DNA metabarcoding reveals spatial and temporal variation of fish eye fluke communities in lake ecosystems.

Eye flukes (Diplostomidae) are diverse and abundant trematode parasites that form multi-species communities in fish with negative effects on host fitness and survival. However, the environmental factors and host-related characteristics that determine species diversity, composition, and coexistence in such communities remain poorly understood. Here, we developed a cost-effective cox1 region-specific DNA metabarcoding approach to characterize parasitic diplostomid communities in two common fish species (Eurasian perch and common roach) collected from seven temperate lakes in Estonia. We found considerable inter- and intra-lake, as well as inter-host species, variation in diplostomid communities. Sympatric host species characterization revealed that parasite communities were typically more diverse in roach than perch. Additionally, we detected five positive and two negative diplostomid species associations in roach, whereas only a single negative association was observed in perch. These results indicate that diplostomid communities in temperate lakes are complex and dynamic systems exhibiting both spatial and temporal heterogeneity. They are influenced by various environmental factors and by host-parasite and inter-parasite interactions. We expect that the described methodology facilitates ecological and biodiversity research of diplostomid parasites. It is also adaptable to other parasite groups where it could serve to improve current understanding of diversity, distribution, and interspecies interactions of other understudied taxa.

ERECTION OF BOTHRIOCESTUS N. GEN. (CESTODA: BOTHRIOCEPHALIDEA) AND REDESCRIPTION OF BOTHRIOCESTUS CUSPIDATUS (COOPER, 1917) (SYN. BOTHRIOCEPHALUS CUSPIDATUS) FROM WALLEYE, SANDER VITREUS, (PERCIFORMES: PERCIDAE) IN NORTH AMERICA.

Based on previous molecular phylogenetic analyses, Bothriocestus n. gen. is erected to accommodate bothriocephalid tapeworms that have an elongate scolex, a well-developed apical disc, and a narrow neck region, parasitize freshwater fishes in the Holarctic, and were previously placed in the polyphyletic genus Bothriocephalus Rudolphi, 1808 (Cestoda: Bothriocephalidea). Bothriocestus claviceps (Goeze, 1782) n. comb., a parasite of eels (Anguilla spp.) in the Holarctic region, is designated as the type species. Another species of the new genus, Bothriocestus cuspidatus (Cooper, 1917) (syn. Bothriocephalus cuspidatusCooper, 1917) is redescribed from type and voucher specimens, and new material from the type host, the walleye, Sander vitreus (Mitchill, 1818) (Perciformes: Percidae), in Manitoba and Ontario (where the type locality is located) (Canada) and in New York state and Wisconsin. Bothriocestus cuspidatus of S. vitreus is characterized primarily by the possession of a narrow, long strobila (total length up to 18 cm) composed of distinctly craspedote, trapezoidal proglottids, with primary, secondary, and tertiary proglottids differing in size, and by an arrow-shaped (=cuspidatus) scolex that is distinctly broader than the first proglottids, widest near the base in lateral view and gradually becoming broader toward the anterior end in dorsoventral view. A "dwarf" form of B. cuspidatus (total length of 9-27 mm) from Johnny darter, Etheostoma nigrum Rafinesque, 1820, and tessellated darter, Etheostoma olmstedi Storer, 1842 (both Percidae: Etheostominae), is also characterized morphologically in the present paper.

OCCURRENCE OF EUSTRONGYLIDES EXCISUS (NEMATODA:DIOCTOPHYMATIDAE) IN EUROPEAN PERCH (PERCA FLUVIATILIS) AND GREAT CORMORANT (PHALACROCORAX CARBO) IN LAKE ANNONE, NORTHERN ITALY.

The genus Eustrongylides includes zoonotic nematodes that infect fish species and fish-eating birds of freshwater ecosystems. This study aimed to evaluate the occurrence of Eustrongylides in the paratenic host Perca fluviatilis (European perch) and in the definitive host, Phalacrocorax carbo sinensis (great cormorant), in Lake Annone, a shallow eutrophic lake located in the pre-mountainous area of the Alps in northwest Italy where wintering cormorants coexist with new breeding colonies. A total of 114 European perch and 48 cormorants were examined for the occurrence of Eustrongylides. All parasites collected were identified with microscopic examination and molecular analysis. Overall, 11 specimens of European perch (9.6%) and 13 individuals of cormorants (27%) harbored nematodes identified as fourth-stage larvae and adults of Eustrongylides excisus. The observed prevalence of Eustrongylides spp. appears to be intermediate between the higher values in cormorant breeding areas in northern Europe and the lower prevalence observed in their wintering sites in southernmost Europe. Considering the eutrophication status of freshwater ecosystems and the increasing population of the cormorants, Eustrongylides has an increasing potential range of dispersion in Europe, including Italy; thus an extensive surveillance should be carried out, especially given the zoonotic potential of this nematode.

Fish Parasite Community of Three Lakes with Different Trophic Status in Mecklenburg-Western Pomerania, Germany.

PURPOSE: The present study investigates the fish parasite fauna from Lake Tollense, Mecklenburg-Western Pomerania, Germany. METHODS: A total of 117 perch (Perca fluviatilis), bream (Abramis brama) and roach (Rutilus rutilus) were sampled for parasites during 2018 and 2019 from Lake Tollense and compared with earlier data from Lake Malchin and Lake Hohen Sprenz in 2011 and 2014, respectively. Parasites were identified based on morphological and molecular characters. RESULTS: A total of 32 parasite species were isolated from fishes of Lake Tollense, predominated by digeneans. Diplostomum baeri was found only in perch while D. spathacaeum was isolated from bream and roach. Parasite comparison of similar hosts between lakes revealed highly abundant Ichthyocotylurus spp. in perch and bream of Lake Malchin while Aspidogaster limacoides was most common in roach from Lake Tollense. Diversity indices of roach showed significant variation between localities. However, NMDS graph revealed separation of the three freshwater habitats based on the parasite fauna of perch, bream and roach. CONCLUSION: Based on the multivariate statistical analysis, the three natural inland water bodies could be distinguished based on the parasite communities of perch, bream and roach. The potential to utilise fish parasites as biological indicators in freshwater ecosystem is discussed.

Eye fluke effects on Danish freshwater fish: Field and experimental investigations.

Eye flukes in fish are common in freshwater lakes. Fish become infected by the penetration of cercariae released from freshwater snails, and high infection pressures may be associated with mortalities in a Danish lake. Examination of two other freshwater lakes, combined with laboratory study, supported the notion. We investigated 77 freshwater fish from two lakes and the infection level suggested the occurrence of a high cercarial infection pressure in the Danish lakes. Dominant genera were Tylodelphys and Diplostomum covering a range of species identified by PCR and sequencing of the 18S (partial)-ITS1-5.8S-ITS2-28S (partial) of the rDNA. Cercariae of the prevalent species Diplostomum pseudospathaceum were used to infect zebrafish Danio rerio for the elucidation of short-term effects on the fish host. Zebrafish did not display abnormal behaviour when exposed to 200-400 cercariae, but a dosage of 600 and 1,000 cercariae/fish proved lethal. When fish were exposed to sublethal dosages, 19 out of 27 immune genes were significantly regulated and three genes encoding cytokine (IL 4/13B, IL-6 and IL-8) were upregulated at 3 hr post-infection (hpi), whereas others were downregulated especially at a later time point. We suggest that direct massive cercarial penetration of fish surfaces may be detrimental and may represent a threat to fish populations.

Morphological and molecular characterization of Eustrongylides excisus larvae (Nematoda: Dioctophymatidae) in Sander lucioperca (L.) from Northern Turkey.

The genus Eustrongylides Jägerskiöld, 1909 includes parasitic nematodes (Dioctophymatidae) affecting various fish species and piscivorous birds of freshwater ecosystems. Currently, there is little information on the molecular characterization of E. excisus based on nuclear ribosomal internal transcribed spacer (ITS) rDNA regions. However, before the present study, there had been no reports of characterizing the E. excisus using nuclear small subunit ribosomal RNA (SSU rRNA) and mitochondrial cytochrome c oxidase subunit I (COI) genes sequences. In the present study, Eustrongylides spp. larvae were collected from pike-perch Sander lucioperca (L.) in Northern Turkey, and characterized by sequencing of ITS regions, SSU rRNA, and COI markers. Larvae herein morphologically identified as the fourth stage of Eustrongylides spp. were genetically identified as E. excisus based on the ITS sequence analysis. This study is the first record of SSU rRNA and COI sequences for E. excisus in GenBank. This is also a molecular characterization of E. excisus for the first time in Turkey. The ITS, SSU rRNA, and COI sequences of E. excisus can be used to establish the phylogenetic relationships of Eustrongylides species from Turkey and worldwide for further studies.

Parasite diversity of introduced fish Lates niloticus, Oreochromis niloticus and endemic Haplochromis spp. of Lake Victoria, Kenya.

This study reports on the status of metazoan fish parasites in Lake Victoria following the establishment of introduced Lates niloticus (Latidae) and Oreochromis niloticus (Cichlidae) and changes in environmental quality. For this study, 412 fish specimens were examined for parasites: 103 L. niloticus, 165 O. niloticus, 82 Haplochromis piceatus and 62 H. humilior (endemic cichlids). In total, 25 parasite taxa were identified: Lates niloticus (6), O. niloticus (19) and Haplochromis spp. (13). The myxosporean Henneguya ghaffari, prevalence (P) = 79% and the monogenean Diplectanum lacustris (P = 34%), were the dominant parasites on L. niloticus. Myxobolus sp. (P = 44%) was dominant on O. niloticus while for the haplochromines, metacercariae of 'Neascus' sp. (Diplostomidae) was dominant (P = 37%). Contrary to reports of high diversity of monogeneans on endemic species, the haplochromines harboured only Cichlidogyrus gillardinae (P = 6.9%). Oreochromis niloticus harboured seven monogenean species: Cichlidogyrus sclerosus, C. halli, C. tilapiae, C. quaestio, Scutogyrus longicornis, Gyrodactylus cichlidarum and G. malalai-they appear to have been co-introduced with the fish. Cichlidogyrus gillardinae, C. quaestio, G. malalai and the acanthocephalan Acanthogyrus (Acathosentis) tilapiae are recorded for the first time in Lake Victoria, representing new geographical records. Presence of A. (A) tilapiae in Haplochromis spp. indicates possible spillover from O. niloticus. Low prevalence and diversity of monogeneans, crustaceans and glochidia on the fish corresponded with increased pollution in the lake. Overall, changing environmental conditions and introductions of fish species have contributed to parasite community changes in Lake Victoria.

Urocleidus sayani n. sp. (Monogenea: Dactylogyridae) from Gills of Pirate Perch (Aphredoderus sayanus).

Urocleidus sayani n. sp. is described from the gills of pirate perch (Aphredoderus sayanus) in the Wisconsin backwaters of the upper Mississippi River and was found in samples from the Southeastern United States. Urocleidus sayani n. sp. is the second monogenean described from the pirate perch and the first for this host within Dactylogyridae. The description includes a partial 18S rRNA gene sequence (623 bp), filling a void in sequence data from North American monogeneans.

Eye fluke infection changes diet composition in juvenile European perch (Perca fluviatilis).

Intraspecific diet specialization, usually driven by resource availability, competition and predation, is common in natural populations. However, the role of parasites on diet specialization of their hosts has rarely been studied. Eye flukes can impair vision ability of their hosts and have been associated with alterations of fish feeding behavior. Here it was assessed whether European perch (Perca fluviatilis) alter their diet composition as a consequence of infection with eye flukes. Young-of-the-year (YOY) perch from temperate Lake Müggelsee (Berlin, Germany) were sampled in two years, eye flukes counted and fish diet was evaluated using both stomach content and stable isotope analyses. Perch diet was dominated by zooplankton and benthic macroinvertebrates. Both methods indicated that with increasing eye fluke infection intensity fish had a more selective diet, feeding mainly on the benthic macroinvertebrate Dikerogammarus villosus, while less intensively infected fish appeared to be generalist feeders showing no preference for any particular prey type. Our results show that infection with eye flukes can indirectly affect interaction of the host with lower trophic levels by altering the diet composition and highlight the underestimated role of parasites in food web studies.

Phyllodistomum kupermani n. sp. from the European perch, Perca fluviatilis L. (Perciformes: Percidae), and redescription of Phyllodistomum macrocotyle (Lühe, 1909) with notes on the species diversity and host specificity in the European Phyllodistomum spp. (Trematoda: Gorgoderidae).

BACKGROUND: European species of the large genus Phyllodistomum Braun, 1899 had historically been erected based solely on morphological characters. Unfortunately, many of them are still poorly known and inadequately described. Molecular approaches are critical to delineate species which were impossible to differentiate based on morphology alone. METHODS: New samples of adult Phyllodistomum spp. were collected from the urinary bladder and/or ureters of European freshwater fishes and fixed to conduct a light and scanning electron microscopy study, and to obtain sequences of nuclear (ITS2 spacer and 28S rRNA gene), to be analysed in the context of a molecular phylogeny. RESULTS: Based on new findings, a new species of Phyllodistomum from the urinary bladder of the European perch, Perca fluviatilis, in Volga River basin, Russia, is described. Additionally, new data on the morphology and tegumental surface topography of P. macrocotyle (Lühe, 1909) Odhner, 1911 from ureters of the common rudd, Scardinius erythrophthalmus, is presented. The host range of P. folium, confirmed by DNA analysis, is extended to other cyprinid fish species. CONCLUSIONS: The present study has again shown that species of the genus Phyllodistomum are in dire need of revision based on both molecular analysis and detailed morphological redescriptions of the forms attributed to the genus. Morphologically, P. kupermani n. sp. most closely resembles P. pseudofolium, a highly host-specific parasite of Gymnocephalus cernuus (L.), but molecular phylogenetic analyses based on ITS2 and 28S rDNA sequences showed that these species are distantly related. Phyllodistomum kupermani n. sp. was found to be phylogenetically most closely related to the type-species of Phyllodistomum, P. folium. Phylogenetic analyses revealed that Phyllodistomum kupermani n. sp. and P. folium formed a clade with other freshwater species for which cystocercous cercariae develop in bivalves of the family Sphaeriidae. The micromorphology and tegumental surface topography of P. macrocotyle revealed in the present study provide a valuable taxonomic criterion for congeneric species differentiation.

Ex Uno Plures? Morphotype and Lineage Diversity of Bothriocephalus (Cestoda: Bothriocephalidea) in North American Freshwater Fishes.

Morphological and molecular evaluation of tapeworms of the genus Bothriocephalus Rudolphi, 1808 (Cestoda: Bothriocephalidea), based on newly collected and uniformly fixed worms from freshwater fishes in Canada and the United States has revealed unexpected diversity. With a combination of selected morphological features and 4 molecular markers (18S rDNA V8 region, ITS1, ITS2, and COI gene sequences), the following morphotypes and lineages of the Bothriocephalus cuspidatus Cooper, 1917 complex were identified, several of which are specific to their respective fish definitive hosts and may represent separate species: B. cuspidatus sensu stricto from walleye, Sander vitreus (type host), which likely includes a miniature morphotype from Johnny darter, Etheostoma nigrum (both Percidae); Bothriocephalus morphotype from pumpkinseed, Lepomis gibbosus (Centrarchidae); and Bothriocephalus morphotype from rock bass, Ambloplites rupestris (Centrarchidae). The Bothriocephalus morphotype from goldeye, Hiodon alosoides (Hiodontidae), may also represent a separate lineage (possibly Bothriocephalus texomensisSelf, 1954) but requires additional studies. A morphotype from smallmouth bass, Micropterus dolomieu, based on a single specimen, is morphologically and genetically very similar to the morphotype from rock bass. Morphological study of the scolex and strobila of heat-killed and fixed specimens has revealed consistent differences, often subtle, that allowed us to differentiate between these morphotypes.

Morphological abnormalities in fish parasites: a potential tool for biomonitoring natural contaminants?

A comparative study on the strobilar morphology of the tapeworm Proteocephalus percae (Müller, 1780) (Cestoda), a parasite of the perch Perca fluviatilis (L.), showed a high percentage of abnormally developed parasite individuals. The evaluation of biological samples showed seven types of morphological abnormalities, mostly related to reproductive organs of the model tapeworm species. The most commonly identified deformity was an incomplete segmentation of the strobila. A malformed ovary, which is a structural anomaly linked with proglottization and maturation of the strobila, was also shown to be rather frequent. Offish hosts (P. percae) were collected from two localities with different levels of heavy metal pollution, the highly contaminated water reservoir Ruzín and a control locality, the water reservoir Palcmanská Masa, which belongs to the European network of protected areas in Slovakia. Tapeworm abnormalities occurred more frequently in individuals from the contaminated environment (29.9%) compared with individuals from the control site (4.9%). The concentrations of heavy metals found in the parasites and their fish hosts from the heavily polluted reservoir support our assumption that the occurrence of abnormalities could be linked with the destructive effect of toxic substances. The present study also demonstrates that the enumeration of body deformities exceeding the common level of phenotypic variability of particular parasitic species could potentially be used as an indicator of environmental problems.

Humic-acid-driven escape from eye parasites revealed by RNA-seq and target-specific metabarcoding.

BACKGROUND: Next generation sequencing (NGS) technologies are extensively used to dissect the molecular mechanisms of host-parasite interactions in human pathogens. However, ecological studies have yet to fully exploit the power of NGS as a rich source for formulating and testing new hypotheses. METHODS: We studied Eurasian perch (Perca fluviatilis) and its eye parasite (Trematoda, Diplostomidae) communities in 14 lakes that differed in humic content in order to explore host-parasite-environment interactions. We hypothesised that high humic content along with low pH would decrease the abundance of the intermediate hosts (gastropods), thus limiting the occurrence of diplostomid parasites in humic lakes. This hypothesis was initially invoked by whole eye RNA-seq data analysis and subsequently tested using PCR-based detection and a novel targeted metabarcoding approach. RESULTS: Whole eye transcriptome results revealed overexpression of immune-related genes and the presence of eye parasite sequences in RNA-seq data obtained from perch living in clear-water lakes. Both PCR-based and targeted-metabarcoding approach showed that perch from humic lakes were completely free from diplostomid parasites, while the prevalence of eye flukes in clear-water lakes that contain low amounts of humic substances was close to 100%, with the majority of NGS reads assigned to Tylodelphys clavata. CONCLUSIONS: High intraspecific diversity of T. clavata indicates that massively parallel sequencing of naturally pooled samples represents an efficient and powerful strategy for shedding light on cryptic diversity of eye parasites. Our results demonstrate that perch populations in clear-water lakes experience contrasting eye parasite pressure compared to those from humic lakes, which is reflected by prevalent differences in the expression of immune-related genes in the eye. This study highlights the utility of NGS to discover novel host-parasite-environment interactions and provide unprecedented power to characterize the molecular diversity of cryptic parasites.

First Report of Clinostomum complanatum (Trematoda: Digenea) in European Perch (Perca fluviatilis) from an Italian Subalpine Lake: A Risk for Public Health?

Clinostomum complanatum, a digenean trematode of the Clinostomidae family, is a fish-borne zoonotic parasite responsible for Halzoun syndrome in humans and is transmitted through the consumption of raw or undercooked freshwater fish. Of the total of 112 specimens of European perch (Perca fluviatilis) sampled from a subalpine lake (Lake Endine) in North Italy in 2019, 21 (18.75%) tested positive for encysted metacercariae in the fillet. This study reports the first isolation of C. complanatum in P. fluviatilis and highlights the possible zoonotic risk for consumers, since P. fluviatilis is a food fish used in the traditional local cuisine.

Myxobolus spp. (Cnidaria: Myxozoa) in introduced yellow perch Perca flavescens (Mitchill, 1814).

We surveyed introduced yellow perch Perca flavescens (Mitchill, 1814) from the Willamette River, OR, USA, to determine if these fish have co-introduced myxosporean parasites. Mature parasite myxospores were observed in brains of 3/19 fish, and were morphologically and molecularly consistent with Myxobolus neurophilus (Guilford 1963), a parasite known from yellow perch in their native range. We identified another Myxobolus species from the gill filaments of 1/22 fish. The spores from the gill filaments were oval-shaped, 11.7 (10.7-12.3) µm long × 8.6 (7.7-9.0) µm wide × 5.2 (4.6-5.6) µm thick, with two oval-shaped polar capsules 5.7 (5.1-6.5) µm × 2.7 (2.4-3.2) µm, each containing a polar tubule with 8-9 turns. Small-subunit ribosomal DNA sequences from each of four plasmodia were identical, and 4.0% different (over 1800 nucleotides) from the closest known myxosporeans. Interestingly, these sequences had overlapping peaks in their chromatograms, which suggested that DNA from multiple species was present. Hence, we isolated and sequenced three individual myxospores and found that they too had mixed chromatograms, which indicated presence of at least two sequence types of small-subunit ribosomal DNA in each spore (GenBank accession MK592012, MK592013), a rare character among described myxosporeans. The spore morphology, morphometry, tissue tropism, and DNA sequence supported a diagnosis of a novel species, Myxobolus doubleae n. sp. This parasite is unknown from yellow perch in its native range, despite extensive historical surveys, which suggests that introduced yellow perch might have acquired an endemic Myxobolus species via spillback from another fish host.

Two Novel Myxozoans from Pirate Perch Aphredoderus sayanus (Gilliams, 1824) in the Upper Mississippi River, Including the First North American Species of Hennegoides Lom, Tonguthai, & Dyková, 1991.

The pirate perch Aphredoderus sayanus is a relatively small fish species found in rivers throughout much of the eastern United States. Due to their cryptic nature, relatively little is known regarding their parasite fauna. A survey of pirate perch from the upper Mississippi River revealed 2 novel myxozoans. Hennegoides flockae n. sp. was observed in heavily infected gills where the lamellae featured irregular expansion by bulbous myxozoan polysporic plasmodia, typically affecting the middle to distal half of the filaments. When severe, infection of sequential filaments was such that the filaments were fused, forming what appeared as multicystic/lobular parasitic aggregates subdivided by fine epithelial cords. The total myxospore length of Hennegoides flockae was 35.4-46.4 (41.3 ± 3.3) and the spore body, asymmetrically ovoid in valvular view, was 15.4-18.7 (17.0 ± 0.7) × 7.1-8.7 (7.9 ± 0.4). Henneguya marcquenskiae n. sp. was observed in the liver with plasmodia present randomly and infrequently in the hepatocellular parenchyma. The total myxospore length for Henneguya marcquenskiae was 39.5-55.9 (48.4 ± 4.2), with the spore body being lanceolate, 13.9-16.5 (15.4 ± 0.7) × 7.1-9.0 (8.3 ± 0.5). Phylogenetic analysis of the SSU rRNA gene placed both Hennegoides flockae and Henneguya marcquenskiae as sisters to each other in a clade containing other Myxozoans known to infect the gills of esocids, percids, and centrarchids. These parasites represent the first reports of Henneguya and Hennegoides from pirate perch, with the latter being the first report of this genus outside of the Asian continent.

The Proteocephalus Species-Aggregate in Freshwater Centrarchid and Percid Fishes of the Nearctic Region (North America).

In the present paper, species of the Proteocephalus-aggregate de Chambrier, Zehnder, Vaucher, and Mariaux, 2004 (Cestoda: Proteocephalidae) reported from centrarchid and percid fishes in North America are reviewed, and their taxonomic status is critically assessed based on a study of type specimens and new material from Canada and the United States. The following 3 species, supposedly strictly specific to their fish definitive hosts, are recognized as valid: (1) Proteocephalus fluviatilis Bangham, 1925 (new synonyms Proteocephalus osburni Bangham, 1925 and Proteocephalus microcephalus Haderlie, 1953 ; Proteocephalus 'robustus' nomen nudum) from the smallmouth and largemouth bass, Micropterus dolomieu (Lacépède) (type host) and Micropterus salmoides (Lacépède) (both Centrarchidae); (2) Proteocephalus luciopercae Wardle, 1932 (new synonym Proteocephalus stizostethi Hunter and Bangham, 1933 ) from the walleye, Sander vitreus (Mitchill) (type host), and sauger, Sander canadensis (Griffith et Smith) (Percidae); and (3) Proteocephalus pearsei La Rue, 1919 , a parasite of the yellow perch, Perca flavescens Mitchill (Percidae). All species are illustrated based on new, properly heat-fixed material. Scanning electron micrographs of the scoleces of percid tapeworms P. luciopercae and P. pearsei, as well as the bass tapeworms P. fluviatilis and Proteocephalus ambloplitis ( Leidy, 1887 ), the latter of which does not belong to this Proteocephalus-aggregate, are provided for the first time together with a simple key to species identification of proteocephalids from centrarchiform and perciform teleost fishes.

Eye fluke (Tylodelphys clavata) infection impairs visual ability and hampers foraging success in European perch.

Visual performance and environmental conditions can influence both behavioral patterns and predator-prey interactions of fish. Eye parasites can impair their host's sensory performance with important consequences for the detection of prey, predators, and conspecifics. We used European perch (Perca fluviatilis) experimentally infected with the eye fluke Tylodelphys clavata and evaluated their feeding behavior and competitive ability under competition with non-infected conspecifics, in groups of four individuals, for two different prey species (Asellus aquaticus and Daphnia magna). To test whether the effect of T. clavata infection differs at different light conditions, we performed the experiments at two light intensities (600 and 6 lx). Foraging efficiency of perch was significantly affected by infection but not by light intensity. The distance at which infected fish attacked both prey species was significantly shorter in comparison to non-infected conspecifics. Additionally, infected fish more often unsuccessfully attacked A. aquaticus. Although the outcome of competition depended on prey species, there was a general tendency that non-infected fish consumed more of the available prey under both light intensities. Even though individual prey preferences for either A. aquaticus or D. magna were observed, we could not detect that infected fish change their prey preference to compensate for a reduced competitive foraging ability. As infection of T. clavata impairs foraging efficiency and competitive ability, infected fish would need to spend more time foraging to attain similar food intake as non-infected conspecifics; this presumably increases predation risk and potentially enhances transmission success to the final host.

Influence of an extreme climatic event on bilateral asymmetry and occurrence of eye flukes in perch (Perca fluviatilis) and roach (Rutilus rutilus) from a lake in southeast England.

Eye flukes are common infections of freshwater fish and their occurrence as metacercarial stages may occur non-randomly resulting in an asymmetrical distribution within the host eyes. However, from previous studies the presentation of bias by these trematodes lacks consistency suggesting that congenital asymmetrical effects are unlikely to be the cause and exogenous factors, such as environmental stress, may be more influential. The present study, undertaken over a 4-year period, investigates the impact of an extreme heatwave and drought on the annual bilateral asymmetry and occurrence of two eye fluke species (Diplostomum sp. in the lens and Tylodelphys sp. in the vitreous humour) from perch (Perca fluviatilis) and roach (Rutilus rutilus). The onset of the extreme climatic event resulted in a significant negative effect on the occurrence of the eye flukes. Bilateral asymmetry, which was present within both trematode species and hosts, appeared to be highly variable between eye fluke and fish species and also year of study. However, during the pre-drought period both host species demonstrated significant asymmetry for Tylodelphys sp. but not for Diplostomum sp. while during the drought this bias was reversed. The potential role of fluctuating asymmetry of fish hosts in structuring the bilateral asymmetry of eye flukes is discussed.

Endohelminths of European Perch (Perca fluviatilis) from Selected Localities in Poland with an Emphasis on Search of the Broad Fish Tapeworm Dibothriocephalus latus.

INTRODUCTION: A piscivorous fish European perch (Perca fluviatilis) is present in all types of lakes and brackish waters in Poland. Previous ichthyoparasitological surveys revealed broad spectrum of endohelminths in perch from different aquatic environments. Among them, detection of Diphyllobothrium sp. and Dibothriocephalus latus (syn. Diphyllobothrium latum) in the Pomeranian Bay is of particular interest, since D. latus is one of the causative agents of diphyllobothriasis, fish-borne parasitic zoonosis. Besides, D. latus eggs were previously detected in coprological samples of otter, wolf and lynx from the Bialowieza Primeval Forest. AIM: To conduct parasitological examinations of European perch from the Pomeranian Bay in order to detect the spectrum of its endohelminths and to provide a pilot study on helminths of perch from different water bodies in the Bialowieza Primeval Forest. Due to zoonotic character of D. latus, we have focused our attention to this tapeworm. RESULTS: The larvae of tapeworm Triaenophorus nodulosus and thorny-headed worm Acanthocephalus lucii were detected in perch from the Pomeranian Bay. In perch from different localities in the Bialowieza Primeval Forest, T. nodulosus, A. lucii and tapeworm Proteocephalus percae were detected. D. latus plerocercoids were found neither in musculature nor in peritoneal cavity and other internal organs of any of the fish examined from both studied localities in Poland. CONCLUSION: Future screening implementing morphological and molecular markers is needed in order to understand the current distribution of D. latus in Europe.