Shift in parasite load in native and non-native Eupercarian fish species living in sympatry

151 Shift in parasite load in native and non-native Eupercarian fish species living in sympatry Maria Yu. Tkachenko1, Michal Hnilička1,2 , Michal Janáč1, Yuriy Kvach1,3 , Lukáš Vetešník1, Markéta Ondračková1 1 InstituteofVertebrateBiologyoftheCzechAcademyofSciences,Květná8,60300Brno,CzechRepublic 2 DepartmentofBotanyandZoology,FacultyofScience,MasarykUniversity,Kotlářská2,61137,Brno,CzechRepublic 3 InstituteofMarineBiology,NationalAcademyofSciencesofUkraine,37ItalyiskaSt.,65048Odesa,Ukraine Correspondingauthor:MarkétaOndračková(ondracko[email protected]) Copyright: © Maria Yu. Tkachenko et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract Non-native species have the potential to alter host community structure, which, in turn, may affect parasite transmission, diversity and distribution, particularly when the introduced host is ecologically or phylogenetically related to its native host(s). In this study, we examine whether the introduction of a competent non-native fish host shifts parasite load and community composition in native fish species. Specifically, we focus on two Eupercarian species: the native Eurasian perch (Perca fluviatilis) and the non-native pumpkinseed sunfish (Lepomis gibbosus), which was introduced into Europe from North America. Our results show that the parasite community of native perch co-occurring with non-native pumpkinseed (sympatric populations) differ significantly from those of perch inhabiting sites unaffected by pumpkinseed introduction (allopatric populations). Moreover, sympatric perch populations exhibit significantly higher parasite abundance, infracommunity richness and dominance, along with lower equitability, compared to allopatric populations. These findings suggest that the introduction of pumpkinseed may have contributed to shifts in both parasite abundance and diversity in native fish hosts. Additionally, our study indicates a potential parasite spillback of the trematode Bucephalus polymorphus and spill-over of the Asian invasive parasitic copepod Neoergasilus japonicus, both associated with pumpkinseed establishment in the study area. The successful inclusion of non-native fish species into the life cycles of several independently introduced and unrelated invasive parasites underscores the importance of introduced hosts in driving changes within local parasite communities. Key words: Freshwater fish, parasite community, perch, pumpkinseed, species introduction, spill-back Introduction The introduction of non-native freshwater fish species has increased significantly across the world over the past few decades, leading to negative socio-economic, ecological and evolutionary impacts on local biodiversity (Hulme 2009; Vitule et al. 2009; Gozlan et al. 2010; Cucherousset and Olden 2011). Local ecosystems may be affected by invasive species through a range of mechanisms, including predation, competition for food and space, hybridisation, alterations in trophic relationships and modification of biochemical cycles (Leunda 2010; Cucherousset and Olden 2011). The success of an invasive species may also be partially linked to parasites and pathogens. For example, introduced species can gain an advantage over native species due to the absence of their Academic editor: Filipe Ribeiro Received: 30 January 2025 Accepted: 16 April 2025 Published: 7 October 2025 Citation: Tkachenko MYu, Hnilička M, Janáč M, Kvach Yu, Vetešník L, Ondračková M (2025) Shift in parasite load in native and nonnative Eupercarian fish species living in sympatry. In: Anastácio P, Ribeiro F, Chainho P (Eds) Invasions in Aquatic Systems. NeoBiota 102: 151–171. https://doi.org/10.3897/ neobiota.102.148301 NeoBiota 102: 151–171 (2025) DOI: 10.3897/neobiota.102.148301 Advancing research on alien species and biological invasions A peer-reviewed open-access journal NeoBiota 152 NeoBiota 102: 151–171 (2025), DOI: 10.3897/neobiota.102.148301 Maria Yu. Tkachenko et al.: Shift in parasite load in native and non-native Eupercarian fish natural parasites (the so-called enemy release hypothesis; Keane and Crawley (2002)) or through the introduction of invasive parasites or pathogens that are more harmful to native competitors (Lymbery et al. 2014). While the role of parasites in the success of invasive species is widely recognised (e.g. Torchin et al. (2003); Prenter et al. (2004); Dunn (2009)), the impact of invasive species on parasite communities in native hosts has received less attention (Hohenadler et al. 2019). Non-native species can alter local parasite communities through the introduction of non-native parasites capable of parasitising local hosts or by interacting with native parasite species. New parasite species co-introduced to the invaded ecosystem may either be host-specific and remain part of the parasitic fauna of their original host (Lymbery et al. 2014) or they may infect native species (Taraschewski 2006), potentially resulting in subsequent changes to local parasite diversity. For example, the introduction of Eurasian perch (Perca fluviatilis; Eupercaria, Percidae) into African lakes led to the co-introduction of several cestodes that later became established in native fish populations (Loker and Hofkin 2015). In France, monogenean parasites of the invasive common nase (Chondrostoma nasus) infected the threatened native south-west European nase (Parachondrostoma toxostoma) at sites where the two species co-occurred (Šimková et al. 2012). Following a successful introduction, therefore, non-native parasite species may be expected to transmit from the invasive species to native hosts, typically those that are ecologically similar or taxonomically close (Mack et al. 2000). This has led to the concept of “parasite spill-over” (Cleaveland et al. 2002; Prenter et al. 2004), which suggests that non-native parasites can infect native host species in invaded habitats, thus accelerating their range expansion (Galli et al. 2005). Furthermore, these interactions may benefit the introduced host if the parasite negatively affects its new host (Tompkins et al. 2003). However, interactions between non-native hosts and local parasite fauna appear to occur more frequently than the above-mentioned spill-over (Kelly et al. 2009). In this context, Kelly et al. (2009) introduced the concept of “parasite spill-back”, which suggests that, when a non-indigenous species serves as a competent host for a native parasite, the presence of the additional host can amplify disease impacts on native species, affecting both individual hosts and entire populations. For example, in German rivers affected by the invasion of Ponto-Caspian species (gobiids and amphipods), local fish species exhibited higher infection rates by the nematode Raphidascaris acus, which successfully used Ponto-Caspian gobies as paratenic hosts, compared to rivers that had not been invaded (Hohenadler et al. 2019). A specific case of parasite spill-back is the hypothesis of “invasional meltdown” (Simberloff and von Holle 1999), which proposes that multiple species simultaneously colonising a new environment can enhance each other’s chances of invasion. Certain ecological relationships that may develop amongst invasive species, such as predator-prey interactions, symbiosis and parasitism, could facilitate the establishment of invader parasites, even if the parasites were introduced through other vectors (Taraschewski 2006; Emde et al. 2014). This is particularly relevant for parasites with complex life cycles as their invasion success depends on the availability of suitable intermediate hosts. Interestingly, invasional meltdown may also occur within parasite-parasite relationships, as exemplified by the eel nematode Anguillicola crassus, which escapes the host’s immune response by invading the cyst of another invasive parasite, the acanthocephalan Pomphorhynchus laevis (Hohenadler et al. 2018). Non-native species can also alter local parasite communities through the “dilution effect”. In this case, if they are not competent hosts for the acquired parasites, 153 NeoBiota 102: 151–171 (2025), DOI: 10.3897/neobiota.102.148301 Maria Yu. Tkachenko et al.: Shift in parasite load in native and non-native Eupercarian fish non-native species may act as parasite sinks, or dilution agents, reducing parasite prevalence in local populations (Keesing et al. 2006; Gendron and Marcogliese 2017). Non-native species can also disrupt parasite transmission pathways. For example, fish can decrease the prevalence of trematodes in native fish populations by preying on the parasite’s intermediate hosts, such as snails (Poulin and Mouillot 2003). Additionally, native specialist parasites may decline if non-native hosts replace native hosts, particularly when the non-native species are unsuitable hosts, potentially leading to the extinction of certain native parasite species (Hohenadler et al. 2019). This can result in the homogenisation of parasite communities, favouring generalist parasites (Dunn 2009). Finally, changes in host availability and community structure can shift competitive dynamics amongst parasite species, for example, when a diverse native host community is supplanted by a dominant non-native species, following which parasite diversity is reduced, promoting the dominance of a few generalist parasite species (Gendron et al. 2012). Aquatic ecosystems are highly vulnerable to the introduction of non-native species, whether as hosts, such as fish, molluscs or crustaceans (Bódis et al. 2012) or their associated parasites (Esposito et al. 2023). Furthermore, the introduction of species into freshwater ecosystems can have numerous impacts on native communities and on ecosystem functioning (Havel et al. 2015). The Eurasian perch is a widely distributed predatory freshwater species (Stepien and Haponski 2015) inhabiting both lentic and lotic waterbodies (Baruš and Oliva 1995). It frequently co-occurs with the non-native pumpkinseed sunfish (Lepomis gibbosus; Eupercaria, Centrarchidae), a North American freshwater species (Scott and Crossman 1973) introduced into Europe as an ornamental fish in the late 19th century which has had negative impacts on local ecosystems (Soes et al. 2011). The pumpkinseed is now widespread across many European countries and was added to the list of Invasive Species of Union Concern in 2019 (European Commission 2019). In its native North American range, the pumpkinseed commonly occurs sympatrically with the yellow perch (Perca flavescens), a sister species of the Eurasian perch (Collette and Banarescu 1977; Shcherbukha 1993), with both species sharing similar habitat and food preferences (Sun and Harvey 1986; Runciman and Leaf 2009). Despite millions of years of isolation, the parasite assemblages of these two perch species, composed of non-host-specific parasites, are not random, but contain key components that are predictable at the continental scale (Carney and Dick 1999). As a result, pumpkinseed in Europe share several common parasites with the Eurasian perch, such as the cestodes Triaenophorus nodulosus and Bothriocestus claviceps and the nematode Camallanus lacustris (Carney and Dick 1999; Masson et al. 2015; Ondračková et al. 2019a; Kvach et al. 2021a). Building on present knowledge of the sympatric occurrence of these two Eupercarian fish species, this study aims to evaluate potential changes in parasite communities between populations where either perch or pumpkinseed live without their Eupercarian relative (allopatric sites) and populations where both species co-exist (sympatric sites). In doing so, we: 1) compare the abundance and species diversity of parasite communities to assess the ability of non-native pumpkinseed to acquire local parasite species, 2) evaluate the similarity of parasite communities between sympatric and allopatric populations in both host species and identify parasite species that significantly contribute to shifts in the parasite community of perch and 3) focus on parasite species shared between the perch and pumpkinseed and discuss their relevance in the context of potential parasite spill-back, spill-over, dilution or invasional meltdown. 154 NeoBiota 102: 151–171 (2025), DOI: 10.3897/neobiota.102.148301 Maria Yu. Tkachenko et al.: Shift in parasite load in native and non-native Eupercarian fish Material and methods Fish and parasite sampling Fish were collected from eight small lentic waterbodies (oxbows and sand pits) in the lower Morava River Basin in the Czech Republic, three sites having allopatric populations of perch and of pumpkinseed (six sites in total) and two sites having sympatric populations of both species (Table 1). Fish were collected using electrofishing gear (SEN, fa Bednář R., Olomouc, Czech Republic; frequency 75–85 Hz; maximum output 225/300 V) or seine nets (7 m long, 1 cm mesh size), depending on habitat type. After collection, the fish were transported alive in aerated river water to the facilities at the Institute of Vertebrate Biology, where they were placed in outdoor tanks, separated by fish species and locality. Fifteen fish per species were collected for each site (75 perch and 75 pumpkinseed in total) and examined for parasites (Table 1). Following humane dispatch, standard length (SL, mm) was measured for each fish, after which the fins, skin, gills, gill covers, eyes, muscle and internal organs (intestine, liver, spleen, heart, kidney, gonads) were inspected for the presence of parasites. All fish were dissected within three days of sampling to ensure maximum parasite recovery (Kvach et al. 2016). Table 1. List of sites sampled for parasite communities of native European perch Perca fluviatilis and non-native pumpkinseed sunfish Lepomis gibbosus in the Basin of the lower Morava River, Czech Republic, showing number of fish dissected (n), fish standard length (SL in mm (mean, range)), parasite prevalence (%), total abundance of parasites and parasite species richness. Locality name Coordinates Fish species n Fish SL Parasite prevalence Parasite abundance Parasite species richness Sympatric sites Babice oxbow 49.117968, 17.488028 P. fluviatilis 15 69 (61-98) 100 209 12 L. gibbosus 15 74 (63-81) 100 532 9 U Jezu sandpit 49.131407, 17.510408 P. fluviatilis 15 71 (63-79) 100 358 18 L. gibbosus 15 75 (65-82) 100 989 10 Allopatric sites Dedava sandpit 48.633176, 16.959805 P. fluviatilis 15 71 (56-114) 100 123 10 Kanada oxbow 49.102925, 17.492513 P. fluviatilis 15 71 (63-76) 100 129 11 Penne oxbow 49.156364, 17.511674 P. fluviatilis 15 71 (66-87) 93 118 11 Dyje oxbow D3 48.684344, 16.916031 L. gibbosus 15 68 (52-84) 100 6623 4 Dyje oxbow D6 48.717369, 16.888908 L. gibbosus 15 59 (55-64) 100 97 5 Pohansko oxbow 48.725360, 16.890936 L. gibbosus 15 60 (53-70) 100 285 8 Parasite processing Non-metazoan parasites were examined alive under light microscopy and their presence/absence recorded. Numbers of Ichthyophthirius multifiliis and Dermocystidium were recorded. Metazoan parasites were collected, counted and preserved for species identification. Monogeneans and myxosporeans were preserved in GAP (glycerine-ammonium-picrate) and prepared as semi-permanent slides (Malmberg 1970), while cestodes, trematodes, acanthocephalans and nematodes were preserved in hot 4% formaldehyde. Prior to species identification, cestodes and trematodes were stained with iron acetic carmine, dehydrated in ethanol of increasing concentration and mounted in Canada balsam as permanent slides (Georgiev et al. 1986; Cribb and Bray 2010). Acanthocephalans were pressed between two slides and preserved in 70% ethanol. Both preserved acanthocephalans and nematodes were mounted in glycerol as temporary slides for light microscopy. Crustaceans and bivalves were 155 NeoBiota 102: 151–171 (2025), DOI: 10.3897/neobiota.102.148301 Maria Yu. Tkachenko et al.: Shift in parasite load in native and non-native Eupercarian fish preserved in 4% formaldehyde and identified under light microscopy. All preserved parasites were identified to species level, or to the lowest possible taxonomic level when species-level identification was not feasible. An Olympus BX53 light microscope (Olympus Optical Co., Hamburg, Germany) equipped with phase-contrast, differential (Nomarski) interference contrast and image analysis software (CellSens Standard digital image analysis package; Olympus Optical Co., Hamburg, Germany) was used for species identification. Identification followed the keys of Bauer (1987), Beverley-Burton (1984), Moravec (2013) and Sudarikov et al. (2002). A subsample of larval cestodes (Bothriocephalidae, Proteocephalidae) and trematodes (Diplostomum), for which species identification could not be determined morphologically, was preserved in 96% ethanol for further molecular analysis. Subsequently, DNA was extracted using the Invisorb® Spin Forensic Kit (STRATEC Molecular, Germany) following the standard protocol. For Diplostomum metacercariae, the forward primer BD1 (5′-GTCGTAACAAGGTTTCCGTA-3′) and reverse primer 4S (5′-TCTAGATGCGTTCGAA(G/A)TGTCGATG-3′) (Bowles et al. 1993) were used to obtain the ITS1 partial sequence, with an annealing temperature of 58 °C (PCR details in Dudliv et al. (2024)), while the forward primer 18S9F and reverse primer 18S637R (Moszczynska et al. 2009) were employed to obtain the 18S partial sequence for the larval cestodes (Bothriocephalidae, Proteocephalidae). The PCR reaction mix contained 4 μl of extracted DNA, 0.3 μl of each primer (10 μM), 2 μl of buffer A, 0.2 μl of dNTPs (10 mM), 0.2 μl of MgCl2 (25 mM), 0.5 U/μl of Taq polymerase and ddH2O up to a total volume of 10 μl. PCR was performed using the KAPA2G Robust HotStart PCR Kit (Kapabiosystems, USA) in a Mastercycler ep gradient S thermocycler (Eppendorf, Germany) with the following temperature profile: 95 °C for 2 min, 33 cycles of 95 °C for 30 s, 58 °C for 30 s and 72 °C for 5 min, with a final extension at 72 °C for 5 min. All PCR products were purified using an ExoSAP-IT Kit (Affymetrix Inc., Santa Clara, USA) following the manufacturer’s protocol. PCR products were commercially sequenced at Eurofins Genomics Germany GmbH. The sequences were checked and aligned using Geneious® v.9.0.5 and compared with the NCBI database using BLASTn to assess sequence similarity. Similarity 99.8–100% and 99.5–100% for ITS/18S and COI, respectively, was applied to perform species assignment. Newly-generated sequences were submitted to GenBank under the accession numbers: PV402115, PV402120-1, PV402623, PV402631 and PV402405-6. Data analysis Only those parasites that could be counted were used for quantitative analysis (i.e. kinetoplastids and ciliates, except Ichthyophthirius multifiliis, were excluded from the analysis), while all parasite taxa were included in the qualitative analysis. Non-native parasites included species co-introduced with pumpkinseed from North America (according to Hoffman (1999)) and species introduced historically to the study area from Asia by other vectors. Acquired parasites included both European (Moravec 2001) and Asian species (Wielgoss et al. 2008; Konečný et al. 2018; Ondračková et al. 2025) infecting pumpkinseed and Asian species infecting perch. Parasite load was expressed in terms of prevalence, intensity of infection and abundance, calculated following Bush et al. (1997). Mean abundance was calculated as the average number of parasites per host in each locality, intensity of infection as the average number of parasites per infected hosts and prevalence as the percentage of infected hosts. Differences between parasite communities were analysed at both 156 NeoBiota 102: 151–171 (2025), DOI: 10.3897/neobiota.102.148301 Maria Yu. Tkachenko et al.: Shift in parasite load in native and non-native Eupercarian fish the infracommunity level (parasites found on a single host) and component community level (parasites found in a host population) (Bush et al. 1997). Analysis at the infracommunity level was conducted using generalised linear mixed models (GLMM), which were used to compare parasite abundance (negative binomial distribution) and infracommunity species richness (Poisson distribution) between species (perch/pumpkinseed) and between type of population (allopatric/sympatric), with site as a random factor. The same approach was applied to compare the abundance and richness of acquired parasites, with the exception of comparisons between acquired parasites in sympatric and allopatric populations of perch, where the low incidence of acquired parasites made testing for differences in abundance pointless. Instead, we tested for differences in prevalence using GLMM with a Bernoulli distribution. Differences in parasite diversity at the component community level were assessed using three diversity indices, i.e. the Shannon-Wiener index (H), which indicates average niche width; the dominance index (D), which expresses the extent to which a few species dominate the community; and the equitability index (J), which measures the evenness with which individuals are distributed amongst the taxa present (Magurran 2004). Permutation tests (1000 permutations) were used to compare diversity indices between groups (host species, allopatric/sympatric localities). All diversity analyses were performed using PAST software (PAlaeontologicalSTatistics, v.3.22; Hammer et al. (2001)). Differences in parasite community composition between sympatric and allopatric populations in both host species were tested using permutational multivariate analysis of variance (PERMANOVA; Anderson (2001)) and visualised using non-metric multidimensional scaling (NMDS). Both NMDS and PERMANOVA were conducted separately on two distance matrices, i.e. a matrix based on Jaccard distances (binary) and one based on Bray-Curtis distances (quantitative, abundance data fourth-root transformed), with PERMANOVA based on 999 permutations. All statistical analyses were performed using R v.4.3.1 (R Core Team 2023), with the MASS (Venables and Ripley 2002), lme4 (Bates et al. 2015), lmtest (Zeileis and Hothorn 2002) and vegan (Oksanen et al. 2022) packages. Results Parasite community composition in native and introduced hosts All pumpkinseed and all, but one perch were infected with at least one parasite taxon. In total, 38 parasite taxa were found, 31 infecting perch and 18 infecting pumpkinseed (Table 2). Three parasite species co-introduced with pumpkinseed from North America, the monogeneans Onchocleidus dispar and O. similis and the trematode Posthodiplostomum centrarchi, were found solely on their natural host, representing 85% of all pumpkinseed parasites. Three additional species, A. crassus, Neoergasilus japonicus and Sinanodonta woodiana, were introduced to the study area from Asia, contributing 5% and 10% to the parasite communities of perch and pumpkinseed, respectively. Trematode metacercariae dominated the parasite communities of both host species, representing 55% of metazoan parasites in perch (nine species) and 75% in pumpkinseed (four species). The maximum intensity of infection in pumpkinseed was by metacercariae of P. centrarchidae (up to 1111 parasites per fish), whereas in perch, it was metacercariae of B. polymorphus (up to 33 parasites). 157 NeoBiota 102: 151–171 (2025), DOI: 10.3897/neobiota.102.148301 Maria Yu. Tkachenko et al.: Shift in parasite load in native and non-native Eupercarian fish Table 2. List of parasite species found in Eurasian perch Perca fluviatilis and pumpkinseed Lepomis gibbosus in lentic waterbodies in the basin of the lower Morava River. Perca fluviatilis Lepomis gibbosus ALLOPATRIC SYMPATRIC ALLOPATRIC SYMPATRIC Prevalence Abundance Prevalence Abundance Prevalence Abundance Prevalence Abundance Opisthoconta Ichthyosporea Dermocystidium spp. 15.6% 0.4 Excavata Kinetoplastida Cryptobiidae spp. 3.3% 3.3% SAR Oligohymenophorea Apiosoma spp. 33.3% 56.7% Capriniana piscium 3.3% Epistylis spp. 2.2% Scyphidia spp. 15.6% 20.0% Trichodinidae spp. 40.0% 63.6% 13.3% Ichthyophthirius multifiliis 3.3% 0.03 15.6% 0.8 Animalia Myxosporea Myxosporidia sp. (muscle) 13.3% 0.3 Monogenea Onchocleidus dispar 57.8% 20.1 Onchocleidus similis 20.0% 0.6 50.0% 10.0 Trematoda Bucephalus polymorphus 100% 6.5 76.7% 14.6 Rhipidocotyle campanula 2.2% 0.02 Diplostomum pseudospathaceum 26.7% 0.3 3.3% 0.03 Diplostomum sp. Lineage 3 sensu Faltýnková et al. 2022 6.7% 0.07 Tylodelphys clavata 46.7% 3.3 6.7% 0.13 Clinostomum complanatum 51.1% 2.4 26.7% 0.4 Cyathocotyle prussica 3.3% 0.03 Holostephanus cobitidis 3.3% 0.03 Holostephanus luhei 10.0% 0.1 Metorchis xanthosomus 3.3% 0.03 Posthodiplostomum centrarchi 88.9% 131.8 16.7% 0.2 Cestoda Bothriocestus claviceps 6.7% 0.1 Proteocephalus percae 4.4% 0.1 23.3% 0.5 Triaenophorus nodulosus 11.1% 0.1 8.9% 0.3 Valipora campylancristrota 6.7% 0.1 Nematoda Anguillicola crassus 4.4% 0.2 13.3% 0.4 20.0% 0.5 Camallanus truncatus 26.7% 0.8 Philometra obturans 15.6% 0.5 Philometra sp. 2.2% 0.02 Schulmanela petruschewskii 8.9% 0.3 6.7% 0.07 Acanthocephala Acanthocephalus lucii 11.1% 0.3 3.3% 0.03 Pomphorhynchus laevis 3.3% 0.3 Bivalvia Sinanodonta woodiana 23.3% 0.4 26.7% 1.2 3.3% 0.07 Crustacea Ergasilus sieboldi 57.8% 1.4 50.0% 4.4 3.3% 0.03 Neoergasilus japonicus 6.7% 0.07 60.0% 24.7 Paraergasilus longidigitus 23.3% 0.8 Branchiura Argulus foliaceus 6.7% 0.1 3.3% 0.03 13.3% 0.2 3.3% 0.03 Nine parasite species were found in both perch and pumpkinseed hosts, i.e. the ciliates Ichthyophthirius multifiliis; the larval trematodes Bucephalus polymorphus and Tylodelphys clavata; the larval cestode Triaenophorus nodulosus; the larval nematode A. crassus, bivalve glochidia of S. woodiana, the copepods Ergasilus sieboldi and N. japonicus and the branchiuran Argulus foliaceus (Fig. 1). Additionally, representatives of the Trichodinidae and Cryptobiidae families were found in both hosts, although species identification was not performed (Table 2). 158 NeoBiota 102: 151–171 (2025), DOI: 10.3897/neobiota.102.148301 Maria Yu. Tkachenko et al.: Shift in parasite load in native and non-native Eupercarian fish Parasite abundance and diversity in native and introduced hosts Mean parasite abundance differed significantly between host species (GLMM, Z = 5.931, p < 0.001), being ten times higher in pumpkinseed (113.6 ± 233.1 S.D.) compared to perch (10.6 ± 8.8). Parasite infracommunity species richness ranged from 1–10 in perch and 1–7 in pumpkinseed, with significantly higher species richness in perch (3.9 ± 1.9) than pumpkinseed (2.8 ± 1.2) (GLMM, Z = 3.849, p < 0.001). Parasite diversity indices differed significantly between native and non-native hosts (permutation tests, p < 0.0001 for all comparisons). Both the Shannon diversity index and equitability were higher in perch than pumpkinseed (Shannon diversity index: perch H = 2.524, pumpkinseed H = 1.099; equitability: perch J = 0.735, pumpkinseed J = 0.380), while dominance was lower in perch (D = 0.118) than pumpkinseed (D = 0.509). Allopatric vs. sympatric populations Parasite abundance was significantly higher in sympatric than allopatric perch populations (GLMM: Z = 4.081, p < 0.001), but did not differ in pumpkinseed populations (GLMM, Z = 0.195, p = 0.846; Fig. 2A). Similarly, infracommunity richness was significantly higher in sympatric than allopatric perch populations (GLMM, Z = 3.206, p < 0.001), while no differences were found in pumpkinseed populations (GLMM, Z = 0.174, p = 0.862; Fig. 2B). However, sympatric pumpkinseed populations were parasitised by a significantly higher abundance of acquired parasites (including both European and Asian species) than allopatric populations (GLMM, Z = 2.864, p = 0.004). Perch from sympatric populations hosted acquired parasites significantly more often than those from allopatric populations (GLMM, df = 1, 73, P = 0.001). Figure 1. Proportion of metazoan parasite taxa in infracommunities of native European perch Perca fluviatilis and non-native pumpkinseed Lepomis gibbosus examined at eight localities: 1) with sympatric occurrence of perch and pumpkinseed: A = U Jezu, B = Babice; 2) with allopatric perch: C = Dedava, D = Kanada, E = Penne; and 3) with allopatric pumpkinseed: F = Pohansko, G = Oxbow D3, H = Oxbow D6. 159 NeoBiota 102: 151–171 (2025), DOI: 10.3897/neobiota.102.148301 Maria Yu. Tkachenko et al.: Shift in parasite load in native and non-native Eupercarian fish Parasite species richness was higher in sympatric than allopatric populations of both species (18 and 13 in perch, 13 and 9 in pumpkinseed), despite just two sympatric sites being compared to three allopatric sites. While Shannon diversity did not differ in perch (H = 1.905 and 1.781 for allopatric and sympatric populations), it was significantly higher in sympatric (H = 1.173) compared with allopatric (H = 0.527) pumpkinseed populations. In pumpkinseed, equitability was higher in sympatric (J = 0.458) compared to allopatric populations (J = 0.240), while dominance was higher in allopatric populations (D = 0.736) compared to sympatric (D = 0.361; p < 0.001 for all comparisons). In contrast, significantly lower dominance (D = 0.210; p = 0.01) and higher equitability (J = 0.743; p < 0.001) were observed in allopatric populations compared to sympatric ones in perch (D = 0.244; J = 0.616). The composition of parasite communities in both perch and pumpkinseed differed significantly between sympatric and allopatric populations. This was confirmed for both Jaccard (Fig. 3A) and Bray-Curtis (Fig. 3B) distance matrices (PERMANOVA, df = 1, 73, P = 0.001 for all analyses). Discussion In this study, we tested whether the introduction of a competent non-native fish host, the pumpkinseed, might shift the parasite load and community composition of native fish species, such as the Eurasian perch. We found that the parasite communities of native perch co-occurring with non-native pumpkinseed at sympatric sites differed significantly from those of perch at allopatric sites, which were not influenced by the introduction of pumpkinseed. Specifically, parasite communities of perch populations living in sympatry with pumpkinseed exhibited significantly higher parasite abundance, infracommunity richness and dominance, along with lower equitability compared to those in allopatric populations. These findings suggest that the introduction of pumpkinseed may have contributed to shifts in parasite abundance, diversity and community composition in the native fish species. Figure 2. Parasite abundance (A) and infracommunity species richness (B) in sympatric and allopatric populations of introduced pumpkinseed Lepomis gibbosus (green bars) and native European perch Perca fluviatilis (blue bars) in small lentic waterbodies in the lower Morava River Basin. Parasite abundance is expressed with a logarithmic scale. Horizontal line: median, box: interquartile range, whiskers: non-outlier range, points: outliers. 166 NeoBiota 102: 151–171 (2025), DOI: 10.3897/neobiota.102.148301 Maria Yu. Tkachenko et al.: Shift in parasite load in native and non-native Eupercarian fish Bowles J, Hope M, Tiu WU, Liu XS, McManus DP (1993) Nuclear and mitochondrial genetic markers highly conserved between Chinese and Philippine Schistosoma japonicum. Acta Tropica 55(4): 217–229. https://doi.org/10.1016/0001-706X(93)90079-Q Bush AO, Lafferty KD, Lotz JM, Shostak AW (1997) Parasitology Meets Ecology on Its Own Terms: Margolis et al. Revisited. 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