown. The probable life-cycle (based on the developmental stages observed in daughter-redia) appears to eliminate the cercarial stage and, instead, a cercariaeum (a type of cercaria without a tail) may develop directly from germ balls or, rarely, through the stages of an ophthalmoxiphidiocercaria that transforms into a young caudate cercariaeum. Their morphology and development are shown to be consistent with the family Allocreadiidae. The probable lack of a second intermediate host in the life-cycle is discussed.

Sexual differences in larval life history traits of acanthocephalan cystacanths.

Sexual differences in life history traits, such as size dimorphism, presumably arise via sexual selection and are most readily observed in adults. For complex life-cycle parasites, however, sexual selection may also have consequences for larval traits, e.g., growth in intermediate hosts. Two acanthocephalan species (Acanthocephalus lucii and Echinorhynchus borealis) were studied to determine, whether larval life histories differ between males and females. The size of female A. lucii cystacanths had a much stronger relationship with intermediate host size than males, suggesting females invest more in growth and are consequently more limited by resources. No relationship between host size and cystacanth size was observed for E. borealis. For both species, female cystacanths survived longer in a culture medium composed entirely of salts, which could suggest that females have greater energy reserves than males. A comparative analysis across acanthocephalan species indicated that sexual size dimorphism at the adult stage correlates with cystacanth dimorphism. However, the relationship was not isometric; cystacanths do not reach the same level of sexual dimorphism as adults, possibly due to resource constraints. Our results suggest that larval life histories diverge between males and females in some acanthocephalans, and this is seemingly a consequence of sexual selection acting on adults.