Host diversity and latitude drive trematode diversity patterns in the European freshwater fauna

Aim We investigated the relationship between host and parasite diversity as well as latitudinal gradients in parasite diversity on a continental scale in European freshwater trematodes. Location European freshwaters. Methods We extracted distributional data for 564 freshwater trematodes across 25 biogeographical regions in Europe from the Limnofauna Europaea and used multiple regression analyses to test for correlations between the diversity of definitive (vertebrates) or first intermediate (gastropods) hosts and that of trematodes, and for latitudinal gradients in trematode diversity. In particular, we investigated patterns in beta diversity among latitudinal bands and between trematode species that parasitize host groups with low (autogenic) and high (allogenic) dispersal capacity. We also tested for a latitudinal gradient in the proportional representation of these two trematode groups within regional faunas. Results Latitude or first intermediate host richness had no effect on trematode richness, but definitive host richness was a strong predictor of trematode richness, among both allogenic and autogenic parasites. We found that beta diversity of trematode faunas within latitudinal bands decreased to the north, with similar values for allogenic and autogenic trematodes. Finally, we observed an increasing proportion of autogenic species toward the north of Europe. Main conclusions The richness of definitive hosts appears to be the driver of trematode diversity at a continental scale. The latitudinal gradient in beta diversity reflects patterns observed in free‐living species and probably results from recolonization in the aftermath of the ice ages. The similar beta‐diversity patterns of allogenic and autogenic trematodes and the increasing proportion of autogenic trematodes with increasing latitude are surprising. We suggest that the geographical scale of our analysis or confounding factors such as differences in habitat utilization and specialization may partly explain these patterns.     

Swimming against the current: genetic structure, host mobility and the drift paradox in trematode parasites

Life-cycle characteristics and habitat processes can potentially interact to determine gene flow and genetic structuring of parasitic species. In this comparative study, we analysed the genetic structure of two freshwater trematode species with different life histories using cytochrome c oxidase I gene (COI) sequences and examined the effect of a unidirectional river current on their genetic diversity at 10 sites along the river. We found moderate genetic structure consistent with an isolation-by-distance pattern among subpopulations of Coitocaecum parvum but not in Stegodexamene anguillae. These contrasting parasite population structures were consistent with the relative dispersal abilities of their most mobile hosts (i.e. their definitive hosts). Genetic diversity decreased, as a likely consequence of unidirectional river flow, with increasing distance upstream in C. parvum, which utilizes a definitive host with only restricted mobility. The absence of such a pattern in S. anguillae suggests that unidirectional river flow affects parasite species differently depending on the dispersal abilities of their most mobile host. In conclusion, genetic structure, genetic diversity loss and drift are stronger in parasites whose most mobile hosts have low dispersal abilities and small home ranges. An additional prediction can be made for parasites under unidirectional drift: those parasites that stay longer in their benthic intermediate host or have more than one benthic intermediate hosts would have relatively high local recruitment and hence increased retention of upstream genetic diversity.     

Metazoan parasites of African annual killifish (Nothobranchiidae): abundance,diversity, and their environmental correlates

Estimates of biodiversity and its global patterns are affected by parasite richness and specificity. Despite this, parasite communities are largely neglected in biodiversity estimates, especially in the tropics. We studied the parasites of annual killifish of the genus Nothobranchius that inhabit annually desiccating pools across the African savannah and survive the dry period as developmentally arrested embryos. Their discontinuous, non‐overlapping generations make them a unique organism in which to study natural parasite fauna. We investigated the relationship between global (climate and altitude) and local (pool size, vegetation, host density and diversity, and diversity of potential intermediate hosts) environmental factors and the community structure of killifish parasites. We examined metazoan parasites from 21 populations of four host species (Nothobranchius orthonotus, N. furzeri, N. kadleci, and N. pienaari) across a gradient of aridity in Mozambique. Seventeen parasite taxa were recorded, with trematode larval stages (metacercariae) being the most abundant taxa. The parasites recorded were both allogenic (life cycle includes non‐aquatic host; predominantly trematodes) and autogenic (cycling only in aquatic hosts; nematodes). The parasite abundance was highest in climatic regions with intermediate aridity, while parasite diversity was associated with local environmental characteristics and positively correlated with fish species diversity and the amount of aquatic vegetation. Our results suggest that parasite communities of sympatric Nothobranchius species are similar and dominated by the larval stages of generalist parasites. Therefore, Nothobranchius serve as important intermediate or paratenic hosts of parasites, with piscivorous birds and predatory fish being their most likely definitive hosts.     

Past and present patterns of connectivity among populations of four cryptic species of freshwater mussels Velesunio spp. (Hyriidae) in central Australia

We examined genetic structure and levels of connectivity among subpopulations within each of four cryptic species belonging to the freshwater mussel genus Velesunio. We used allozymes and a fragment of the mitochondrial cytochrome c oxidase I gene to examine genetic variation in populations from isolated waterholes, belonging to four major inland drainages in eastern Australia. Based on evidence from other invertebrates in the region we predicted that, in each species, we would find evidence of historical connectivity among populations from different drainages. This was clearly not the case, as for the two species that occurred in more than one drainage there was evidence of both current and past restriction to gene flow. Moreover, given the potential for extensive dispersal of these mussels through the river systems during flood times via their fish hosts, we predicted low levels of genetic variation among populations from waterholes in the same drainage. Contrary to our expectations, all four species showed some evidence of restricted gene flow among waterholes within drainages. This suggests that either (a) mussel larvae are not produced during flood times, when their fish hosts would be free to move between waterholes, or (b) mussel larvae are attached to their hosts at these times, but the fish movement is limited between waterholes.     

Range size patterns in European freshwater trematodes

While patterns in geographic range sizes in free‐living species have received much attention, little is known on the corresponding patterns in parasites. For the first time, we report on patterns in geographic range sizes and dimensions of endoparasites, using published species lists of freshwater trematodes in 25 biogeographical regions of Europe. In general, the range sizes of trematodes showed a typical hollow curve frequency distribution, with most species having small ranges. Contrary to expectations, there were no differences in range sizes among trematodes using hosts with high (birds) and limited dispersal capacity (e.g. fish). This suggests that the well known importance of host dispersal capacity for parasite dispersal at small spatial scales is overridden by other factors on larger scales. Regression analyses and Rohde plots showed that the relationship between the latitudinal centre and trematode range size was hump‐shaped in all host groups except for reptiles, for which it was linear. Most of the variation fell within the expectations given by null models, suggesting that the patterns mainly result from the geographic properties of the European continent and the biogeographical regions. Finally, trematode ranges tended to stretch more in east‐west than in north‐south directions, indicating dispersal barrier effects for parasite faunas, probably resulting from the geographical idiosyncrasies of the European continent.     

Metazoan parasite species richness and genetic variation among freshwater fish species: cause or consequence?

The factors responsible for the maintenance of genetic variation among natural populations remain a mystery. Recent models of host-parasite co-evolution assume that parasites exert frequency-dependent selection on their hosts by favouring rare alleles that may confer resistance against infection. We tested this prediction in a comparative analysis that sought relationships between levels of genetic variation and the number of metazoan parasite species exploiting each host species. We used data on 40 species of North American freshwater fishes. After controlling for sampling effort and phylogenetic influences, we found no relationship between genetic polymorphism and parasite species richness among fish species. However, we found a marginal negative correlation between parasite species richness and heterozygosity. This result goes against the prediction that increased selective pressure by parasites should be associated with higher levels of genetic variation. Instead, it suggests that parasites may be colonising host species showing low levels of genetic variation with greater success than genetically more variable host species.     

Does a facultative precocious life cycle predispose the marine trematode Proctoeces cf. lintoni to inbreeding and genetic differentiation among host species?

Intraspecific variability in parasite life cycle complexity (number of hosts and species of hosts in the life cycle) may have an impact how parasite genetic variation is partitioned among individual parasites, host individuals or host species within a given area. Among digenean trematodes, a three-host life cycle is common. However, a few species are precocious and may reach sexual maturity in what is typically regarded as the second intermediate host. The objective of this study was to determine whether a precocious life cycle predisposes digeneans to possible inbreeding or genetic subdivision among host species. As a study system, we used the digenean Proctoeces cf. lintoni whose metacercariae precociously mature (facultative) without a cyst wall in the gonads of multiple sympatric species of keyhole limpets (Fissurella spp.), typically regarded as the second intermediate hosts. Genotyped parasites were collected from four species of limpets and the clingfish Sicyases sanguineus, the third and final host where sexual maturity occurs. We found very high microsatellite diversity, Hardy–Weinberg equilibrium over all genotyped individuals, and little to no genetic structuring among parasites collected from the different host species. The fact that metacercariae do not encyst in the keyhole limpets, coupled with the high mixing potential of an aquatic environment, likely promote panmixia in local populations of P. cf. lintoni.     

Can parasites synchronise the population fluctuations of sympatric tetraonids? –examining some minimum conditions

Sympatric populations of tetraonid birds tend to fluctuate in synchrony, at least on local scales. If shared parasites among sympatric populations of different tetraonid species are to operate as a local, synchronizing factor for population fluctuations at least two conditions should be met: i) the host species should share the same (or similar) parasite species, and ii) geographical location should contribute significantly more to the variation in the parasite species composition and abundance than differences among host species. We examined these conditions among subpopulations of sympatric willow ptarmigan and rock ptarmigan and found that host species shared a common pool of parasite species, and geographic location was more important than host species in determining parasite abundance across locations. There was no time lag between density oscillations in the two hosts, suggesting a symmetrical pattern of transmission and maintenance of parasites within habitats governed by the density of hosts and the environment. These findings are consistent with the idea that parasites may play a role in generating synchronous density fluctuations, but large scale experiments are needed to verify this hypothesis.     

Associations between shorebird abundance and parasites in the sand crab, Emerita analoga, along the California coast

I investigated spatial variation in the prevalence and abundance of 4 species of parasites in the sand crab, Emerita analoga, on 8 sandy beaches along 800 km of the California coast, to assess the importance of bird abundance for the distribution of parasites among sand crab populations. I collected sand crabs and counted shorebirds and gulls at each beach during June and November 1994. Sand crabs served as intermediate hosts for 4 species of parasites, including a trematode, Spelotrema nicolli (Cable and Hunnienen, 1938); an acanthocephalan, Polymorphus kenti (Van Cleave, 1947); a nematode, Proleptus sp., and an unidentified trypanorhynch tapeworm. Among sand crab populations, there was substantial spatial variation in the prevalence and abundance of each parasite species. No latitudinal pattern was apparent for any of the 4 species observed. Temporally, parasite prevalence and abundance was significantly different between dates for all 4 parasites. Specifically, sand crab populations experienced higher trematode, nematode, and trypanorhynch prevalence and abundance in November than in June. In contrast, prevalence and abundance of acanthocephalans were higher in June than in November. There were strong positive associations between bird abundance and prevalence of parasitic infection for trematodes and acanthocephalans for some dates but not for nematodes or trypanorhynchs, which use elasmobranchs as definitive hosts. The spatial variation in prevalence and abundance of trematodes and acanthocephalans observed among sand crab populations may be attributed to the distribution and abundance of shorebirds and gulls that serve as definitive hosts.     

Variation in infection parameters among populations within parasite species: intrinsic properties versus local factors

Within any parasite species, variation among populations in standard infection parameters (prevalence, intensity and abundance) is an accepted fact. The proportion of hosts infected and the mean number of parasites per host are not fixed values across the entire geographic range of any parasite species. The question is whether this inter-population variation occurs within a narrow, species-specific range and is thus driven mainly by the biological features of the parasite, or whether it is substantial and unpredictable, leaving population parameters at the mercy of local conditions. Here, the repeatability of estimates of prevalence, intensity and abundance of infection was assessed across populations of the same parasite species, for 77 metazoan parasite species of Canadian freshwater fishes. Overall, parameter values from different populations of the same parasite species were more similar to each other and more different from those of other species, than expected by chance alone. Much of the variation in parameter values in the dataset was associated with differences between parasite species, rather than with differences among populations within species. This was particularly true for intensity and abundance of infection; in contrast, prevalence values, while somewhat repeatable among populations of the same species, still showed considerable variation. Among the higher taxa investigated (monogeneans, trematodes, cestodes, nematodes, acanthocephalans, copepods), there was no evidence that species of one taxon display intrinsically greater variation in population parameters than species of other taxa. Overall, the results suggest that intensity and abundance of infection are real species characters, though somewhat variable. This conclusion supports the view that the biological features of parasite species can potentially override local environmental conditions in driving parasite population dynamics.     

Host dispersal as the driver of parasite genetic structure: a paradigm lost?

Understanding traits influencing the distribution of genetic diversity has major ecological and evolutionary implications for host–parasite interactions. The genetic structure of parasites is expected to conform to that of their hosts, because host dispersal is generally assumed to drive parasite dispersal. Here, we used a meta‐analysis to test this paradigm and determine whether traits related to host dispersal correctly predict the spatial co‐distribution of host and parasite genetic variation. We compiled data from empirical work on local adaptation and host–parasite population genetic structure from a wide range of taxonomic groups. We found that genetic differentiation was significantly lower in parasites than in hosts, suggesting that dispersal may often be higher for parasites. A significant correlation in the pairwise genetic differentiation of hosts and parasites was evident, but surprisingly weak. These results were largely explained by parasite reproductive mode, the proportion of free‐living stages in the parasite life cycle and the geographical extent of the study; variables related to host dispersal were poor predictors of genetic patterns. Our results do not dispel the paradigm that parasite population genetic structure depends on host dispersal. Rather, we highlight that alternative factors are also important in driving the co‐distribution of host and parasite genetic variation.     

THE INFLUENCE OF INTRASPECIFIC VARIATION IN DISPERSAL STRATEGIES ON THE GENETIC STRUCTURE OF PLANTHOPPER POPULATIONS

The hypothesis that levels of gene flow among populations are correlated with dispersal ability has typically been tested by comparing gene flow among species that differ in dispersal abilities, an approach that potentially confounds dispersal ability with other species-specific differences. In this study, we take advantage of geographic variation in the dispersal strategies of two wing-dimorphic planthopper species, Prokelisia marginata and P. dolus, to examine for the first time whether levels of gene flow among populations are correlated with intraspecific variation in dispersal ability. We found that in both of these coastal salt marsh–inhabiting species, population-genetic subdivision, as assessed using allozyme electrophoresis, parallels geographic variation in the proportion of flight-capable adults (macropters) in a population; in regions where levels of macroptery are high, population genetic subdivision is less than in regions where levels of macroptery are low. We found no evidence that geographic variation in dispersal capability influences the degree to which gene flow declines with distance in either species. Thus, both species provided evidence that intraspecific variation in dispersal strategies influences the genetic structure of populations, and that this effect is manifested in population-genetic structure at the scale of large, coastal regions, rather than in genetic isolation by distance within a region. This conclusion was supported by interspecific comparisons revealing that: (1) population-genetic structure (G_ST) of the two Prokelisia species correlated negatively with the mean proportion of flight-capable adults within a region; and (2) there was no evidence that the degree of isolation by distance increased with decreasing dispersal capability. Populations of the relatively sedentary P. dolus clustered by geographic region (using Nei's distances), but this was not the case for the more mobile P. marginata. Furthermore, gene flow among the two major regions we surveyed (Atlantic and Gulf Coasts) has been substantial in P. marginata, but relatively less in P. dolus. The results for P. marginata suggest that differences in the dispersal strategies of Atlantic and Gulf Coast populations occur despite extensive gene flow. We argue that gene flow is biased from Atlantic to Gulf Coast populations, indicating that selection favoring a reduction in flight capability must be intense along the Gulf. Together, the results of this study provide the first rigorous evidence of a negative relationship within a species between dispersal ability and the genetic structure of populations. Furthermore, regional variation in dispersal ability is apparently maintained by selective differences that outweigh high levels of gene flow among regions.     

Genetic variation in the Heterodoxus octoseriatus group (Phthiraptera): a test of Price's model of parasite evolution.

Most of the genetic variation in the H. octoseriatus group is present as fixed gene differences between species which have been described on morphological criteria. Based on allozymes, the taxonomic status of some species was challenged. There was insufficient evidence, however, to demonstrate that these were not 'good' biological species. Overall, the limited intraspecific variation was present as fixed gene differences among lice from different hosts and from different colonies of hosts; heterozygotes were rare. Two predictions derived from Price's model of parasite evolution were met: populations of lice were genetically homogeneous and, where genetic markers were present, we found substantial genetic variation among populations. These data contrast with those for endoparasitic helminths, where, in general, the amount of genetic variation is similar to that of free-living invertebrates.     

The combined influence of trematode parasites and predatory salamanders on wood frog (<Emphasis Type="Italic">Rana sylvatica</Emphasis>) tadpoles

Predators can have important impacts on host–parasite dynamics. For many directly transmitted parasites, predators can reduce transmission by removing the most heavily infected individuals from the population. Less is known about how predators might influence parasite dynamics in systems where the parasite relies on vectors or multiple host species to complete their life cycles. Digenetic trematodes are parasitic flatworms with complex life cycles typically involving three host species. They are common parasites in freshwater systems containing aquatic snails, which serve as obligate first intermediate hosts, and multiple trematode species use amphibians as second intermediate hosts. We experimentally examined the impact of predatory salamanders (Ambystoma jeffersonianum) and trematode parasites (Echinostoma trivolvis and Ribeiroia ondatrae) on short-term survival of wood frog tadpoles (Rana sylvatica) in 150-L outdoor pools. Two trematode species were used in experiments because field surveys indicated the presence of both species at our primary study site. Parasites and predators both significantly reduced tadpole survival in outdoor pools; after 6 days, tadpole survival was reduced from 100% in control pools to a mean of 46% in pools containing just parasites and a mean of 49% in pools containing just predators. In pools containing both infected snails and predators, tadpole survival was further reduced to a mean of 5%, a clear risk-enhancement or synergism. These dramatic results suggest that predators may alter transmission dynamics of trematodes in natural systems, and that a complete understanding of host–parasite interactions requires studying these interactions within the ecological framework of community interactions.     

THE GEOGRAPHY OF COEVOLUTION: COMPARATIVE POPULATION STRUCTURES FOR A SNAIL AND ITS TREMATODE PARASITE

Gene flow and the genetic structure of host and parasite populations are critical to the coevolutionary process, including the conditions under which antagonistic coevolution favors sexual reproduction. Here we compare the genetic structures of different populations of a freshwater New Zealand snail (Potamopyrgus antipodarum) with its trematode parasite (Microphallus sp.) using allozyme frequency data. Allozyme variation among snail populations was found to be highly structured among lakes; but for the parasite there was little allozyme structure among lake populations, suggesting much higher levels of parasite gene flow. The overall pattern of variation was confirmed with principal component analysis, which also showed that the organization of genetic differentiation for the snail (but not the parasite) was strongly related to the geographic arrangement of lakes. Some snail populations from different sides of the Alps near mountain passes were more similar to each other than to other snail populations on the same side of the Alps. Furthermore, genetic distances among parasite populations were correlated with the genetic distances among host populations, and genetic distances among both host and parasite populations were correlated with “stepping-stone” distances among lakes. Hence, the host snail and its trematode parasite seem to be dispersing to adjacent lakes in a stepping-stone fashion, although parasite dispersal among lakes is clearly greater. High parasite gene flow should help to continuously reintroduce genetic diversity within local populations where strong selection might otherwise isolate “host races.” Parasite gene flow can thereby facilitate the coevolutionary (Red Queen) dynamics that confer an advantage to sexual reproduction by restoring lost genetic variation.     

Contrasting definitive hosts as determinants of the genetic structure in a parasite with complex life cycle along the south‐eastern Pacific

The spatial genetic structure (and gene flow) of parasites with complex life cycles, such as digeneans, has been attributed mainly to the dispersion ability of the most mobile host, which most often corresponds to the definitive host (DH). In this study, we compared the genetic structure and diversity of adult Neolebouria georgenascimentoi in two fish species (DHs) that are extensively distributed along the south‐eastern Pacific (SEP). The analysis was based on the cytochrome oxidase subunit I gene sequences of parasites collected between 23°S and 45°S. In total, 202 sequences of N. georgenascimentoi in Pinguipes chilensis isolated from nine sites and 136 sequences of Prolatilus jugularis from five sites were analysed. Our results showed that N. georgenascimentoi is a species complex that includes three different parasite species; however, in this study, only lineage 1 and 2 found in P. chilensis and P. jugularis, respectively, were studied because they are widely distributed along the coastline. Lineage 1 parasites had two common haplotypes with wide distribution and unique haplotypes in northern sites. Lineage 2 had only one common haplotype with wide distribution and a large number of unique haplotypes with greater genetic diversity. Both lineages have experienced recent population expansion. Only lineage 1 exhibited a genetic structure that was mainly associated with a biogeographical break at approximately 30°S along the SEP. Our finding suggests that host access to different prey (=intermediate hosts) could affect the genetic structure of the parasite complex discovered here. Consequently, difference between these patterns suggests that factors other than DH dispersal are involved in the genetic structure of autogenic parasites.     

Variation in life cycle affects the distance decay of similarity among bluegill sunfish parasite communities

Rates of community similarity decay were examined among parasite communities of the bluegill sunfish from isolated populations located throughout the midwestern and southeastern United States. Rates of decay were compared for assemblages composed of all species, autogenic species only, and allogenic species only. Rates of similarity decay were calculated by regressing a matrix of intercommunity distance against a matrix of intercommunity Sorenson similarity for each group of species. Significance was determined with the use of a Mantel test using 1,000 permutations. Significant rates of decay were found for all groups; however, allogenic similarity decayed according to an exponential function, whereas autogenic similarity decayed according to a linear function. Overall, the results suggest that autogenic-allogenic status is an important factor determining the rate at which similarity decays, but illustrate the need for more insight regarding the factors that affect parasite dispersal ability.     

Bottom–up regulation of parasite population densities in freshwater ecosystems

Theory predicts the bottom–up coupling of resource and consumer densities, and epidemiological models make the same prediction for host–parasite interactions. Empirical evidence that spatial variation in local host density drives parasite population density remains scarce, however. We test the coupling of consumer (parasite) and resource (host) populations using data from 310 populations of metazoan parasites infecting invertebrates and fish in New Zealand lakes, spanning a range of transmission modes. Both parasite density (no. parasites per m²) and intensity of infection (no. parasites per infected hosts) were quantified for each parasite population, and related to host density, spatial variability in host density and transmission mode (egg ingestion, contact transmission or trophic transmission). The results show that dense and temporally stable host populations are exploited by denser and more stable parasite populations. For parasites with multi‐host cycles, density of the ‘source’ host did not matter: only density of the current host affected parasite density at a given life stage. For contact‐transmitted parasites, intensity of infection decreased with increasing host density. Our results support the strong bottom–up coupling of consumer and resource densities, but also suggest that intraspecific competition among parasites may be weaker when hosts are abundant: high host density promotes greater parasite population density, but also reduces the number of conspecific parasites per individual host.     

Dispersal in a parasitic worm and its two hosts: consequence for local adaptation

Characterizing host and parasite population genetic structure and estimating gene flow among populations is essential for understanding coevolutionary interactions between hosts and parasites. We examined the population genetic structure of the trematode Schistosoma mansoni and its two host species (the definitive host Rattus rattus and the intermediate host Biomphalaria glabrata) using microsatellite markers. Parasites were sampled from rats. The study was conducted in five sites of the Guadeloupe Island, Lesser Antilles. Mollusks display a pattern of isolation by distance whereas such a pattern is not found neither in schistosomes nor in rats. The comparison of the distribution of genetic variability in S. mansoni and its two host species strongly suggests that migration of parasites is principally determined by that of the vertebrate host in the marshy focus of Guadeloupe. However, the comparison between genetic differentiation values in schistosomes and rats suggests that the efficacy of the schistosome rat-mediated dispersal between transmission sites is lower than expected given the prevalence, parasitic load and migration rate of rats among sites. This could notably suggest that rat migration rate could be negatively correlated to the age or the infection status of individuals. Models made about the evolution of local adaptation in function of the dispersal rates of hosts and parasites suggest that rats and mollusks should be locally adapted to their parasites.     

The scaling of parasite biomass with host biomass in lake ecosystems: are parasites limited by host resources?

The standing crop biomass of different populations or trophic levels reflects patterns of energy flow through an ecosystem. The contribution of parasites to total biomass is often considered negligible; recent evidence suggests otherwise, although it comes from a narrow range of natural systems. Quantifying how local parasite biomass, whether that of a single species or an assemblage of species sharing the same host, varies across localities with host population biomass, is critical to determine what constrains parasite populations. We use an extensive dataset on all free‐living and parasitic metazoan species from multiple sites in New Zealand lakes to measure parasite biomass and test how it covaries with host biomass. In all lakes, trematodes had the highest combined biomass among parasite taxa, ranging from about 0.01 to 0.25 g m^?2, surpassing the biomass of minor free‐living taxa. Unlike findings from other studies, the life stage contributing the most to total trematode biomass was the metacercarial stage in the second intermediate host, and not sporocysts or rediae within snail first intermediate hosts, possibly due to low prevalence and small snail sizes. For populations of single parasite species, we found no relationship between host and parasite biomass for either juvenile or adult nematodes. In contrast, all life stages of trematodes had local biomasses that correlated positively with those of their hosts. For assemblages of parasite species sharing the same host, we found strong relationships between local host population biomass and the total biomass of parasites supported. In these host–parasite biomass relationships, the scaling factor (slope in log‐log space) suggests that parasites may not be making full use of available host resources. Host populations appear capable of supporting a little more parasite biomass, and may be open to expansion of existing parasites or invasion by new ones.