Hidden parasite diversity in a European freshwater system

Modern Strategies for Diagnosis and Treatment of Parasitic Diseases

Parasites are very widely distributed in the environment and form complex relationships with their hosts, forming host-parasite systems [...].

Characterisation of Nematoda and Digenea in selected Australian freshwater snails

Freshwater snails are integral to local ecosystems as a primary food source for various vertebrate species, thereby contributing significantly to ecological food webs. However, their role as intermediate hosts also makes them pivotal in the transmission of parasites. In Australia, research on freshwater snails has predominantly focused on their role as intermediate hosts for livestock parasites, while there has been limited exploration of the impact of these parasites on snail health and population dynamics. The aim of this study was to determine parasitic infection in freshwater snails. This study was conducted in the south-eastern region of Australia, in 2022. A total of 163 freshwater snails from four different species were collected and examined in the Murrumbidgee catchment area in the southeastern part of Australia during the Southern Hemisphere summer and autumn months (February to May). The species included Isidorella hainesii, Glyptophysa novaehollandica, Bullastra lessoni (endemic species), and Physella acuta (an introduced species). Through the analysis of sequence data from the various regions of the nuclear ribosomal DNA, we determined that the Digenea species in this study belonged to three distinct species, including Choanocotyle hobbsi, Petasiger sp. and an unidentified species belonging to Plagiorchioidea. Additionally, analysis of the sequences from Nematoda found in this study, revealed they could be categorized into two separate taxa, including Krefftascaris sp. and an unidentified nematode closely associated with plant and soil nematodes. This research holds significant implications for the future understanding and conservation of Australian freshwater ecosystems. Most parasites found in the present study complete their life cycle in snails and turtles. As many of freshwater snail and turtle species in Australia are endemic and face population threats, exploring the potential adverse impacts of parasitic infections on snail and turtle health, is crucial for advancing our understanding of these ecosystems and also paving the way for future research and conservation efforts. While none of the native snail species in the present study have been listed as endangered or threatened, this may simply be attributed to the absence of regular population surveys.

Environmental influence on abundance and infection patterns of snail intermediate hosts of liver and intestinal flukes in North and Central Vietnam

Liver and intestinal flukes (LIF) are important groups of foodborne zoonotic trematodes (FZTs) in Southeast Asia, including Vietnam. Their complex life cycles require specific freshwater snail species as the obligatory first intermediate hosts. In 2019, we conducted a longitudinal study in Yen Bai and Thanh Hoa provinces in North and Central Vietnam, respectively, to investigate the diversity of LIF and their infection prevalence in relation to snail host abundance and environmental factors. Using a combination of morphological and molecular identification techniques, we identified 10 LIF species infecting 11 snail host species. We observed significant seasonal variation in the mean abundance of several snail host species, with the majority of snails collected during the spring. We also detected seasonal changes in LIF species composition, with the highest species richness reported in the spring. Clonorchis sinensis and Fasciola gigantica, two medically important human liver flukes in Asia, were found only in the spring in Yen Bai. Our study revealed that not all snail host species have the same probability of becoming infected, and we recorded seasonal variations in the prevalence of LIF infection in different snail species in relation to water parameters.

Checklist of digeneans (Platyhelminthes, Trematoda, Digenea) of Georgia

Background

In the present study, we aim to provide an inventory of digenetic trematodes (Platyhelminthes, Trematoda, Digenea) from Georgia including records from the freshwater, marine and terrestrial realms. The checklist is based on a critical review of data from 109 papers, 11 monographs and four Ph.D. theses published between 1935 and 2019 and our new records. The checklist includes information on synonymy, the host species, site of infection, geographical distribution and bibliographical references. The present data will serve as a baseline for further studies on trematodes from Georgia focused on integrative taxonomy, life-cycle elucidation, parasite ecology and epidemiology.

New information

We compiled data on the digenean trematode fauna of Georgia, which is represented by 186 species (of these 173 identified to species level) belonging to 108 genera, 47 families and 17 superfamilies. This is the first checklist of the digeneans of Georgia. The majority of digenean species were recorded as adults (160 species), only a small fraction being found as cercariae (33 species) or metacercariae (24 species), in their first or second intermediate hosts, respectively. Predominantly, records of trematodes (62 species) from birds were found, followed by those parasitising fish (50 species, i.e. 32 species as adults and 18 as metacercariae), mammals (33 species) and amphibians (25 species, i.e. 23 species as adults and 2 as metacercariae), with the least number of species reported from reptiles (12 species, i.e. 9 species as adults and 3 as metacercariae). Adult digeneans recorded together with another life-cycle stage (metacercariae and/or cercariae) comprised 28 species, i.e. for 15% of the total trematode species number, a part of their life-cycle is known.

Molecular epidemiological analyses reveal extensive connectivity between Echinostoma revolutum (sensu stricto) populations across Eurasia and species richness of zoonotic echinostomatids in England

Echinostoma revolutum (sensu stricto) is a widely distributed member of the Echinostomatidae, a cosmopolitan family of digenetic trematodes with complex life cycles involving a wide range of definitive hosts, particularly aquatic birds. Integrative taxonomic studies, notably those utilising nad1 barcoding, have been essential in discrimination of E. revolutum (s.s.) within the 'Echinostoma revolutum' species complex and investigation of its molecular diversity. No studies, however, have focussed on factors affecting population genetic structure and connectivity of E. revolutum (s.s.) in Eurasia. Here, we used morphology combined with nad1 and cox1 barcoding to determine the occurrence of E. revolutum (s.s.) and its lymnaeid hosts in England for the first time, in addition to other echinostomatid species Echinoparyphium aconiatum, Echinoparyphium recurvatum and Hypoderaeum conoideum. Analysis of genetic diversity in E. revolutum (s.s.) populations across Eurasia demonstrated haplotype sharing and gene flow, probably facilitated by migratory bird hosts. Neutrality and mismatch distribution analyses support possible recent demographic expansion of the Asian population of E. revolutum (s.s.) (nad1 sequences from Bangladesh and Thailand) and stability in European (nad1 sequences from this study, Iceland and continental Europe) and Eurasian (combined data sets from Europe and Asia) populations with evidence of sub-population structure and selection processes. This study provides new molecular evidence for a panmictic population of E. revolutum (s.s.) in Eurasia and phylogeographically expands the nad1 database for identification of echinostomatids.

DNA Barcoding of Trichobilharzia (Trematoda: Schistosomatidae) Species and Their Detection in eDNA Water Samples

We designed and tested species-specific PCR primers to detect Trichobilharzia species via environmental DNA (eDNA) barcoding in selected Austrian water bodies. Tests were performed with eDNA samples from the field as well as with artificial samples from the lab, where snails releasing cercariae were kept in aquariums. From two localities, Trichobilharzia was documented based on the release of cercariae from snails, enabling morphological species identification. In both cases, the corresponding species were detected via eDNA: Trichobilharzia szidati and Trichobilharzia physellae. Nonetheless, the stochasticity was high in the replicates. PCR tests with aquarium water into which the cercariae had been released allowed eDNA detection even after 44 days. As in the PCRs with eDNA samples from the field, positive results of these experiments were not obtained for all samples and replicates. PCR sensitivity tests with dilution series of T. szidati genomic DNA as well as of PCR amplification products yielded successful amplification down to concentrations of 0.83 pg/µL and 0.008 pg/µL, respectively. Our results indicate that the presumed species specificity of PCR primers may not be guaranteed, even if primers were designed for specific species. This entails misidentification risks, particularly in areas with incomplete species inventories.

Diversity of trematodes from the amphibian anomaly P hotspot: Role of planorbid snails

Trematode infection of the second intermediate hosts can lead to changes in their fitness and, as a result, a change in the invasion rate of animal communities. It is especially pronounced during the invasion of parasite species that reduce activity due to the manipulation of hosts through the changes of their morphology and physiology. One of these cases is an anomaly P syndrome hotspot found in some populations of water frogs and toads in Europe caused by the trematode Strigea robusta metacercariae. The occurrence of pathogen and their participation in ecosystems are intrigues questions in the anomaly P phenomenon, as well as the role of planorbid snails that serve as the first intermediate hosts for many trematode species. Herein, we focused on trematodes spectra from planorbid snails and amphibians from the anomaly P hosts with the aim to undetected interactions between the pathways of parasites. Emerging cercariae of 6802 planorbid snails of dominant species (Planorbarius corneus, Planorbis planorbis, and Anisus spp.) were detected by both morphological and molecular methods in seven waterbodies in Privolzhskaya Lesostep Nature Reserve (Russia). A total of 95 sequences of 18 species were received, and 48 sequences were unique and did not present in any genetic databases. The 18 species of trematodes from snails and 14 species of trematodes from amphibian hosts (Pelophylax ridibundus; Ranidae; Anura) were detected. Three species (Echinostoma nasincovae, Tylodelphys circibuteonis and Australapatemon burti) was new for the trematode fauna of the Middle Volga River region and Russia as a whole. Eleven species of parasitic flatworms have amphibians in their life cycles and nine species used amphibians as metacercariae hosts: Echinostoma nasincovae, E. miyagawai, Echinoparyphium recurvatum, Tylodelphys circibuteonis, Neodiplostomum spathula, Paralepoderma cloacicola, Macrodera longicollis, Strigea robusta, and Strigea strigis. The occurrence of trematode species from planorbid mollusks and frogs were compared.

Life history strategies of Cotylurus spp. Szidat, 1928 (Trematoda, Strigeidae) in the molecular era – Evolutionary consequences and implications for taxonomy

Species of Cotylurus Szidat, 1928 (Diplostomoidea: Strigeidae) are highly specialized digeneans that parasitize the gastrointestinal tract and bursa of Fabricius of water and wading birds. They have a three-host life cycle; the role of first intermediate host is played by pulmonate snails, while a wide range of water snails (both pulmonate and prosobranch) and leeches are reported as second intermediate hosts.

Unfortunately, species richness, molecular diversity and phylogeny of metacercariae of Cotylurus spp. (tetracotyle) occurring in snails remain poorly understood. Thus, we have performed the parasitological and taxonomical examination of tetracotyles form freshwater snails from Poland, supplemented with adult Strigeidae specimens sampled from water birds. In this study we report our use of recently obtained sequences of two molecular markers (28S nuclear large ribosomal subunit gene (28S rDNA) and the cytochrome c oxidase subunit 1 (CO1) fragment), supplemented by results of a method of species delimitation (GMYC) and haplotype analysis to analyse some aspects of the ecology, taxonomy, and phylogeny of members of the genus Cotylurus.

The provided phylogenetic reconstructions discovered unexpectedly high molecular diversity within Cotylurus occurring in snails, with clearly expressed evidence of cryptic diversity and the existence of several novel-species lineages. The obtained results revealed the polyphyletic character of C. syrius Dubois, 1934 (with three separate molecular species-level lineages) and C. cornutus (Rudolphi, 1809) Szidat, 1928 (with four separate molecular species-level lineages). Moreover, we demonstrated the existence of two divergent phylogenetical and ecological lineages within Cotylurus (one using leeches and other snails as second intermediate hosts), differing significantly in their life history strategies.

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Within Cotylurus existed two divergent phylogenetical and ecological lineages.

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The lineages within Cotylurus sp. differing significantly in their routes of transmission.

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Molecular analysis revealed high genetic diversity with evidence of cryptic species.

Parasite infection influences the biomarker response and locomotor activity of Gammarus fossarum exposed to conventionally-treated wastewater

Modern wastewater treatment plants cannot completely remove pollutants. Often, effluents entering the aquatic environment still contain micropollutants such as pharmaceuticals or pesticides, which may impose adverse effects on aquatic biota. At the same time, a large proportion of free-living aquatic species are known to be infected with parasites, which raises the question of interactions between environmental stressors (such as micropollutants) and parasite infection. We chose the freshwater amphipod Gammarus fossarum (Koch, 1835) as a test organism to investigate potential pollutant-parasite interactions. This gammarid is frequently used in ecotoxicological tests and is also commonly infected with larvae of the acanthocephalan parasite species Polymorphus minutus (Zeder, 1800) Lühe, 1911. We exposed infected and uninfected specimens of G. fossarum to conventionally-treated wastewater and river water in a 22-day flow channel experiment. The test organisms' response was measured as mortality rates, concentrations or activities of five biomarkers, and overall locomotor activity. No significant differences were found between mortality rates of different exposure conditions. Contrastingly, three biomarkers (phenoloxidase activity, glycogen, and lipid concentrations) showed a significant increase in infected gammarids, while the effect of the water type was insignificant. Infected gammarids also showed a significantly higher locomotor activity in both water types. Our results suggest that the response of G. fossarum during the exposure experiments was mainly driven by parasite infection. This implies that parasites may act as additional biotic stressors in multiple stressor scenarios, and therefore, might play an important role when measuring the response of organisms to chemical stressors. Future ecotoxicological studies and assessments thus should consider parasite infection as an additional test parameter.

Rare inventory of trematode diversity in a protected natural reserve

In the face of ongoing habitat degradation and the biodiversity crisis, natural reserves are important refuges for wildlife. Since most free-living organisms serve as hosts to parasites, the diverse communities in protected areas can be expected to provide suitable habitats for a species-rich parasite fauna. However, to date, assessments of parasite diversity in protected nature reserves are rare. To expand our knowledge of parasite communities in natural habitats, we examined 1994 molluscs belonging to 15 species for trematode infections in a central European natural reserve. The parasitological examination revealed an overall prevalence of 17.3% and a total species richness of 40 trematode species. However, the parasite diversity and prevalence did not differ markedly from trematode communities in non-protected environments, which might be partly explained by a dilution effect caused by a high number of non-host organisms in our study system. The proportion of complex and long life cycles of parasites in the present study is high, indicating complex biotic interactions. We conclude that life cycle complexity, in addition to parasite diversity and trematode species richness, can provide valuable information on ecosystem health and should therefore be considered in future studies.

Temporal dynamics of freshwater planktonic parasites inferred using a DNA metabarcoding time-series

Parasites are important components of biodiversity and contributors to ecosystem functioning, but are often neglected in ecological studies. Most studies examine model parasite systems or single taxa, thus our understanding of community composition is lacking. Here, the seasonal and annual dynamics of parasites was quantified using a 5-year metabarcoding time-series of freshwater plankton, collected weekly. We first identified parasites in the dataset using literature searches of the taxonomic match and using sequence metadata from the National Center for Biotechnology Information (NCBI) nucleotide database. In total, 441 amplicon sequence variants (belonging to 18 phyla/clades) were classified as parasites. The four phyla/clades with the highest relative read abundance and richness were Chytridiomycota, Dinoflagellata, Oomycota and Perkinsozoa. Relative read abundance of total parasite taxa, Dinoflagellata and Perkinsozoa significantly varied with season and was highest in summer. Parasite richness varied significantly with season and year, and was generally lowest in spring. Each season had distinct parasite communities, and the difference between summer and winter communities was most pronounced. Combining DNA metabarcoding with searches of the literature and NCBI metadata allowed us to characterize parasite diversity and community dynamics and revealed the extent to which parasites contribute to the diversity of freshwater plankton communities.

Morphological and molecular assessment of the diversity of trematode communities in freshwater gastropods and bivalves in Los Tuxtlas tropical rainforest

Trematode life cycles involve molluscs as first intermediate hosts. The identification of trematodes based on the morphology of cercariae released from molluscs is challenging because the identification relies on adult forms obtained from their definitive hosts. Several studies have recently genetically characterized these larval forms establishing a link with adults sampled from their vertebrate hosts, allowing their identification at species level. In Los Tuxtlas tropical rainforest, in south-eastern Mexico, 57 species of trematodes have been reported from wildlife vertebrates; however, studies evaluating the diversity of trematode cercariae in molluscs are lacking. Here, we studied 11 species of molluscs to assess the diversity of cercariae in two lakes of Los Tuxtlas, Veracruz, Mexico; six of the 11 species were infected. Twelve cercarial morphotypes were collected and characterized morphologically. Sequences of the 28S ribosomal RNA gene were generated to allocate each morphotype into a family using the most recent phylogenetic classification of the Digenea as a framework; molecular work revealed 16 genetic lineages; some cercariae were identified up to genus (Apharyngostrigea, Ascocotyle, Centrocestus, Echinochasmus, Lecithodendrium and Posthodiplostomum), and some to species levels (Gorgoderina rosamondae, Langeronia macrocirra, Oligogonotylus manteri and Phyllodistomum inecoli) based on their phylogenetic position within the tree, and the genetic distance with respect to other sequenced congeners. Therefore, the cercarial morphotypes in the present study represent at least 16 putative species. Our study contributes to a better understanding of the trematode diversity in an area of high vertebrate species diversity, and to the knowledge of trematode life cycles.

Diversity of echinostomes (Digenea: Echinostomatidae) in their snail hosts at high latitudes

The biodiversity of freshwater ecosystems globally still leaves much to be discovered, not least in the trematode parasite fauna they support. Echinostome trematode parasites have complex, multiple-host life-cycles, often involving migratory bird definitive hosts, thus leading to widespread distributions. Here, we examined the echinostome diversity in freshwater ecosystems at high latitude locations in Iceland, Finland, Ireland and Alaska (USA). We report 14 echinostome species identified morphologically and molecularly from analyses of nad1 and 28S rDNA sequence data. We found echinostomes parasitising snails of 11 species from the families Lymnaeidae, Planorbidae, Physidae and Valvatidae. The number of echinostome species in different hosts did not vary greatly and ranged from one to three species. Of these 14 trematode species, we discovered four species (Echinoparyphium sp. 1, Echinoparyphium sp. 2, Neopetasiger sp. 5, and Echinostomatidae gen. sp.) as novel in Europe; we provide descriptions for the newly recorded species and those not previously associated with DNA sequences. Two species from Iceland (Neopetasiger islandicus and Echinoparyphium sp. 2) were recorded in both Iceland and North America. All species found in Ireland are new records for this country. Via an integrative taxonomic approach taken, both morphological and molecular data are provided for comparison with future studies to elucidate many of the unknown parasite life cycles and transmission routes. Our reports of species distributions spanning Europe and North America highlight the need for parasite biodiversity assessments across large geographical areas.

Trematode diversity in freshwater snails from a stopover point for migratory waterfowls in Hokkaido, Japan: An assessment by molecular phylogenetic and population genetic analyses

The cryptic diversity of trematodes was evaluated in the Nagayama-Shinkawa River, an artificial canal of the Ishikari River System of Hokkaido, Japan. Numerous migratory waterfowls use the canal as a stopover point in every spring season. The lymnaeid snail, Radix auricularia, and the semisulcospirid snail, Semisulcospira libertina, colonize the static and flowing water areas, respectively. The trematode fauna of the two snails was assessed by molecular phylogenetic and population genetic analyses. Each of distinctive clades in mitochondrial DNA trees was arbitrarily set as a species. In total, 14 species of the families Diplostomidae, Echinostomatidae, Notocotylidae, Plagiorchiidae, and Strigeidae occurred in R. auricularia, wherease S. libertina harbored 10 species of the families Echinochasmidae, Heterophyidae, Notocotylidae, and Lecithodendridae and Cercaria creta, an unclassified species whose adult stage is still unknown. The species diversity of the larval trematodes could be recognized as a "hot spot", suggesting that the seasonal visit of waterfowls is very important to spread trematodes and to keep their diversity. A high intraspecific genetic diversity was observed in the echinostomatid, notocotylid, echinochasmid, and heterophyid species, whose definitive hosts include birds. It seems likely that each of the parasite populations is always disturbed by repeated visits of waterfowls.

Cercarial trematodes in freshwater snails from Bangkok, Thailand: prevalence, morphological and molecular studies and human parasite perspective

We investigated the prevalence, morphological characters and molecular classifications of trematode cercariae in freshwater snails randomly collected from 59 sampling localities in Bangkok from May 2018 to March 2019. We used a crushing technique to observe the cercarial stage inside each snail body and amplified the internal transcribed spacer 2 regions of cercarial DNA using polymerase chain reaction methodology. The associated phylogenetic tree was reconstructed using Bayesian inference analyses. A total of 517 of 15 621 examined snails were infected with trematode cercariae, and the infected snails were classified into 11 species of seven families with a 3.31% overall prevalence of the infection. The Bithynia siamensis siamensis snail displayed the highest prevalence of infection (16.16%), whereas the Physella acuta snail exhibited the lowest prevalence (0.08%) of infection. Eight morphological types of cercariae were observed. The highest prevalence of infection was observed in mutabile cercaria (1.86%). Based on molecular investigations, the phylogram revealed eight cercaria types assigned to at least nine digenean trematode families, of which five belong to groups of human intestinal flukes. Although, with the exception of schistosome cercaria, trematode cercariae are not known to directly damage humans, understanding the general biology of trematode cercariae (including diversity, distribution, infection rates and host range) is important and necessary for the prevention and control of parasitic transmission that impacts aquatic cultivations, livestock farming and human health.

Trematode diversity reflecting the community structure of Danish freshwater systems: molecular clues

Background

Digenean trematodes are parasitic platyhelminths that use several hosts in their life cycles and are thereby embedded in various ecosystems affected by local environmental conditions. Their presence in a habitat will reflect the presence of different host species and, as such, they can serve as ecological indicators. Only limited information on the occurrence of trematodes and their link to other trophic levels in the Danish freshwater ecosystems is currently available.Therefore, the main aim of the present study was to increase our knowledge in this field.

Methods

Snails were sampled from 21 freshwater lakes in Denmark, following which shedding procedures were performed, cercariae were recoved and the released parasites were identified using molecular tools (PCR and sequencing).

Results

A total of 5657 snail hosts belonging to ten species were identified, revealing a highly diverse parasite fauna comprising 22 trematode species. The overall trematode prevalence was 12.6%, but large variations occurred between host species. The snail host Lymnaea stagnalis showed the highest prevalence and also exhibited the highest diversity, accounting for 47.6% of the species richness.

Conclusions

This survey contributes updated information on parasite–host relations and compatibility and may assist in describing the ecological structure of the investigated Danish freshwater ecosystems.

Some like it hotter: trematode transmission under changing temperature conditions

Climate change-related increases in temperature will influence the interactions between organisms, including the infection dynamics of parasites in ecosystems. The distribution and transmission of parasites are expected to increase with warmer temperature, but to what extent this will affect closely related parasite taxa living in sympatry is currently impossible to predict, due to our extremely limited understanding of the interspecific variation in transmission potential among parasite species in changing ecosystems. Here, we analyse the transmission patterns of four trematode species from the New Zealand mudsnail Potamopyrgus antipodarum with different life cycles and transmission strategies under two temperature scenarios, simulating current and future warmer temperatures. In a comparative experimental study, we investigated the effects of temperature on the productivity, movement and survival of the parasites' transmission stages (cercariae) to quantify the net effect of temperature on their overall transmission potential. Our results show that increases in temperature positively affect cercarial transmission dynamics, yet these impacts varied considerably between the cercariae of different trematode species, depending on their host-searching behaviour. These different species-specific transmission abilities as well as the varying individual patterns of productivity, activity and longevity are likely to have far-reaching implications for disease dynamics in changing ecosystems, since increases in temperature can shift parasite community structure. Due to the parasites' capacity to regulate the functioning of whole ecological communities and their potential impact as disease agents, understanding these species-specific parasite transmission traits remains a fundamental requirement to predict parasite dynamics under changing environmental conditions.

Stable isotope analysis spills the beans about spatial variance in trophic structure in a fish host - parasite system from the Vaal River System, South Africa

Stable isotope analysis offers a unique tool for comparing trophic interactions and food web architecture in ecosystems based on analysis of stable isotope ratios of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) in organisms. Clarias gariepinus were collected from six sites along the Vaal River, South Africa and were assessed for ectoparasites and endoparasites. Lamproglena clariae (Copepoda), Tetracampos ciliotheca and Proteocephalus glanduligerus (Cestoda), and larval Contracaecum sp. (Nematoda) were collected from the gills, intestine and mesenteries, respectively. Signatures of δ¹³C and δ¹⁵N were analysed in host muscle tissue and parasites using bulk stable isotope analysis. Variable stable isotope enrichment between parasites and host were observed; L. clariae and the host shared similar δ¹⁵N signatures and endoparasites being depleted in δ¹³C and δ¹⁵N relative to the host. Differences in stable isotope enrichment between parasites could be related to the feeding strategy of each parasite species collected. Geographic and spatial differences in enrichment of stable isotopes observed in hosts were mirrored by parasites. As parasites rely on a single host for meeting their nutritional demands, stable isotope variability in parasites relates to the dietary differences of host organisms and therefore variations in baseline stable isotope signatures of food items consumed by hosts.

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Stable isotope enrichment in parasites was variable compared to the host.

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Host fish and Lamproglena clariae shared similar trophic level.

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Endoparasites were depleted in δ¹³C and δ¹⁵N compared to the host.

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Spatial differences in host stable isotopes were mirrored by parasites and related to host diet.