Communities of metazoan parasites in seven sympatric skate species (Elasmobranchii, Rajidae) from the English Channel and Celtic Sea differing in conservation status.

Elasmobranch populations are in steep decline mainly due to overfishing bycatch, but parasites may accelerate the collapse of vulnerable and/or highly parasitized species. We therefore studied metazoan parasites of Rajidae from the northeast Atlantic: vulnerable Leucoraja fullonica, near-threatened Raja brachyura, Raja clavata, Raja microocellata and Raja undulata, and least-concerned Raja montagui and Leucoraja naevus. Overall prevalence varied from 19% for R. montagui to 100% for L. fullonica. Parasite communities differed between skate species, and prevalence and abundance were higher for L. fullonica, R. microocellata, and R. undulata. We recorded 11 parasite taxa in the study: three nematodes, six cestodes, one monogenean, and one myxosporean. Whatever the skate species, the parasite component community comprised at least two nematode taxa among Phocanema spp., Proleptus sp. and Anisakis simplex. DNA-sequencing revealed that Phocanema azarasi and Phocanema krabbei both occurred in R. microocellata and R. undulata. Phocanema spp. was first recorded in L. fullonica, L. naevus, R. microocellata, R. montagui, and R. undulata, as Proleptus sp. in L. fullonica, and A. simplex in L. fullonica and R. clavata, Rockacestus sp. and Nybelinia sp. in R. undulata, and gill-myxosporeans on L. fullonica, L. naevus, R. microocellata, and R. undulata. The occurrence of 16 new host-parasite associations suggests potential environmental changes. Information provided by trophically transmitted helminths confirmed an opportunistic skate diet based on crustaceans and fish. We discuss results in terms of host fitness loss, bioindicator role of parasites, and anisakiasis risk. We recommend incorporating parasitology in research to improve elasmobranch conservation.

Low similarity between parasite communities of ten sympatric carangid species.

Host phylogeny and ecological convergence are two factors thought to influence the structure of parasite communities. The aims of this study were to determine the diversity of metazoan parasites of 10 sympatric fish species of the family Carangidae from the southeastern Gulf of California, and to analyze their similarity at infracommunity and component community levels, in order to determine if the host species, particularly those congeneric with similar ecological characteristics, exhibit similar assemblages of parasites. In total, 874 fish specimens were examined and 40 parasite species were identified. The component community was composed by 21 parasite species in Caranx caninus, 20 in C. caballus, 11 in C. vinctus, five in Chloroscombrus orqueta, four in Carangoides otrynter, seven in Hemicaranx leucurus, eight in Selene brevoortii, 14 in S. peruviana, and 11 in Trachinotus rhodopus. The metazoan parasite communities of C. vinctus, Ch. orqueta, H. leucurus, and S. brevoortii are reported here for the first time. The parasite communities of the remaining six carangid species have been reported from regions other than the Gulf of California. All fish species differed significantly regarding the diversity of their parasite infracommunities. This possibly is due to different patterns of habitat use among fish species, and because of the differential host specificity among parasite taxa. Nonetheless, when the analysis was restricted to common parasite species, some fish showed similar parasite infracommunities, particularly congeners of the genus Selene as well as C. caballus and C. vinctus. The component communities of species of Selene were highly similar (>65%), but the three species of Caranx were not. This result supports the hypothesis that congeneric fish species with similar ecological filters harbor similar parasite communities. However, the difference observed between C. caninus and C. caballus suggests that these species, despite being evolutionary and ecologically related, have different physiological or immunological characteristics (compatibility filters) that may result in different parasite communities.

Teleost innate immunity, an intricate game between immune cells and parasites of fish organs: who wins, who loses.

Fish, comprising over 27,000 species, represent the oldest vertebrate group and possess both innate and adaptive immune systems. The susceptibility of most wild fish to parasitic infections and related diseases is well-established. Among all vertebrates, the digestive tract creates a remarkably favorable and nutrient-rich environment, which, in turn, renders it susceptible to microparasites and macroparasites. Consequently, metazoan parasites emerge as important disease agents, impacting both wild and farmed fish and resulting in substantial economic losses. Given their status as pathogenic organisms, these parasites warrant considerable attention. Helminths, a general term encompassing worms, constitute one of the most important groups of metazoan parasites in fish. This group includes various species of platyhelminthes (digeneans, cestodes), nematodes, and acanthocephalans. In addition, myxozoans, microscopic metazoan endoparasites, are found in water-dwelling invertebrates and vertebrate hosts. It is worth noting that several innate immune cells within the fish alimentary canal and certain visceral organs (e.g., liver, spleen, and gonads) play active roles in the immune response against parasites. These immune cells include macrophages, neutrophils, rodlet cells, and mast cells also known as eosinophilic granular cells. At the site of intestinal infection, helminths often impact mucous cells number and alter mucus composition. This paper presents an overview of the state of the art on the occurrence and characteristics of innate immune cells in the digestive tract and other visceral organs in different fish-parasite systems. The data, coming especially from studies employed immunohistochemical, histopathological, and ultrastructural analyses, provide evidence supporting the involvement of teleost innate immune cells in modulating inflammatory responses to metazoan and protozoan parasitic infections.

Trace metal accumulation is infrapopulation-dependent in acanthocephalans parasites of the white mullet (Mugil curema) from an estuarine environment of southeastern Brazil coast.

Here, in an estuarine canal in southeast Brazil, we evaluated the potential for trace metal accumulation of the acanthocephalan parasite Floridosentis mugilis, which infects the fish host Mugil curema. The quantities of the trace metals were quantified using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), which was used to analyze samples of the fish's muscle, intestine, and liver as well as the parasites. The parasites and the host's tissues had significantly different trace metal concentrations, according to our data. Furthermore, some metals have quite high bioconcentration factors, including Al, Cr, Ni, and Cd. We also found that the trace metal concentrations in the parasites were impacted by the sizes of the parasitic infrapopulations, with smaller infrapopulations tending to accumulate more metals. This study shows this acanthocephalan species' effective ability to store metals and is the first to investigate metal accumulation using it as a model.

Host-parasite interactions in perpetual darkness: Macroparasite diversity in the cavefish Astyanax mexicanus.

Astyanax mexicanus has repeatedly colonized cave environments, displaying evolutionary parallelisms in many troglobitic traits. Despite being a model system for the study of adaptation to life in perpetual darkness, the parasites that infect cavefish are practically unknown. In this study, we investigated the macroparasite communities in 18 cavefish populations from independent lineages and compared them with the parasite diversity found in their sister surface fish populations, with the aim of better understanding the role that parasites play in the colonization of new environments. Within the cavefish populations, we identified 13 parasite taxa, including a subset of 10 of the 27 parasite taxa known for the surface populations. Parasites infecting the cavefish belong to five taxonomic groups, including trematodes, monogeneans, nematodes, copepods, and acari. Monogeneans are the most dominant group, found in 14 caves. The macroparasites include species with direct life cycles and trophic transmission, including invasive species. Surprisingly, paired comparisons indicate higher parasite richness in the cavefish than in the surface fish. Spatial variation in parasite composition across the caves suggests historical and geographical contingencies in the host-parasite colonization process and potential evolution of local adaptations. This base-line data on parasite diversity in cavefish populations of A. mexicanus provides a foundation to explore the role of divergent parasite infections under contrasting ecological pressures (cave vs. surface environments) in the evolution of cave adaptive traits.

Parasite communities of the golden snapper Lutjanus inermis (Perciformes: Lutjanidae): inter-annual variations during strange climatic events.

Strange oceanographic events such as El Niño and La Niña may have indirect effects on the local transmission processes of intestinal parasites due to the reduction or increase in populations of potential intermediate or definitive hosts. A total of 713 individuals of Lutjanus inermis (Peters) were collected over an 8-year period (October 2015 to July 2022) from Acapulco Bay, Mexico. Parasite communities in L. inermis were quantified and analysed to determine if they experienced interannual variations in species composition and structure as a result of local biotic and abiotic factors influenced by oceanographic events, such as El Niño-Southern Oscillation (ENSO), or La Niña, the cool phase of the ENSO climate pattern. Twenty-six taxa of metazoan parasites were recovered and identified: two Monogenea, eight Digenea, two Acanthocephala, four Nematoda, one Cestoda, seven Copepoda, and two Isopoda. Species richness at the component community level (8 to 17 species) was similar to reported richness in other species of Lutjanus Bloch. Parasite communities of L. inermis exhibited high inter-annual variation in the abundance of component species of parasite. However, the species richness and diversity were fairly stable over time. Climatic episodes of El Niño and La Niña probably generated notable changes in the structure of local food webs, thus indirectly influencing the transmission rates of intestinal parasite species. Changes in species composition and community structure of parasites possibly were due to variations in feeding behaviour during the events and differences in the host body size.

Ecology and effects of metazoan parasites of fish in transitional waters.

Given the abundance, heterogeneity and ubiquity of parasitic organisms, understanding how they influence biodiversity, evolution, health and ecosystem functionality is crucial, especially currently when anthropogenic pressures are altering host­parasite balances. This review describes the features, roles and impacts of metazoan parasites of fish occurring in transitional waters (TW). These aquatic ecosystems are highly productive and widespread around the globe and represent most favourable theatres for parasitism given the availability of hosts (invertebrates, fishes and birds) and an increased probability of parasite transmission, especially of those having complex life cycles. Fascinating examples of how parasitism can influence different hierarchical levels of biological systems, from host individuals and populations to entire aquatic communities, through effects on food webs come from this kind of ecosystem. Edible fish of commercial value found in TW can harbour some parasite species, significantly reducing host health, marketability and food safety, with possible economic and public health consequences. Many TW are historically exploited by humans as sources of relevant ecosystem services, including fisheries and aquaculture, and they are highly vulnerable ecosystems. Alteration of TW can be revealed through the study of parasite communities, contributing, as bioindicators, for assessing environmental changes, health and restoration. Fish parasites can provide much information about TW, but this potential appears to be not fully exploited. More studies are necessary to quantify the ecological, economic and medical impacts fish parasites can have on these important ecosystems.

Survival of metazoan parasites in fish: Putting into context the protective immune responses of teleost fish.

Defence mechanisms of fish can be divided into specific and non-specific that act in concert and are often interdependent. Most fish in both wild and cultured populations are vulnerable to metazoan parasites. Endoparasitic helminths include several species of digeneans, cestodes, nematodes, and acanthocephalans. Although they may occur in large numbers, helminth infections rarely result in fish mortality. Conversely, some ectoparasites cause mass mortality in farmed fish. Given the importance of fish innate immunity, this review addresses non-specific defence mechanisms of fish against metazoan parasites, with emphasis on granulocyte responses involving mast cells, neutrophils, macrophages, rodlet cells, and mucous cells. Metazoan parasites are important disease agents that affect wild and farmed fish and can induce high economic loss and, as pathogen organisms, deserve considerable attention. The paper will provide our light and transmission electron microscopy data on metazoan parasites-fish innate immune and neuroendocrine systems. Insights about the structure and functions of the cell types listed above and a brief account of the effects and harms of each metazoan taxon to specific fish apparati/organs will be presented.

Biodiversity of anuran endoparasites from a transitional area between the Atlantic Forest and Cerrado biomes in Brazil: new records and remarks.

The richness of metazoan endoparasites associated with 25 anuran species from a transitional area between Cerrado and Atlantic Rain Forest from Brazil is here presented. We present and discuss the type-host and localities, the current taxonomic status, remarks on morphological features, biological cycle, and new records of these parasites. Nine hundred and seventy-eight anurans of four families (Bufonidae, Hylidae, Microhylidae, and Leptodactylidae) were collected during four expeditions in a private forested area from Northwest of São Paulo state. The richness of metazoan parasites was composed of nematodes (21), acanthocephalans (2), digeneans (18), monogeneans (1), cestodes (1), and oligochaetes (1), resulting in 44 taxa that are presented with their respective prevalence and the range of abundance in host populations. Among these parasites, nine were found in immature stages which usually complete their life cycle in fishes, snakes, birds or mammals, attesting the trophic relation of amphibians and their parasites within the regional vertebrate community. We reported 23 anuran species as new hosts resulting in 79 new associations between anurans and parasites. Gorgoderina diaster and Bursotrema tetracotyloides are reported for the first time in Brazil, and our finding of Clinostomum cf. complanatum represents the first in South American anurans.

New Parasite Records for the Sunfish Mola mola in the Mediterranean Sea and Their Potential Use as Biological Tags for Long-Distance Host Migration.

Studies describing the parasite fauna of sunfish species from the Mediterranean Sea are to date limited, despite information gained through parasitological examination may reveal unknown ecological and biological aspects of both hosts and parasites. Moreover, recent molecular studies on sunfish taxonomy revealed the presence of two species belonging to the genus Mola in the Mediterranean basin, namely M. mola and M. alexandrini. These two fish taxa have long been synonymized or confused among them, which implies that the majority of the studies carried out so far reported the parasites infecting both species under a single host species, generally referred to as M. mola. We hereby investigated the parasite fauna of a 43 cm long M. mola specimen from the Mediterranean Sea, whose identification was confirmed by molecular tool, and provided the first evidence of the occurrence of the nematode Anisakis simplex (s.s.) and of the cestode Gymnorhynchus isuri in Mola species anywhere. The use of helminth species as biological tags for the sunfish is also discussed.

Can differences between continental and insular habitats influence the parasites communities associated with the endemic frog Haddadus binotatus?

Habitats characterized by geographic isolation such as islands have been studied using different organisms as models for understanding the dynamic and insular patterns of biodiversity. Determinants of parasite richness in insular host populations have been conducted mainly with mammals and birds, showing that parasite richness decreases in insular areas. In the present study, we predicted that the type of environment (insular or continental) can influence the richness, diversity and abundance of parasites associated with the endemic frog Haddadus binotatus (Spix, 1824). We sampled frogs in two insular and two mainland fragments to survey their helminth parasites. The total richness was composed of 15 taxa of Nematoda and two of Acanthocephala, and the community composition of the two islands had more similarities between them than the two mainland localities. The insular effect was positive for richness and abundance of helminths, and no significant effect was observed on helminth diversity - even the mean diversity presented high numbers for the islands. We presumed that insular hosts could have lost some parasites in the colonization process when these continental islands were separated from the mainland, approximately 11,000 years ago. However, the high richness and abundance on islands can be explained by an epidemiological argument, which considers high population density due to insularity and other features of the host as factors that increase parasite transmission success among individuals.

Metazoan parasite infracommunities of the dusky flounder (Syacium papillosum) as bioindicators of environmental conditions in the continental shelf of the Yucatan Peninsula, Mexico.

BACKGROUND: We assessed metrics of the metazoan parasite infracommunities of the dusky flounder (Syacium papillosum) as indicators of aquatic environmental health of the Yucatan Shelf (YS) prior to oil extraction. We sampled the dusky flounder and its parasites along the YS, mostly during the 2015 north wind season (November-April). Our aims were: (i) to determine whether the parasite infracommunity metrics of S. papillosum exhibit significant differences among YS subregions; (ii) to determine whether the probability of the occurrence of its parasite species and individuals were affected by environmental variables, nutrients, heavy metals and hydrocarbons at the seascape level; and (iii) to determine whether there were statistical differences between the parasite infracommunity metrics of S. papillosum from YS and those of Syacium gunteri from the Campeche Sound. Multivariate statistical analyses and generalised additive models (GAMs) were used to examine the potential statistical associations between the contaminants, environmental variables and parasite community metrics, and the maximum entropy algorithm (MaxEnt) was used to characterise the habitat's suitability for the parasite's probability of occurrence. RESULTS: We recovered 48 metazoan parasite species from 127 S. papillosum, with larval cestodes and digeneans being the most numerically-dominant. Multivariate analyses showed significant differences in parasite infracommunity metrics among Western YS, Mid YS and Caribbean subregions, with the latter being the richest in species but not in individuals. The GAM and MaxEnt results indicated a negative effect of top predators (e.g. sharks and rays) removal on parasite metrics. The parasite infracommunities of S. papillosum were twice as rich in the number of species and individuals as those reported for S. gunteri from the Campeche Sound. CONCLUSIONS: The significant differences among subregions in parasite metrics were apparently due to the interruption of the Yucatan current during the north wind season. The fishing of top predators in combination with an influx of nutrients and hydrocarbons in low concentrations coincides with an increase in larval cestodes and digeneans in S. papillosum. The dusky flounder inhabits a region (YS) with a larger number of metazoan parasite species compared with those available for S. gunteri in the Campeche Sound, suggesting better environmental conditions for transmission in the YS.

Haem Biology in Metazoan Parasites - 'The Bright Side of Haem'.

Traditionally, host haem has been recognized as a cytotoxic molecule that parasites need to eliminate or detoxify in order to survive. However, recent evidence indicates that some lineages of parasites have lost genes that encode enzymes involved specifically in endogenous haem biosynthesis. Such lineages thus need to acquire and utilize haem originating from their host animal, making it an indispensable molecule for their survival and reproduction. In multicellular parasites, host haem needs to be systemically distributed throughout their bodies to meet the haem demands in all cell and tissue types. Host haem also gets deposited in parasite eggs, enabling embryogenesis and reproduction. Clearly, a better understanding of haem biology in multicellular parasites should elucidate organismal adaptations to obligatory blood-feeding.

Metazoan parasite fauna of migrating common garfish, Belone belone (L.), in the Baltic Sea.

A total of 35 common garfish, Belone belone (Linnaeus, 1761), were studied for metazoan parasites on their spawning grounds in the western Baltic Sea. Nine parasite species were found, and six new locality records could be established for German coastal waters (Axine belones, Monogenea; Proteocephalus sp., Cestoda; Anisakis simplex (s.s.), Contracaecum rudolphii A and Hysterothylacium aduncum, Nematoda; Echinorhynchus gadi, Acanthocephala). For the first time, the marine ectoparasite A. belones was recorded from the gills of garfish inside the Baltic Sea, indicating its ability to survive the spawning migration as well as the brackish water conditions at its reproduction grounds. This is alike the endohelminth A. simplex (s.s.), that was identified by molecular analyses of the internal transcribed spacer (ITS-1, 5.8S, ITS-2) region. Almost all isolated metazoans were parasites commonly recorded from the northeast Atlantic Ocean and the North Sea. The lower number of typical generalist Baltic Sea parasites indicates the rapid migration of common garfish onto the spawning grounds, reducing the access and uptake of these species.

Metazoan parasite communities in Alosa alosa (Linnaeus, 1758) and Alosa fallax (Lacépède, 1803) (Clupeidae) from North-East Atlantic coastal waters and connected rivers.

Metazoan parasites were studied in 96 Alosa alosa and 78 Alosa fallax from North-East Atlantic coastal waters and connected rivers (among them three sympatric sites) in order to increase knowledge on these anadromous endangered fish and measure the parasitic impact on host condition. All shads were infected by one to six metazoan parasite taxa among the 12 identified in the whole sampling, with a mean abundance of parasites higher for A. alosa (167 ± 10) than for A. fallax (112 ± 11). Helminths, mostly trophically transmitted, were the best represented (eight taxa, prevalence up to 99%) in contrast with crustaceans and Petromyzontidae that rarely occurred (four taxa, prevalence <6%). Despite some quantitative differences, metazoan parasite communities of A. alosa and A. fallax remained stable in composition whatever the host developmental stage, sex, sample site, and salinity. Among the nine parasite taxa harbored by each Alosa species, six were shared with some differences in distribution patterns including in sympatric conditions, suggesting increasing dissimilarities between A. alosa and A. fallax with the age. Information on feeding ecology provided by trophically transmitted helminths confirmed euryphagous opportunistic diet of immatures and adults of both shad species, and assessed feeding of adults during spawning migrations. Our study also revealed the significant negative impact of Hemiurus appendiculatus on A. alosa and Pronoprymna ventricosa on A. fallax. Because helminth parasites are omnipresent in the shads and decrease their fitness, parasitological data must be included in further investigations and management programs on A. alosa and A. fallax.

Parasite infracommunities of Leporinus friderici: A comparison of three tributaries of the Jurumirim Reservoir in southeastern Brazil

ABSTRACT The degradation and homogenization of natural habitats is considered a major cause of biotic homogenization. Many studies have been undertaken on the effects of dams on aquatic wildlife, in particular fish assemblages. But how do dams affect the parasitic fauna of such fish? The aim of the present study was to examine parasitic similarity, comparing the diversity and structure of parasite communities of Leporinus friderici (Characiformes, Anostomidae) in three upstream tributaries under the influence of the Jurumirim Dam on the Upper Paranapanema River in southeastern Brazil. The present study did not find any significant differences in parasite communities among populations of L. friderici in the three upstream tributaries. This result highlights that dams promote and facilitate the dispersal of organisms between localities, and therefore the spatial homogenization of parasite communities. Overall, the results suggest that fish parasite assemblages can provide suitable data for evaluating biotic homogenization caused by dams.

Consistency of trematode infection prevalence in host populations across large spatial and temporal scales.

Parasites can impart heavy fitness costs on their hosts. Thus, understanding the spatial and temporal consistency in parasite pressure can elucidate the likeliness of parasites' role as agents of directional selection, as well as revealing variable environmental factors associated with infection risk. We examined spatiotemporal variation in digenetic trematode infection in 18 populations of an intertidal host snail (Littorina littorea) over a 300 km range at an 11-yr interval, more than double the generation time of the snail. Despite a complete turnover in the snail host population, the average change in infection prevalence among populations was <1% over the 11-yr span, and all but three populations remained within 5 percentage points. This consistency of prevalence in each population over time suggests remarkable spatiotemporal constancy in parasite delivery vectors in this system, notably gulls that serve as definitive hosts for the parasites. Thus, despite gulls' high mobility, their habitat usage patterns are ostensibly relatively fixed in space. Importantly, this spatiotemporal consistency also implies that sites where parasites are recruitment limited remain so over time, and likewise, that parasite hotspots stay hot.

Endoparasite fauna of Brycon amazonicus and B. melanopterus (Characidae, Bryconinae) from Negro and Solimões rivers, Amazon, Brazil / Fauna endoparasitária de Brycon amazonicus e B. melanopterus (Characidae, Bryconinae) dos rios Negro e Solimões, Amazônia, Brasil

Congeneric host species present similar biological and behavioral aspects, what may favor the presence of a similar parasite fauna. The aim of the present study was to compare the composition and structure of the parasite community from congeneric species, Brycon amazonicus and B. melanopterus, collected on the Negro and Solimões Rivers. The fish internal organs were longitudinally opened and analyzed under stereomicroscope. The examination revealed that B. amazonicuswas parasitized by Procamallanus(Spirocamallanus) inopinatus (26.7% and 35.5%), Contracaecum type 2 (10% and 16.13%) and Rhabdochona acuminata (0% and 3.23%), prevalence values for Negro and Solimões River, respectively. The analysis of B. melanopterus, a fish species found only in the Solimões River, revealed P.(S.) inopinatus(33.3%), Contracaecum type 1 (60%) and R. acuminata (3.34%). These results indicate that the taxonomic proximity of the hosts was a stronger influence on the parasite species than external host environment.

Espécies hospedeiras congenéricas apresentam aspectos biológicos e comportamentais semelhantes, o que pode favorecer a presença de uma fauna de parasitas semelhantes. O objetivo do presente estudo foi comparar a composição e a estrutura da comunidade parasitária de espécies congêneres, Brycon amazonicuse B. melanopterus, coletados nos rios Negro e Solimões. Os órgãos internos dos peixes foram longitudinalmente abertos e analisados com o auxílio de estereomicroscópio. O exame revelou que B. amazonicus foi parasitada por Procamallanus (Spirocamallanus) inopinatus (26,7% e 35,5%), tipo Contracaecum 2 (10% e 16,13%) e Rhabdochona acuminata (0% e 3,23%), os valores de prevalência para os rios Negro e Solimões, respectivamente. A análise de B. melanopterus, uma espécie de peixe encontradas somente no Rio Solimões, revelou P. (S.) inopinatus (33,3%), Contracaecum tipo 1 (60%) e R. acuminata (3,34%). Estes resultados indicam que a proximidade taxonômica dos hospedeiros foi uma influência mais forte sobre as espécies de parasitas do que ambiente externo ao peixe.

Does the parasite-mediated selection drive the MHC class IIB diversity in wild populations of European chub (Squalius cephalus)?

The genes of major histocompatibility complex (MHC) provide an excellent opportunity to study host-parasite relationships because they are expected to evolve in response to parasites and variation in parasite communities. In this study, we investigated the potential role of parasite-mediated selection acting on MHC class IIB (DAB) genes in European chub (Squalius cephalus) natural populations. We found significant differences between populations in metazoan parasites, neutral and adaptive genetic diversities. The analyses based on pairwise data revealed that populations with dissimilar MHC allelic profiles were geographically distant populations with significantly different diversity in microsatellites and a dissimilar composition of parasite communities. The results from the generalized estimating equations method (GEE) on the level of individuals revealed that metazoan parasite load in European chub was influenced by the diversity of DAB alleles as well as by the diversity of neutral genetic markers and host traits reflecting condition and immunocompetence. The multivariate co-inertia analysis showed specific associations between DAB alleles and parasite species. DAB1-like alleles were more involved in associations with ectoparasites, while DAB3-like alleles were positively associated with endoparasites which could suggest potential differences between DAB genes caused by different selection pressure. Our study revealed that parasite-mediated selection is not the only variable affecting MHC diversity in European chub; however, we strongly support the role of neutral processes as the main driver of DAB diversity across populations. In addition, our study contributes to the understanding of the evolution of MHC genes in wild living fish.