Digenean Trematodes Research Articles

Helminth Parasites of Invasive Freshwater Fish in Lithuania.

Freshwater ecosystems in Lithuania are threatened by the introduction of invasive fish species including Neogobius fluviatilis, N. melanostomus, Perccottus glenii, and Pseudorasbora parva. Data on helminth parasites of these fishes have not been comprehensively studied, with only two reports on parasites of N. melanostomus from the Curonian Lagoon and Baltic Sea, Lithuania. We examined 278 fish individuals representing 4 invasive species from 13 waterbodies. Using morphological and molecular analyses, we identified 29 helminth taxa representing 15 digenean trematodes, 6 nematodes, 4 cestodes, 2 acanthocephalans, and 2 monogeneans. With 18 species, N. fluviatilis had the highest helminth diversity, followed by N. melanostomus (11 species) and Ps. parva (8 species). Perccottus glenii was found to be free from helminth infection. The availability of historical information on the native fish parasites in Lithuania allowed us to conclude that out of the 29 recorded species, invasive fish serve as hosts for 22 local fish helminth species, while 7 helminth species have been reported exclusively in invasive fish. Based on newly obtained and previously published data, a total of 34 helminth species parasitise invasive fish in Lithuania, of which 30 use these fish as intermediate or paratenic hosts.

Fasciola hepatica and Fasciola hybrid form co-existence in yak from Tibet of China: application of rDNA internal transcribed spacer.

Fasciolosis is a parasitic disease affecting humans and livestock, caused by digenean trematodes of the genus Fasciola, primarily F. hepatica and F. gigantica. This study investigates the coexistence of these species and their hybrids in yaks from Tibet, China. We analyzed the nuclear rDNA internal transcribed spacer (ITS) regions, including ITS1 and ITS2, through Sanger sequencing and Next-Generation Sequencing (NGS) to assess single-nucleotide polymorphisms (SNPs). Our results reveal that one specimen (NM008B) is identical to pure F. hepatica, while another (NM008A) contains genetic information from both F. hepatica and F. gigantica, indicating potential hybridization or introgression. The morphological analysis reveals that the collected adult F. hepatica specimens exhibit distinct characteristics, while the hybrid specimens display "intermediate" features of F. hepatica and F. gigantica. This study is the first to document the coexistence of F. hepatica and hybrid Fasciola forms in a single yak. The findings underscore the complexities of hybridization dynamics and the necessity for advanced molecular techniques in accurately identifying Fasciola species. Future research should focus on mitochondrial DNA and other nuclear gene analysis to further elucidate the nature of these hybrids and their ecological implications.

Complete mitochondrial genome and phylogenetic analysis of Dollfustrema vaneyi (Trematoda: Bucephalidae)

BackgroundThe Bucephalidae is a large family of digenean trematodes but most previous analyses of its phylogenetic position have relied on a single mitochondrial gene or morphological features. Mitochondrial genomes (mitogenomes) remain unavailable for the entire family. To address this, we sequenced the complete mitogenome of Dollfustrema vaneyi and analyzed the phylogenetic relationships with other trematodes.ResultsThe circular genome of Dollfustrema vaneyi spanned 14,959 bp and contained 12 protein-coding genes, 22 transfer RNA genes, 2 ribosomal RNA genes, and a major non-coding region. We used concatenated amino acid and nucleotide sequences of all 36 genes for phylogenetic analyses, conducted using MrBayes, IQ-TREE and PhyloBayes. We identified pronounced topological instability across different analyses. The addition of recently sequenced two mitogenomes for the Aspidogastrea subclass along with the use of a site-heterogeneous model stabilized the topology, particularly the positions of Azygiidae and Bucephalidae. The stabilized results indicated that Azygiidae was the closest lineage to Bucephalidae in the available dataset, and together, they clustered at the base of the Plagiorchiida.ConclusionsOur study provides the first comprehensive description and annotation of the mitochondrial genome for the Bucephalidae family. The results indicate a close phylogenetic relationship between Azygiidae and Bucephalidae, and reveal their basal placement within the order Plagiorchiida. Furthermore, the inclusion of Aspidogastrea mitogenomes and the site-heterogeneous model significantly improved the topological stability. These data will provide key molecular resources for future taxonomic and phylogenetic studies of the family Bucephalidae.

Survey of trematodes in Manila clam Ruditapes philippinarum on the west coast of Korea: A preliminary study

The Manila clam Ruditapes philippinarum on the west coast of Korea harbors several digenetic trematodes. However, most studies in this region have been restricted to a few sampling sites and the current species designation of some trematodes is largely based on morphology, leaving the molecular phylogenetic position among the Digenea unsolved. Thus, we first provide both morphology and molecular phylogeny of some components in the trematodes community in the Manila clam based on a large-scale survey of 26 sites on the west coast, where well-developed tidal flats serve as large commercial clam culture beds. Our study revealed that the trematodes community in the clams consisted of at least 5 species that belong to 3 families (Himasthlidae, Gymnophallidae, Baccigeridae) and 1 superfamily (Monorchioidea). The life mode of the 5 different species included the metacercaria and sporocyst, with one species (Parvatrema duboisi) utilizing the clams as both the first and/or second intermediate host. Trematode infection prevalences were not evenly distributed among the study sites, although the reasons behind this are yet to be determined. Morphological identification was confirmed with the molecular analyses based on ITS and 28S rDNA; phylogenetic analysis also revealed that Cercaria pectinata infecting the clam gonad should be referred to as Bacciger bacciger hereafter. The present preliminary study provides a crucial baseline that could be further developed in a future study on the digenean trematodes community in the Manila clam.

Viability of Metagonimus romanicus (Ciurea, 1915) metacercariae after physico-chemical treatments

Digenean (Heterophyidae) trematodes include several zoonotic species such as the genus Metagonimus (Katsurada, 1912). Metagonimus romanicus (Ciurea, 1915) is a species widely distributed in Europe, whose metacercariae can be found on cyprinoids from the River Danube in Hungary. The aim of the study was to measure the viability of Metagonimus romanicus metacercariae by physical (freezing, heating, desiccation) and chemical (acetic acid and NaCl solutions) treatments. The methods were chosen as a model for procedures commonly used in traditional food preservation (such as freezing, salting, pickling and smoking) to measure the survival rate of metacercariae under different conditions. Most physical treatments (freezing of metacercariae at −20 °C, keeping them at 40 °C and 60 °C and desiccation) and chemical treatments (2.5%, 5%, 10% acetic acid solution and 5% and 10% NaCl solution) killed the metacercariae in a relatively short time (from 30 min to 6 days depending on the treatment) so their effects eliminated the risk of zoonotic infection. On the other hand, the metacercariae survived at room temperature and at 4 °C up to one month. Therefore, storing unprepared fish in domestic refrigerators cannot prevent infection with metacercariae in humans.

Incidence and pathological effects during encysted progenetic Euclinostomum heterostomum metacercariae infection in the freshwater fish spotted snakehead (Channa punctatus)

Aim: To assess the incidence and pathological effects during encysted progenetic Euclinostomum heterostomum metacercariae infestation in the freshwater fish spotted snakehead (Channa punctatus). Methodology: Samples of Channa punctatus were collected from Tilaiya Reservoir, Koderma, Jharkhand, India, during pre-monsoon, monsoon, and post-monsoon seasons. The encysted metacercariae trematodes were isolated from Channa punctatus. Characterization was performed based on the morphology and sequencing analysis of a partial 18S rRNA gene. Further, the kidney and liver samples from infected fish were subjected to histopathological studies. Prevalence, abundance, and Intensity of infection were further determined. Results: Morphological identification suggests that the encysted metacercariae belonged to Digenean trematodes of genera Euclinostomum, and amplification of partial sequence of 18S rRNA, and phylogenetic analysis revealed the isolated metacercariae were Euclinostomum heterostomum. The histopathological study of the kidney showed hepatocyte degeneration, cytoplasmic vacuolation, and nuclear transformations. Liver fibrosis was observed in Channa punctatus. The prevalence during pre-monsoon, monsoon, and post-monsoon season ranged from 66.1% to 81.1%, abundance from 0.74 to 0.92, and intensity ranged from 1 to 1.14 using the Margolis formula. Interpretation: The study highlights the infestation of Euclinostomum heterostomum in Channa punctatus during the pre- monsoon climate appears favorable for the abundance of parasites. Hence, a proper inspection before consumption is necessary. Key words: Euclinostomum heterostomum, Infestation, Metacercariae, Parasites, Progenetic infection

Cannibalism and competition can increase parasite abundance for parasites with complex life history strategies

AbstractEcological interactions among hosts are critical to consider when predicting disease dynamics. Most theory predicts that intraguild predation (IGP) and cannibalism negatively impact parasite populations, but this is based primarily on assumptions of simple or single‐host life cycles. Here we investigate the effects of cannibalism in a size‐structured host population on two digenean trematodes that have complex, multihost life cycles. A high incidence of cannibalism among paratenic hosts produced higher parasite infection loads and abundance, whereas cannibalism among obligate hosts reduced parasite abundances. We attributed this difference to trophic transmission aggregating parasites in larger, potentially fitter hosts and also to transmission among paratenic hosts via cannibalism. Moreover, we found evidence of indirect competitive interactions between parasites that can also increase infections at small scales. Our results show there are multiple mechanisms through which high cannibalism environments can benefit parasites that use paratenic hosts and trophic transfer to complete their life cycles.

A sea of change: Tracing parasitic dynamics through the past millennia in the northern Adriatic, Italy

Abstract Our study uses data from Holocene core samples and modern death assemblages to understand how human-induced environmental change in the northern Adriatic Sea (Italy) may have affected parasite-host dynamics in the economically important bivalve Chamelea gallina. Thirty-one radiocarbon dates confirm temporal distinctness between the periods before and after the onset of significant human influence and confirm that trematode prevalence has decreased by an order of magnitude over the past ~2 k.y. The median number of parasite-induced pits per bivalve host and parasite aggregation has also decreased significantly, signaling a substantial decrease in the effective population size of digenean trematodes. Gaussian finite mixture modeling of pit size does not support the hypothesis of parasite extinction. Combined, these results indicate the (potentially ongoing) collapse of parasite-host interactions in C. gallina in concert with human influence on the Adriatic and its transition to an “urban sea.”

Histopathological Lesions Caused by a Digenean Trematode in a Pest Apple Snail, Pomacea canaliculata, in Its Native Geographic Distribution Area.

Pomacea canaliculata is one of the most dangerous invasive species. Morphological and molecular analyses have revealed that a digenean species belonging to the family Echinostomatidae parasitizes this snail at two sites in Buenos Aires Province, Argentina, South America. Molecular results confirmed that the species belongs to a genus closely related to Patagifer. Analysis of the 28S rDNA showed that the sequences of the rediae and metacercariae are identical, indicating that the apple snail acts as the first and second intermediate host. The cercariae may encyst as metacercaria inside the redia and also emerge and re-infect the same snail or another snail. The prevalence of digeneans was higher in one of the sampling locations (15.1% vs. 0.72%), probably because the bird species that acts as the definitive host is more abundant in that area. Histopathological examination showed that the parasite quickly invades multiple host organs (gills, intestines, albumen gland, lung, kidney, and mantle border) besides the gonad and digestive gland, as is usual in digeneans. In addition, the partial or total castration of snails was observed in cases of moderate and high infection intensity. In males, there was loss of integrity in testicular tubules, while in females, the replacement of ovarian tissue by rediae was found.

Transversotrema hafniensis n. sp. infection in Poecilia reticulata by cercariae released from Melanoides tuberculata in Denmark

BackgroundExotic and ornamental fish are highly popular companion animals resulting in a significant transcontinental trade of fish, invertebrates and aquatic plants. A major issue is the diseases associated with these organisms, as they have a major impact on health of the fish in both public and private household aquaria. A secondary issue is the trade with these products, which potentially may expand the distribution area and spread a range of diseases to new habitats.ResultsWe here describe how Poecilia reticulata (guppy), produced in a private household aquarium, were invaded by cercariae of an exotic trematode released by imported Melanoides tuberculata snails. The fish presented with severe clinical signs (tremor, flashing, scraping of body against objects). A standard parasitological examination and morphometric identification showed scale pocket infections with a digenean trematode species within the genus Transversotrema. Molecular identification by PCR, sequencing and phylogenetic analyses of a 2646 bp sequence encoding ribosomal RNA (partial 18 S, ITS1, 5.8 S, ITS2, partial 28 S) was performed. The 1107 bp sequence of mitochondrial DNA (cox1) showed that the parasite differed from previously described Transversotrema species in M. tuberculata. Morphometrics of adult and larval specimens of this isolate also differed from previously described freshwater species within the genus. The new species was described and is named after Copenhagen, for its geographic origin.ConclusionsThe genus Transversotrema comprises a range of species, adapted to a microhabitat in scalepockets of teleosts. A combination of morphological and molecular characterization techniques has been shown to provide a good differentiation between species. The fish were not purchased from a pet shop but produced in the home aquarium. This indicated that an infection pressure existed in the aquarium, where the source of infection was found to be exotic intermediate host snails M. tuberculata, which originally were imported and purchased from a pet shop. The potential spread of fish diseases associated with trade of fish and snails to new geographic regions, where climate conditions are favourable, is discussed.

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

In the present study, we aim to provide an inventory of digenetic trematodes (Platyhelminthes, Trematoda, Digenea) from Georgia including records from the freshwater, marine and terrestrial realms. The checklist is based on a critical review of data from 109 papers, 11 monographs and four Ph.D. theses published between 1935 and 2019 and our new records. The checklist includes information on synonymy, the host species, site of infection, geographical distribution and bibliographical references. The present data will serve as a baseline for further studies on trematodes from Georgia focused on integrative taxonomy, life-cycle elucidation, parasite ecology and epidemiology. We compiled data on the digenean trematode fauna of Georgia, which is represented by 186 species (of these 173 identified to species level) belonging to 108 genera, 47 families and 17 superfamilies. This is the first checklist of the digeneans of Georgia. The majority of digenean species were recorded as adults (160 species), only a small fraction being found as cercariae (33 species) or metacercariae (24 species), in their first or second intermediate hosts, respectively. Predominantly, records of trematodes (62 species) from birds were found, followed by those parasitising fish (50 species, i.e. 32 species as adults and 18 as metacercariae), mammals (33 species) and amphibians (25 species, i.e. 23 species as adults and 2 as metacercariae), with the least number of species reported from reptiles (12 species, i.e. 9 species as adults and 3 as metacercariae). Adult digeneans recorded together with another life-cycle stage (metacercariae and/or cercariae) comprised 28 species, i.e. for 15% of the total trematode species number, a part of their life-cycle is known.

Host and parasite identity interact in scale-dependent fashion to determine parasite community structure.

Understanding the ecological assembly of parasite communities is critical to characterise how changing host and environmental landscapes will alter infection dynamics and outcomes. However, studies frequently assume that (a) closely related parasite species or those with identical life-history strategies are functionally equivalent, and (b) the same factors will drive infection dynamics for a single parasite across multiple host species, oversimplifying community assembly patterns. Here, we challenge these two assumptions using a naturally occurring host-parasite system, with the mussel Anodonta anatina infected by the digenean trematode Echinoparyphium recurvatum, and the snail Viviparus viviparus infected by both E. recurvatum and Echinostoma sp. By analysing the impact of temporal parasite dispersal, host species and size, and the impact of coinfection (moving from broader environmental factors to within-host dynamics), we show that neither assumption holds true, but at different ecological scales. The assumption that closely related parasites can be functionally grouped is challenged when considering dispersal to the host (i.e. larger scales), while the assumption that the same factors will drive infection dynamics for a single parasite across multiple host species is challenged when considering within-host interspecific competition (i.e. smaller scales). Our results demonstrate that host identity, parasite identity and ecological scale require simultaneous consideration in studies of parasite community composition and transmission.

Reproduction cycle, trematode parasitism and growth of freshwater mussel Anodonta anatina (Bivalvia: Unionidae)

Anodonta anatina, a widely distributed Eurasian freshwater mussel, has dramatically declined and listed as endangered species in IUCN Red List in some parts of the Europe. To better understand its basic biology and possible sex-dependent effects of parasitism on reproduction, we investigated the reproductive traits, trematode parasitism and growth between November 2020 and October 2021. Our data from a Finnish population indicates male-biased sex ratio and the gonadal development lasting throughout the year with a bradytictic brooding pattern. Gonadosomatic index of both sexes peaked in May and declined in June. Mature oocytes were smaller sized than those of previously studied unionids, fecundity was fairly low, and the maximum predicted lifespan was 14 years. Gonads were infected with digenean trematode parasites, Rhipidocotyle campanula being the dominant species. Interestingly, trematode infection rate was higher in females than in males. In July, when the water temperature reached its annual maximum, the trematode prevalence also peaked, which was accompanied with 16% of mussels being sterile. The trematode parasitism was thus negatively associated with reproduction, that might become even more affected if parasitism increases along with the predicted climate warming, which should be considered in possible future conservation and aquaculture efforts of this species.

Characterization of the Complete Mitochondrial Genome and Phylogenetic Analyses of Eurytrema coelomaticum (Trematoda: Dicrocoeliidae).

Eurytrema coelomaticum, a pancreatic fluke, is recognized as a causative agent of substantial economic losses in ruminants. This infection, commonly referred to as eurytrematosis, is a significant concern due to its detrimental impact on livestock production. However, there is a paucity of knowledge regarding the mitochondrial genome of E. coelomaticum. In this study, we performed the initial sequencing of the complete mitochondrial genome of E. coelomaticum. Our findings unveiled that the mitochondrial genome of E. coelomaticum spans a length of 15,831 bp and consists of 12 protein-coding genes, 22 tRNA genes, two rRNA genes, and two noncoding regions. The A+T content constituted 62.49% of the genome. Moreover, all 12 protein-coding genes of E. coelomaticum exhibit the same arrangement as those of E. pancreaticum and other published species belonging to the family Dicrocoeliidae. The presence of a short string of additional amino acids (approximately 20~23 aa) at the N-terminal of the cox1 protein in both E. coelomaticum and E. pancreaticum mitochondrial genomes has contributed to the elongation of the cox1 gene in genus Eurytrema, surpassing that of all previously sequenced Dicrocoeliidae. The phylogenetic analysis displayed a close relationship between E. coelomaticum and E. pancreaticum, along with a genus-level association between Eurytrema and Lyperosomum. These findings underscore the importance of mitochondrial genomic data for comparative studies of Dicrocoeliidae and even Digenea, offering valuable DNA markers for future investigations in the systematic, epidemiological, and population genetic studies of this parasite and other digenean trematodes.

TAXONOMIC REASSESSMENT AND MORPHOLOGICAL REDESCRIPTION OF NEMATOSTRIGEA SERPENS ANNULATA (DIGENEA: STRIGEIDAE) FROM OSPREY IN NORTH AMERICA.

Digenean trematodes in the strigeid genus Nematostrigea are parasites of various birds, most often ospreys (Pandion haliaetus). Nematostrigea currently contains 2 species, Nematostrigea serpens and Nematostigea hepatica. Nematostrigea serpens is divided into 2 subspecies: N. serpens serpens from the Palearctic and Indomalayan realms and N. serpens annulata from the Nearctic realm. In the present work, we studied the type material of N. serpens annulata and collected new specimens from an osprey in Wisconsin close to the type locality. The original description and illustration of N. serpens annulata were incomplete. Herein we provide a detailed redescription and new illustrations of N. serpens annulata on the basis of the type material as well as newly collected and sequenced specimens. Nematostrigea serpens annulata has a distinct tegumental thickening near the mid-length of the genital cone, which is absent in its congeners. Partial sequences of the large ribosomal subunit (28S) DNA and cytochrome c oxidase subunit 1 (COI) mitochondrial DNA gene sequences were generated from our specimens of N. serpens annulata. Comparisons with previously published sequences of N. serpens serpens from the Palearctic demonstrated 2.2% difference in 28S and 3.7% in COI. On the basis of morphological and molecular comparisons, we elevate N. serpens annulata to species status and recognize it as Nematostrigea annulata Dubois and Rausch, 1948.

Prevalence and Morphological Identification of Digenean Trematode Species Recovered from Infected Organs of Lizardfish (Saurida undosquamis)

Helminthic parasites of the marine fish (Saurida undosquamis) are investigated. Out of 192 specimens of the Saurida undosquamis were collected from the local market of Misurata city in Libya. The prevalence rate of Trematoda helminths in examined fish Saurida undosquamis, was high. Based on the morphological and morphometric characterization represented different species of Trematoda (Lecithochirium sp., Lecithochirium grandiporum, Aponurus laguncula, Neotorticaecum sp, Allocreadium sp, and Gonocerca sp.) was isolated of the total examined fish (185%, 96.4%) were infected with different species of Trematoda. The density of Trematoda was around 9390, and they were collected from the gills, stomach, and anterior and posterior parts of the intestine in all seasons, but Trematode infection was observed in the liver and muscles, the high-density rate in winter (4199), followed by autumn (3291), with non-significant differences between density rates in all seasons (P > 0.05). This study found a high infection rate with Lecithochirium sp. in the examined Saurida undosquamis.

Effects of Cymatocarpus solearis (Trematoda: Brachycoeliidae) on its second intermediate host, the Caribbean spiny lobster Panulirus argus.

Many digenean trematodes require three hosts to complete their life cycle. For Cymatocarpus solearis (Brachycoeliidae), the first intermediate host is unknown; the Caribbean spiny lobster Panulirus argus is a second intermediate host, and the loggerhead turtle Caretta caretta, a lobster predator, is the definitive host. Trophically-transmitted parasites may alter the behavior or general condition of intermediate hosts in ways that increase the hosts' rates of consumption by definitive hosts. Here, we examined the effects of infection by C. solearis on P. argus by comparing several physiological and behavioral variables among uninfected lobsters (0 cysts) and lobsters with light (1-10 cysts), moderate (11-30 cysts), and heavy (>30 cysts) infections. Physiological variables were hepatosomatic index, growth rate, hemocyte count, concentration in hemolymph of cholesterol, protein, albumin, glucose, dopamine (DA) and serotonin (5-HT). Behavioral variables included seven components of the escape response (delay to escape, duration of swimming bout, distance traveled in a swimming bout, swim velocity, acceleration, force exerted, and work performed while swimming). There was no relationship between lobster size or sex and number of cysts. Significant differences among the four lobster groups occurred only in concentration of glucose (lower in heavily infected lobsters) and 5-HT (higher in heavily and moderately infected lobsters) in plasma. As changes in 5-HT concentration can modify the host's activity patterns or choice of microhabitat, our results suggest that infection with C. solearis may alter the behavior of spiny lobsters, potentially increasing the likelihood of trophic transmission of the parasite to the definitive host.

Urinary schistosomiasis and associated microbial infections among pre-school aged and school-aged children in Ipogun, Nigeria

Urinary schistosomiasis is a parasitic infection caused by digenean trematode. In Nigeria, Schistosoma haematobium has been incriminated as the cause of urinary schistosomiasis infection and has been reported from different parts of the country with high prevalence especially in Ondo State. Endemicity in Ipogun village is over two decades due to cultural beliefs and behavioural attitude of the community dwellers to the disease. The study assessed the prevalence of urinary schistosomiasis and associated microbial infections among Pre-School Aged and School-Aged Children (SAC) in Ipogun, Ondo State. Urine samples were collected from 492 children in the village and analysed using the centrifugation method. Similarly, urine samples were cultured using standard microbial methods and examined for the presence of bacteriuria and fungi. The results showed that out of the 492 urine samples examined for parasitology, 306 (62.2%) were positive for urinary schistosomiasis with mean intensity of 22.3 in the sampled population. Haematuria was more prevalent among the male pupils (n =190; 12.2%) than the female (n =116; 6.5%). The prevalence peaked within the age group 16-20 at 82.2%. Staphylococcus aureus, Escherichia coli, Proteus mirabilis, Pseudomonas aeruginosa and Klebsiella sp were isolated as bacteria and Aspergillus sp and Candida sp as fungi solely from the schistosomiasis positive urine samples. This study revealed the prevalence of urinary schistosomiasis in the study area and the predisposition of the infected children to microbial infections. Results suggest routine uninterrupted chemotherapeutic control and the consideration of antibiotics in the treatment regime to control opportunistic microbial infections.

Helminths of the Bluntnose Sixgill Shark, Hexanchus griseus (Bonnaterre, 1788), from the Strait of Messina (Sicily, Southern Italy).

Bluntnose sixgill shark, Hexanchus griseus (Bonnaterre, 1788), is a little-known elasmobranch in the Mediterranean Sea. Given the lack of information about H. griseus, the aim of our study was to describe the helminth fauna of this species. In March 2023, one H. griseus juvenile female specimen was found off the coast of Messina (Italy) and referred by the Italian Coast Guard to our laboratory for necropsy and parasitological evaluation. After necropsy, the specimen's gills, stomach and spiral valve were investigated for parasite presence. All collected parasites were stored in 70% ethanol for routine parasitological analysis. No lesions due to parasites were found in the gills or skin. Three species of helminths were found in one studied female specimen of Hexanchus griseus, namely, two cestode species (Phyllobothrium sinuosiceps and Nybelinia sp., larvae) and one trematode (Otodistomum veliporum). Among them, five Trypanorhyncha plerocercoid larvae were found attached to the stomach mucosa, and six adult cestodes and one digenean trematode were collected from the spiral valve. No other parasite taxa were found in the celomic organs. This study reports new information regarding the parasitic fauna of H. griseus from the central Mediterranean Sea.

New species, new story: The impact of invasive non‐host predators on host‐trematode interactions

Abstract Predation is considered one of the key drivers shaping the relationships between animals. There is increasing evidence that non‐host aquatic taxa use free‐living parasite stages as prey, including motile cercariae of digenean trematodes. However, species‐specific predator differences in cercarial consumption are poorly understood, especially when invasive consumers are involved. We used experimental approaches to investigate the possible predator‐induced reduction in trematode‐host encounters in freshwater benthic communities. Four gammarid species, two native European (Gammarus jazdzewskii and G. pulex) and two invasive Ponto‐Caspian (Dikerogammarus villosus and Pontogammarus robustoides), were selected as predators of echinostomatid cercariae (Echinoparyphium aconiatum). We hypothesised that (i) all predators will prey on cercariae, but the amount of prey consumed will vary, the invaders will be more voracious than native species, and (ii) the effect of predator‐induced dilution of cercariae will depend on the environmental condition, i.e., the presence of the natural target host (the planorbid snail Planorbarius corneus) and alternative food resource; the exposure of predators to these two basic environmental factors will reduce their consumption rates of cercariae. The extent of the dilution effect caused by gammarids was highly predator species‐dependent and also varied depending on experimental conditions. Overall, all gammarids preyed on cercariae and reduced infection levels in the target snail host, but the invaders were more efficient than the native species. Furthermore, decreased consumption rates of cercarial prey were observed in the presence of both the target snail hosts and alternative food resources, except for invasive P. robustoides consuming more cercariae when a snail host was present. The observed differences among predators in the consumption of cercariae may be a result of complex interaction traits of parasites, predators and target hosts, such as different predatory behaviour of invasive and native amphipods, change in parasite behaviour in response to the presence/chemical signals of the target host and a predator‐induced “ecology of fear” interfering with the dilution effect of parasites. We provided evidence that gammarids can cause a significant reduction in parasite‐host encounters. The high consumption rates, especially in the presence of easily available alternative food resources, emphasise the importance of free‐living cercariae as an energy resource in aquatic food webs. The observed predator differences in the consumption of cercariae are further evidence of the higher competitive ability of invasive species. Simultaneously, our results indicate that the appearance of new predators in the environment may interfere with established host‐parasite interactions.