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This is the first detailed study assessing the morphological features of the testis and testicular sperm of members of the freshwater blood flukes Sanguinicola sp. from Leuciscus idus (Cyprinidae) and for S. volgensis from Pelecus cultratus (Cyprinidae), and the marine blood fluke Aporocotyle simplex from Hippoglossoides platessoides (Pleuronectidae). The present study reports a unique feature in the distribution of germinal cellular components in freshwater Sanguinicola sp., showing the presence of the individual spermatocytes or their clusters in the testicular lobes, and the gathering of spermatid rosettes and spermatozoa within middle testicular lumen, which extends along the entire length of the testis. In contrast, each testis of marine A. simplex contains the usual mixed distribution of germ cells at various stages of development. The first TEM data on spermatozoon character of studied species has shown, unusual for digenean sperm structure, the absence of cortical microtubules in sperm principal region. Moreover, a variation in axoneme patterns is revealed in the studied aporocotylids, belonging to the different aporocotylid lineages a 9 + 0 axonemal type observed for freshwater teleost-infective species, Sanguinicola sp. and S. volgensis, and a 9 + ‘1’ axonemal type revealed in spermatozoa of marine teleost-infective species A. simplex. The results discussed with the published data on the digenean sperm structure and the testicular patterns in the Aporocotylidae likely represent additional characteristics supporting the divergent evolutionary lineages of freshwater and marine aporocotylids. We anticipate future morphological studies of the sperm structure in aporocotylids of three lineages for an understanding of their phylogenetic relationships.

Currently, little is known about inhibitory substances enabling tapeworms to settle in fish intestines thereby avoiding proteolysis. Contrary to previous studies with certain host-parasite pairs, this research compares the inhibitory capacities in three tapeworm species of the same genus Proteocephalus from four different fishes (P. torulosus from dace and zope, P. sagittus from stone loach and P. cernuae from ruffe). The tapeworm extracts studied significantly reduced the activity of commercial trypsin (although to a lesser degree than the synthetic inhibitor of serine proteinases PMSF), displaying clear inter-specific variation in worms' inhibitory ability. We also measured the proteolytic activity of the host intestinal mucosa exposed to tapeworm extracts which served as inhibitors. Based on per cent inhibition values, all tapeworm extracts significantly suppressed the mucosal proteolytic activity, with marked differences between certain host-parasite pairs. SDS-PAGE electrophoresis of the incubation media and extracts detected in each tapeworm species 20-36 protein bands with apparent molecular weights from 10-12 to 312.5 kDa, mostly below 50 kDa. The incubation medium and extract of each parasite shared one to six bands ranging from 12 to 35 kDa, depending on its species, with only four bands common for two or more species. The band profiles suggest that in various Proteocephalus species inhibitory capacities against host proteinases can be ensured by different proteins.

Abstract Based on the polymorphism of multilocus DNA markers (inter simple sequence repeat (ISSR)), we assessed the level of genetic variability and differentiation of populations of two invasive species, the Amur sleeper (Percottus glenii) and its specific parasite, Nippotaenia mogurndae, from five reservoirs in the acquired part of the range, the Irtysh River basin, Western Siberia. We revealed a high genetic polymorphism of neutral DNA markers in P. glenii. In general, the proportion of polymorphic loci was 92%, the average heterozygosity was 0.32. These parameters were lower in particular populations of P. glenii and varied within 53–74% and 0.20–0.23, respectively. The interpopulation component accounted for approximately 35% of the variability (GST = 0.35). In N. mogurndae, polymorphism was 73.7%, heterozygosity was 0.18, varying within 18–45% and 0.06–0.15, depending on the population. The level of genetic differentiation of N. mogurndae (GST = 0.38) was similar to that of P. glenii, which indicates that the processes of genetic differentiation of the parasite and the host are synchronous. Polymorphism and genetic distances of the parasite were lower than those of the host. The population genetic structure of P. glenii was partially consistent with the structure of the hydrographic network, in contrast to N. mogurndae. The data obtained indicate that during dispersal, no significant reduction in genetic polymorphism occurred in the Amur sleeper, and over several decades a population-genetic structure characteristic of this species in the native range was formed in the acquired part of the range.

The long-term dynamics (over more than 70 years) of pike infection by cestodes Triaenophorus crassus and T. nodulosus in the Rybinsk Reservoir (the Volga River) has been studied in a comparative aspect. Cestode T. crassus is an invasive species that has entered the reservoir together with vendace Coregonus albula (the second intermediate host) from Lake Beloe; T. nodulosus is a native species that develops with the involvement of perch as a second intermediate host. Climate warming in the Rybinsk Reservoir manifests itself in a consistently growing average water temperature. This has led to a gradual decrease in the rate of pike infection by cestode T. crassus as a result of disturbances in the set of complex ecological relationships between the parasite and its hosts: a decrease in the abundance of large pike, fluctuations in the reservoir water level, and mass death of vendace in years with abnormally hot summers and the alternation of strong and weak year-classes in this short-cycle species (age 1+ to 3+). The long-term dynamics of pike infection by T. nodulosus is characterized by its consistently high rates and slight fluctuations. The abundance of perch population in the Rybinsk Reservoir remains high, and its age composition is stable, which contributes to a high rate of infection of both perch and pike by T. nodulosus. Cestode T. crassus occurs only in large pike (30 cm long or more); the rate of infection increases with age; T. nodulosus parasitizes mainly young pike (starting from young of the year), with the infection rate decreasing in large pike.

Abstract Data on trematode life cycles, fish host distribution, transmission, and fish and snail infection are reported in this review. European freshwater trematodes of the genus Sanguinicola (Aporocotylidae) remain an insufficiently studied group of trematodes. Five species of Aporocotylidae (Sanguinicola armata, S. inermis, S. intermedia, S. volgensis, and S. rutili) in freshwater fish of Europe are described. In addition, they have been found in the water bodies of Central Asia and West Siberia (Ob-Irtysh River basin). The life cycle allowing us to assign the cercariae and adults to a certain species is known only for S. armata, S. inermis, and S. rutili. Trematodes of the genus Sanguinicola are found in 26 fish species assigned to 7 families and 4 orders and 24 gastropod species assigned to 7 families. With few exceptions, the sanguinicolid infection of fish and snails is rather low in the natural water bodies.

The paper reports the study of macroparasite load and aggressiveness of the European perch Perca fluviatilis juveniles of different size captured in the Rybinsk Reservoir (Volga River). A total of 13 macroparasite species have been identified with the Tylodelphys sp. and Ichthyocotylurus sp. trematodes and the Triaenophorus nodulosus cestode being the most abundant ones. A statistically significant correlation between the invasion intensity and fish size and aggressiveness has been found only for T. nodulosus. The possible mechanisms underlying T. nodulosus plerocercoid effects on the size and behavior of their fish hosts, the parasite transmission to the final host, and phenotypic differentiation in perch populations are discussed.

This work represents the first ultrastructural study of the tegument of a freshwater teleost-infecting aporocotylid blood fluke and the first detailed observations of the surface sensory receptors of any aporocotylid. The work was carried out on Sanguinicola inermis from the branchial arteries of the pike, Esox lucius. Light and scanning electron microscopy observations failed to show the presence of protruding surface spines in S. inermis. Transmission electron microscopy studies revealed, deep beneath the distal cytoplasm of the tegument, sporadic, membrane-bound sarcoplasmic processes, each containing a single spine with the typical crystalline structure of digenean spines. On rare occasions, the spine may protrude above the body surface, but in such cases the distal tegumental cytoplasm is extremely flattened. This finding supports a unique, deep origin of aporocotylid spines when compared to those of other digenean families. This character may well prove to be an autapomorphy of the Aporocotylidae, and the position of the base of aporocotylid spines within sarcoplasmic processes supports their origin as a muscle derivative. Unlike other digenean groups, the distal region of the cytoplasmic processes, connecting sunken tegumental perikarya with the syncytial tegumental cytoplasm, is strengthened by peripheral microtubules. Three types of ciliated and three types of non-ciliated sensory receptors were distinguished in the tegument of S. inermis. One of the distinctive features of the ciliated receptors is the unusual position of the basal region of the cilium within a tegumental outgrowth. Such a position has previously been described for certain receptors types in larval and adult schistosomatids and larval diplostomids, suggesting that this feature, in adult worms, might prove to be an apomorphic trait for some or all groups of blood flukes.

A variety of symbionts inhabit colonies of scleractinian corals, the main component of coral reef ecosystems. Obligate symbionts spend almost their whole lives (except the pelagic larvae stage) on or in the host, while facultative symbionts are less tightly associated with the host and easily leave them when food becomes scarce and competition and predation increase. The influence of parasites on the life traits of obligate and facultative symbionts has not yet been studied. The first step in this direction is the study of basic parameters of parasite infracommunities associated with coral symbionts. It was suggested that obligate symbionts that are tightly associated with their hosts and their symbiotic communities are more often infected. An alternative hypothesis suggests that facultative symbionts are more frequently infected because they easily change habitats and consume a broad food spectrum. For several years, we studied parasites of obligate and facultative symbionts (fish, decapods crustaceans, mollusks) associated with the corals Acropora and Pocillopora in the Nha Trang Bay (South China Sea). The highest abundance and diversity of macroparasites (Monogenea, Cestoda, Trematoda, Nematoda, Copepoda) were found in facultative symbionts of corals, fish-parabionts. Obligate fish-inbionts were infected much less often (Cestoda, Trematoda, Nematoda). Obligate symbionts predominated in the invertebrate symbionts, which were also weakly infected. The low abundance and diversity of parasites in obligate symbionts suggest that there should be significant investments in antiparasitic defense, which, together with efficient antipredation defense, minimize the mortality of obligate symbionts. Their fecundity and population number are usually low. Facultative symbionts presumably suffer higher mortality than obligate symbionts, but a more intensive use of resources and higher population numbers may compensate for this. A significant increase in the infection rate with an increase in host body size is typical of facultative symbionts, which are less protected than obligate symbionts. The latter showed low infection levels in all size groups. The results suggest that the role of parasites in shaping the life strategies of symbionts is no less important than the role of predators.

—Long-term observations (1968–2019) on the dynamics of infection by the cestode Eubothrium rugosum in the burbot (Lota lota) have been performed in the Rybinsk Reservoir on the Upper Volga. The results show that a significant and consistent decrease in the prevalence of infection has occurred in the mid-to-late 1990s and is related to global warming. Because of rise in water temperature, large zones of oxygen depletion in the bottom water layer have been formed in the reservoir, leading to mass mortality of the intermediate hosts of the cestode (ruffe and copepods). This has sharply reduced the probability of transmission of its larvae to the final host (burbot). The predicted further climate warming with abnormally hot summers may provide for further decrease in the prevalence of E. rugosum infection in the burbot.

A study of 2536 fish specimens of 22 species from Lake Tana in Ethiopia has revealed the presence of five species of clinostomid metacercariae represented by two genera: Clinostomum complanatum (from Enteromius humilis, E. tanapelagius, and Garra tana), Euclinostomum sp. and Clinostomum brieni (from Clarias gariepinus), and C. phalacrocoracis and Euclinostomum heterostomum (from Oreochromis niloticus). Morphological descriptions of the detected metacercariae, including the indication of their occurrence, mean invasion intensities, and tissue-use, are given.

Parasites in 61 individuals of eight species of coral reef fish (Caracanthus maculatus, Scorpaenodes sp., Sebastapistes cyanostigma, Gobiodon unicolor, Paragobiodon modestus, P. echinocephalus, Pseudocheilinus hexataenia, and Halichoeres sp.) were studied in April–May 2012–2014 and November 2017. All studied fish were symbionts of corals of the genera Acropora and Pocillopora. The parasites found were the nematodes Spirocamallanusistiblenii, Raphidascaris (?) sp., and Ascarophis sp., a larva of the cestode Nybelinia sp., as well as metacercariae of the trematodes Stephanostomum sp. 1, Stephanostomum sp. 2, and Trematoda gen. sp. The low prevalence, infection rate, and diversity of parasites could be related to low population numbers of fish hosts, their efficient individual anti-parasitic defense, as well as to predation by corals that prey on dispersal stages of parasites.

This study is devoted to the investigation of Azygia (Digenea: Azygiidae) species diversity using classical morphological, recent molecular tools (28S rRNA and cox1 mtDNA for genetic-based inference) and robust statistical techniques (Principal component analysis, PCA). The analysis revealed that the genus Azygia included four valid species: A. lucii, A. longa, A. hwangtsiyui, and A. sibirica n. sp. The distribution of the type species A. lucii was confirmed in the largest Russian rivers: the Volga and the Ob. The worms isolated from Perccottus glenii were determined as the Chinese species A. hwangtsiyui, according to the genetic data for the cox1 mtDNA gene, at 1.32–1.56%. The new species, Azygia sibirica n. sp, was described from Esox lucius in the Ob River and differentiated from the type species A. lucii by the smaller ovary, testes and prostatic sac, wider body, very narrow pharyngeal lumen and form of anterior margin of ovary. In addition, multivariate analysis and three methods for species delimitation (ABGD, GMYC, bPTP) showed the subdivision of A. lucii and A. sibirica n. sp. into two separate groups, one from the Volga River and another from the Ob River, respectively. To conclude, A. lucii infects Esox lucius in the western (European part of Russia, the Volga River basin), and northern (Western Siberia, the Ob River basin) parts of Russia; A. sibirica n. sp. has also been found to infect Esox lucius in the Ob River, while A. hwangtsiyui infects Perccottus glenii in the South of the Russian Far East (the Artymovka River basin).

In summer and autumn 2016–2017, the parasitic fauna in four anemonefish species (Amphiprionclarckii (53 ind.), A. polymnus (58 ind.),A. perideraion (15 ind.), and A. frenatus (10 ind.) (Pomacentridae)) from the areas close to Nha Trang, South China Sea, Vietnam, were studied. In total 11 parasite species were found in the studied fish with 3–8 parasite species per fish species. The survey revealed two species of copepods (Hatschekia sp. 1 and Hatschekia sp. 2), six digeneans (Macvicaria sp. 1, Macvicaria sp. 2, Aponurus piriformis,Aponurus laguncula, Hysterolecitha nahaensis, Bivesicula sp. metacercariae), two nematodes (Spirocamallanus istiblenni and Hysterothylacium sp. J4), and one acanthocephalan (Echinorhynchus sp.). The trematode Hysterolecitha nahaensis and the nematode Spirocamallanus istiblenni were the most abundant and prevalent parasites. These parasites reached a prevalence of 64% and 90% with an intensity of infection of 1–6 ind. and 1–37 ind., respectively. Two ectoparasite taxa were found: Hatschekia sp. 1 (a single specimen found in A.clarckii) and Hatschekia sp. 2. Among them, Hatschekia sp. 2 was the most frequent and abundant species in A. polymnus (prevalence 96.6%, intensity of infection 0–51).

The results of original research and published data on the number of alien parasite species of fish recorded in the Volga River basin are generalized. At present, 47 alien species of fish parasites are recorded for the Volga River basin. The problems of species status determination (native or nonnative) are discussed using specific examples. The species originating from the Amur River complex (33 species) dominate in the alien parasite fauna of fish in the Volga River basin. The Ponto-Caspian complex is represented by eight species. The ways of introduction of the alien species of parasites and possibilities of their naturalization in the Volga River are considered.

Accepted manuscript version, licensed CC BY-NC-ND 4.0. The muscle cells and extracellular matrices (ECMs) of two teleost-infecting blood flukes belonging to distinct evolutionary lineages of the Aporocotylidae (Digenea) were examined using Transmission Electron Microscopy. Four morphotypes of muscle cells were found in the freshwater species Sanguinicola sp., but were considered to be various developmental stages of a single cisternic type. In the marine species Aporocotyle simplex, three types of muscle cells were apparent, one of which is cisternic. The first ultrastructural evidence is presented for the exocytosis of the moderately dense contents of dilated cisternae of cisternic muscle cells into the extracellular space in both Sanguinicola sp. and A. simplex. The basal matrices of aporocotylids support various types of epithelia. In Sanguinicola sp., beneath the distal tegumental cytoplasm, there is a thin lamina densa, whereas the intestinal epithelium is supported by a lamina reticularis. In A. simplex, both a thin lamina densa and a thick lamina reticularis underlie the distal cytoplasm of the tegument and are present at the periphery of the ovary, but beneath the epithelial lining of the caeca and both genital and excretory ducts there is only a lamina reticularis. Significant variation in the development and amount of the ECM in marine and freshwater aporocotylids is described, since A. simplex has a much better developed ECM than occurs in Sanguinicola sp. Moreover, thin myofilaments of muscle fibres participate in the ECM formation in A. simplex and represent its dominant component. The presence of two mechanisms for ECM formation in A. simplex, as opposed to a single mechanism in Sanguinicola sp., may represent further evidence for the affiliation of these two taxa to divergent evolutionary lineages. The data presented are discussed in relation to available information on these structures in other neodermatan groups.

During a parasitological survey of nonnative fish species—rudd Scardinius erythrophthalmus—in the Rybinsk Reservoir, the metacercariae Ornithodiplostomum scardinii were found in its brain. This is the first find of this helminth in the Upper Volga basin. The rudd was infected with the prevalence 31.3%, with an intensity of 1–12 parasites per fish. Data on the current distribution of the trematode O. scardinii in the Volga River basin are provided.

Metacercariae of two new trematode species of the genus Austrodiplostomum are described from fishes of Tana Lake in Ethiopia: Austrodiplostomum sp. 1 from cranial cavity of Oreochromis niloticus and Austrodiplostomum sp. 2 from vitreous humor of Varicorhinus beso. New species metacercariae differs from A. mordax metacercariae in the shape of body, the size of body and organs, the shape of anterior extremity, and site of infection. This is the first record of Austrodiplostomum metacercariae from fish in Africa

The data on the finding of Eustrongylides excisus in the Rybinsk Reservoir (Yaroslavl oblast, Russia) are presented. Larvae-IV were recorded in 2015 in ruff. The appearance of this nematode in the reservoir is associated with the expansion of the range of cormorant, its main definitive host. E. excisus is one more parasite species exhibiting northward expansion in the Volga Basin.

It is revealed that the infection of ruff with cestodes Proteocephalus cernuae affects the activity of its intestinal proteolytic enzymes. The influence of cestodes on the activity of intestinal proteinases depends on the sum length of the worms occupying the gut. Proteinase activity decreases at a smaller total length of the worms and increases at larger total length, with serine proteinases being mostly involved. The essential share of activity is presented with metalloproteinases, which can indirectly evidence the important role of microbiota in the digestion of ruff. The small share of cysteine proteinases in both uninfected and infected fish may indicate minor intestinal damages caused by the attachment structures of cestodes.

All of the 166 Clarias gariepinus catfishes in Lake Tana, Ethiopia, were examined for trematode infestation in 2006—2009. Seven trematode species—Eumasenia bangweulensis, Astiotrema reniferum, Orientocreadium batrachoides, Paralecithodendrium chilostomum, Phyllodistomum bavuri, P. tana, and Cladorchiidae gen. sp.—as adult were found. The common catfish parasites were Eumasenia bangweulensis (20% prevalence and 1—62 intensity of invasion), Orientocreadium batrachoides (30% prevalence and 1–31 intensity of invasion), Phyllodistomum bavuri (24.8% prevalence and 1–8 intensity of invasion), Ph. tana (17.6% prevalence and 1–23 intensity of invasion), and Ph. bavuri. Astiotrema reniferum (three specimens were only found) was a rare species; Paralecithodendrium chilostomum was an accidental parasite of catfish. All these trematodes were first recorded in Ethiopia and Eastern Africa.

The metazoan parasites of the invasive goby species Proterorhinus semilunaris were studied in August 2015. In total, 101 specimens sampled at four sites of Rybinsk Reservoir were analyzed. Six parasite species were found: Unionidae gen. sp., Paracoenogonimus ovatus, Diplostomum sp., Ichthiocotylurus platycephalus, Posthodiplostomum cuticola, and Camallanus lacustris. Trematode metacercariae were the dominant parasites in all the studied samples. The specific parasites, monogenean Gyrodactylus proterorhini and cestode Proteocephalus gobiorum, were not recorded. The study summarizes the published and original data on the parasite faunae of P. semilunaris and P. marmoratus in Eurasia. The parasite fauna of P. marmoratus comprises 40 species (Microspora—1 species, Ciliophora—4, Myxozoa—4, Monogenoidea—1, Cestoda—4, Trematoda—17, Nematoda—4, Acanthocephala—4, Arthropoda—1). The parasite fauna of P. semilunaris has 92 species (Kinetoplastida—1, Sporozoa—2, Microspora—4, Ciliophora—11, Myxozoa—2, Monogenoidea—2, Cestoda—8, Trematoda—35, Nematoda—12, Acanthocephala—5, Annelida—1, Mollusca—4, and Arthropoda—5).

This is the first study assessing the cytoarchitecture of the vitellarium of members of the freshwater, teleost-infecting lineage of blood-flukes (Aporocotylidae). The vitelline cytoarchitecture of two innominate species of Sanguinicola from freshwater fishes in Russia showed that vitelline cells at different stages of maturation are widely distributed throughout much of the body and are mixed with other cell types. The latter feature indicates that use of the term "follicular vitellarium" is inappropriate for species of this genus. An additional characteristic of the vitelline cells in these Sanguinicola spp. is their ability to form long, pseudopodia-like extensions of the peripheral cytoplasm that contact neighbouring vitelline cells and sarcoplasmic extensions, forming both heterologous and homologous intercellular junctions. Within the vitelline duct lumen, the cytoplasm of mature vitelline cells is filled with regular clusters (0.5-1.0 μm in diameter), comprising 10-30 vitelline globules, which have heterogeneous contents and electron-lucent lipid droplets (1.1-1.7 μm in diameter), but no apparent modifications of vitelline globules occur within the vitelline duct. The flattened, ciliated, epithelial lining of the common vitelline duct contains intra-epithelial nuclei, its luminal surface bears shallow lamellae and adjacent cells are adjoined by apical septate junctions. All of these observations, when compared to the marine Aporocotyle simplex, likely represent additional characteristics supporting the divergent evolutionary lineages of marine and freshwater aporocotylids.Cytoarchitecture inhabituelle des « follicules vitellins » chez les digènes infectant le sang des poissons d’eau douce, du genre Sanguinicola (Digenea, Aporocotylidae).Il s’agit de la première étude évaluant la cytoarchitecture du vitellarium des membres de la lignée des digènes (Aporocotylidae) infectant le sang des téléostéens d’eau douce. La cytoarchitecture vitelline de deux espèces non nommées de Sanguinicola de poissons d’eau douce de Russie a montré que les cellules vitellines à différents stades de maturation sont largement distribuées dans une grande partie du corps et sont mélangées avec d’autres types cellulaires. Cette dernière caractéristique indique que l’utilisation du terme « vitellarium folliculaire » est inappropriée pour les espèces de ce genre. Une caractéristique supplémentaire des cellules vitellines de ces Sanguinicola spp. est leur capacité à former de longues extensions du cytoplasme périphérique ressemblant à des pseudopodes, qui entrent en contact avec les cellules vitellines voisines et les extensions sarcoplasmiques, formant des jonctions intercellulaires à la fois hétérologues et homologues. Dans la lumière du canal vitellin, le cytoplasme des cellules vitellines mûres est rempli de grappes régulières (0,5 à 1,0 μm de diamètre), comprenant 10 à 30 globules vitellins, qui ont des contenus hétérogènes et des gouttelettes lipidiques (1,1 à 1,7 μm de diamètre) transparentes aux électrons, mais aucune modification apparente des globules vitellines ne se produit dans le canal vitellin. Le revêtement épithélial aplati et cilié du canal vitellin commun contient des noyaux intra-épithéliaux, sa surface luminale porte des lamelles peu profondes et les cellules adjacentes sont jointes par des jonctions septées apicales. Toutes ces observations, comparées à Aporocotyle simplex qui est marin, représentent probablement des caractéristiques supplémentaires soutenant que les lignées évolutives des Aporocotylidae marins et d’eau douce sont divergentes.

The influence of infection with trematode metacercariae on some physiological and biochemical characteristics has been studied in the underyearlings of roach. The parameters that were studied are found to depend on the parasite localization. The body length and weight values of infected underyearlings with muscular localization of metacercariae are lower than in uninfected fish and the glycogen content is higher. On the contrary, individuals with infected eyes display greater body length and weight when coupled with lower glycogen content in comparison with parasite-free fish. The infection with metacercariae has no effect on the activity of glycosidases. Different glycogen levels in the underyearlings with muscular and ocular localization of metacercariae are presumably linked with the locomotor activity of the fish.

Genetic markers of some European Gorgoderidae species, including potential adults of Cercaria duplicata, were obtained and used to clarify phylogenetic affinities within the genus Phyllodistomum and to verify conflicting data existing on their life cycles. Molecular data and karyotype, 2n = 18, provide further support for ascription of C. duplicata to the Gorgoderinae. Sequences of C. duplicata form a robustly supported major clade, phylogenetically distinct from other known gorgoderid species in both ITS2- and 28S-based phylogenetic trees. The molecular data revealed no match between C. duplicata and any species of Phyllodistomum, including adults found in the experimental studies. One of them, P. elongatum, showed no differences from type species P. folium. Other, P. angulatum, forms a robustly supported clade, which is closely related to P. macrocotyle clade in all phylogenetic trees. This study supports the concept that only P. macrocotyle is a parasite of Dreissena polymorpha among the phyllodistomes and life cycle described for the type species P. folium by Sinitsin (1905) can apparently be discounted. Previously reported low host specificity of P. folium was justified. Adults of P. folium were detected in eight teleost species from five families and four orders. Cystocercous cercariae of P. folium were recorded in sphaeriid bivalves of the genus Sphaerium and Pisidium. According to our molecular data, P. simile, parasite of bullhead, must be regarded as synonym of P. folium. Phyllodistomum umblae is most closely related to P. folium in the all phylogenetic analyses. Molecular phylogenies support a presumption that Phyllodistomum species with cystocercous cercariae developing in sphaeriid bivalves should be regarded as Phyllodistomum sensu stricto. The results reveal a clear need for reconsideration of the knowledge on gorgoderid life cycles based on experimental studies and re-evaluation of the validity of some nominal Phyllodistomum species.

A taxonomic revision of cestodes of the genus Paracaryophyllaeus Kulakowskaja, 1961, parasites of cobitid fishes, has been performed. Three species (Paracaryophyllaeus gotoi (Motomura, 1927), P. kulakowskae sp. n., and P. misgurni sp. n.) are considered valid species; two are described as new species. P. dubininorum Kulakowskaja, 1961 is a young synonym of P. gotoi. The taxonomic status for P. gotoi sensu Scholz, 1989, P. gotoi sensu Scholz et al., 2001, and Paracaryophyllaeus sp. 1 and Paracaryophyllaeus sp. 2 (described in present work) needs specification.

Four new species of trematodes of the genus Diplostomum are described from metacercariae found in fishes from Lake Tana in Ethiopia: Diplostomum garrae sp. n. from the lens of Garra dembecha, D. longicollis sp. n. from the lens of Barbus humilis, D. montanum sp. n. from the lens of Barbus humilis, and D. tilapiae sp. n. from the vitreous body of Oreochromis niloticus. The metacercariae of the new species considerably differ in morphology and size from those of all known Holarctic species. It is the first African record of metacercariae of the genus Diplostomum in fish.

The intrauterine eggs of the pleurogenid trematode Brandesia turgida ( Brandes, 1888 ), exhibiting advanced stages of miracidial differentiation and fully formed, ciliated miracidia, were examined by means of transmission electron microscopy (TEM). Each embryonated egg is composed of a mature miracidium surrounded by a four-layered egg wall: (1) an outer, anucleate layer external to the eggshell, which forms a thick cocoon; (2) the operculate egg-shell; (3) a small remnant of the compact, granular cytoplasm of the outer embryonic envelope ( sensu stricto ); and (4) a relatively distinct cellular remnant of the inner embryonic envelope. Layers enveloping the egg apparently play an important role in the protection, metabolism and storage of nutritive reserves for the developing miracidium. The outer, anucleate layer, or cocoon, situated externally to the eggshell and composed of a transparent, electron-lucent substance with numerous dense, osmiophilic islands attached to its peripheral membrane, has never previously been seen in TEM studies of the eggs of parasitic platyhelminths. The origin, formation, functional ultrastructure and chemical composition of this peculiar layer remain enigmatic, although its function appears to be protective. The thick, electron-dense eggshell resembles that of other trematodes, exhibiting a characteristic fissure zone around the operculum. The very small, indistinct remnants of the outer embryonic envelope appear in the form of a very thin, compact, granular cytoplasm closely attached to the inner surface of the eggshell. Conversely, the inner embryonic envelope is frequently apparent at one or both poles of the developed egg as a syncytial envelope formed by the fusion of mesomeres. This envelope, even in eggs containing a fully formed miracidium, still has the features of a metabolically active layer with an energy storage capability. Lysosome-like structures observed in some eggs may be involved in the autolysis of the embryonic envelopes.

Population characteristics of macroparasite infection (prevalence and intensity) in juvenile roach Rutilus rutilus have been studied in one of the tributaries of the Rybinsk Reservoir. The level of infection and parasite diversity is higher in the mouth part than in the upper reaches of the river. Habitats in the upper reaches are characterized by a high degree of irregularity of the risk of infection. The intensity of parasitic infection varies from the absence of parasites to 100% infection. Our data confirm the hypothesis about adap� taive value of the spawning migration to habitats with lower risk for the infection of offspring with parasites. Heterogeneous conditions in the upper reaches of the river make it possible for juvenile fish from a migrating part of the population to grow in relatively safe conditions. Differences in the strategy of the spawning migra� tion can be based on the ecological differentiation as a result of which the groups oriented either at the local adaptation or at migration to the other habitat are formed in the population. DOI: 10.1134/S003294521305007X

Infection by parasites has been studied in 189 specimens of Barbatula barbatula (Linnaeus, 1758) from eight rivers in Yaroslavl province over 4 years. A total of 41 parasite species are identified in stone loach. The following parasite taxa have been found: parasitic Protozoa (Kinetoplastidea (2), Myxozoa (6), and Ciliophora (7)) and parasitic Metazoa (Monogenea (5), Cestoda (4), Trematoda (9), Nematoda (4), Acanthocephala (3), and Bivalvia (1)). Stone loach is recorded for the first time as a new host for seven parasite species: Chloromyxum truttae Leger, 1906, Gyrodactylus cobitis Bychowsky, 1933, Paracaryophylaeus gotoi (Motomura, 1927), Neogryporhynchus cheilancristrotus (Wedl, 1955), Paracoenogonimus ovatus Katsurada, 1914, Camallanus truncates (Rudolphi, 1814), and Neoechinorhynchus crassus Van Cleave, 1919. The most abundant parasites are Raphidascaris acus (Bloch, 1779) (28.6–100%, 1–64), Crepidostomum oschmarini Zhokhov et Pugacheva, 1998 (18.2–71.4%, 1–40), Paracoenogonimus ovatus (3.13–71.4% 1–34), and Gyrodactylus nemachili Bychowsky, 1936 (3.85–100%, 1–65).

Transmission electron microscopical observations were made on the vitelline structure of the digenean Brandesia turgida (Brandes, 1888) collected from crypts within the intestinal wall of the frog Pelophylax ridibundus (Pallas, 1771). Ultrastructural details of the vitelline follicles of B. turgida include: (a) the presence within the vitellarium of a single type of cell, i.e. vitellocytes at different stages of their development; (b) a narrow region between the vitellocytes filled with the processes of surrounding parenchymal cells; and (c) the occurrence of the junctional complexes between the vitellocytes and the surrounding parenchymal cells at the periphery of the vitelline follicles. It is shown that the vitelline globules and a few lipid droplets are the main inclusions resulting from vitellocyte synthetic activity. The limited amount of nutritive reserves in the vitellocytes can be explained by the nature of the parasite’s life-cycle, which is characterized by fully-embryonated intrauterine eggs containing a fully-formed miracidium. Despite the small number of digenean species studied in relation to their vitelline cytoarchitecture, two structural patterns of the vitellarium can be elucidated; these are examined in terms of digenean systematic relationships.

The infection rate by Ligula intestinalis has been studied in 14 fish species (Varicorhinus beso, Garra dembecha, Labeobarbus intermedius, L. crassibarbis, L. tsanensis, L. megastoma, L. brevicephalus, L. nedgia, L. acutirostris, L. gorgorensis, L. dainellii, L. macrophthalmus, Barbus humilis, and B. tanapelagius) from Lake Tana (Ethiopia). Plerocercoids have been found only in the fishes of gg. Labeobarbus and Barbus. Ligula has been found in seven out of ten studied large barbels. Barbels larger than 23 cm are not infected with ligula. It is revealed that a specific "synergism" exists between Ligula intestinalis and cestode Khawia sp. par� asitizing the intestine of barbels. This synergism is manifested as a regular cooccurrence of these parasites. In juvenile Labeobarbus, fish coparasitized by Khawia sp. and L. intestinalis occurred more often than fish infected with each of this parasite separately.

Phyllodistomum tana sp. nov. is described from the ureters of Clarias gariepinus from Lake Tana, Ethiopia. This species is distinguished from its congeners in Africa in having an oblong body shape, transverse loops of the uterus between the posterior margin of ventral sucker and the vitelline follicles, and the localization exclusively in the ureters of C. gariepinus. The new species differs from P. bavuri and P. vanderwaali in the larger size of body, the position of the testes with respect to the ovary and with each other, the lobed vitelline follicles and the sucker ratio.

Metacercariae of two species of trematodes from the fam. Diplostomidae from the cranial cavity of the African catfish Clarias gariepinus are described. The metacercaria Tylodelphys grandis n. sp. is described for the first time. Data are presented on the infection rate of catfishes in Lake Tana.

This paper deals with the recovery process of ruffe helminth fauna following a catastrophic decrease in the fish population of Rybinsk Reservoir. Mass mortality was observed in ruffe from 1997 to 1999. It led to a decrease in parasite species diversity. However, two to three years later about 60% of the former parasitic species had recovered. The reasons for ruffe mass mortality in a range of waterbodies are analyzed based upon literature data. Some regularities of such epizooties in ruffe are revealed. It is suggested that epizooties are due to some unknown viral or bacterial infection, which may be facilitated by a specific combination of several factors.

The communities of intestinal helminths found in the "winter" sample of ide (December-May, 45 fish) and in the "summer" sample (August-September, 42 fish) were compared. They consisted of 10 and 11 species, respectively, with Sphaerostomum globiporum being dominant in both communities. The species Parasymphylodora markewitschi and Capillaria tomentosa were found only in the "summer" community. Dominance and diversity (the Simpson and Brillouin indices) in the "winter" community were higher. All hel- minth species in the intestinal community of ide were of broad specificity. Therefore, host specialization does not play an important role in the forming of intestinal helminth community in this fish species.

The host specificity of flukes belonging to the genus Sphaerostomum (S. bramae and S. globiporum) in the center and at the periphery of the genus range is discussed. Both fluke species are specific parasites of cyprinids. However, fish of nine other families and one cyclostome species have also been recorded as their hosts. At the periphery of the range, the spectrum of definitive hosts is broader than in the center owing to a greater number of unspecific hosts. In the center, unspecific hosts are represented by six fish species (Esox lucius, Misgurnus fossilis, Perca fluviatilis, Lota lota, Gymnocephalus cernuus, and Salmo sp.), four of which are predators. At the periphery, 11 fish species, including six predators (Anguilla anguilla, Barbatula barbatula, Cottus gobio, Cobitis taenia, Coregonus lavaretus, Salmo trutta, Alosa sp., Potamoschistus minutus, E. lucius, P. fluviatilis, and L. lota), and the cyclostome Lampetra fluviatilis are unspecific hosts. In the peripheral populations, compared to central, flukes manifest greater adaptive plasticity and can infest new hosts.

Coral fish experience a very strong predation pres� sure during all their life cycle. This factor is considered to be the major evolutionary and ecological determi� nant in the ecosystem of the coral reef. Recently, mutualism and parasitism increasingly attract atten� tion. The mutualistic relationships of fishes and coral are well known. The coral colonies are reliable shelters for fish, fishes clean the corals from slime, sediments, and parasites, fertilize them, and ventilate the space inside the colony [1, 2]. Being in such a close symbi� otic association with the colony of a host coral and various symbionts, fishes are exposed to infestation with parasites, and the damage may surpass the advan� tages of mutualism. Mutualistc associations of fish with corals differ in the character of topic, trophic, and social relation� ships, as well as the duration of the existence of fish in the colony. Two types of symbiotic relationships are distinguished [3]. Inbiont fish, after the larvae settle from plankton, stay for all their remaining life within the host colony, feed on coral slime and on symbiotic invertebrates. As a rule, they are territorial and never leave the colony. Their abundance in the colony is low. Parabiont fish are closely connected with the coral during much shorter period of their life, namely, in the period of rapid growth. In the period of reproduction, they break symbiotic association and form reproduc� tive groups. Stable groups of parabionts usually consist of a few dozen fish. In the light part of the day, they stay near the host colony and feed on plankton brought by water currents. In the dark part of the day and when frightened, the fish hide inside the colony. Differences in ecology and behavior of inbionts and parabionts are unavoidably reflected in the appearance of communities of parasites in coral fish. It may be expected that inbionts, which are in close con� tact with each other and with other members of the symbiotic association within a limited space of the host coral, are exposed to higher risk of infestation with various parasites. On the other hand, the para� bionts feeding on zooplankton and symbiotic inverte� brates may get more parasites via trophic interconnec� tions. Negative influence of a high infestation with parasites may deteriorate the advantages of the symbi� otic life [4, 5]. Which type of symbiotic relationships of corallobiontic fishes has a higher risk of infestation? To answer this question, we investigated the com� munities of macroparasites of inbiont and parabiont fishes living on the mass species of scleractinian corals Pocillopora and Acropora from the coastal waters of South Vietnam (Nha Trang Bay). The material was collected in April–May 2012–2014 using light diving equipment at depths from 3 to 8 m. Separately situated colonies of corals were covered and wrapped in a piece of plankton gauze (all parabiont fishes which hid among branches of the colony were collected as a gen� eral sample, together with inbionts). Then, they were separated from substrata and placed into vessels with sea water. The fish from each colony were kept in sep� arate aerated aquaria.

The species compositions of fish trematodes in the neighboring Volga and Don basins are similar in many ways, however, there are some differences. One difference is that trematodes of the genera Nicolla and Plagioporus were not previously recorded in the Volga basin. Their absence was accounted for by the fact that their initial hosts, snails of the genera Lithoglyphus (Gastropoda: Lithoglyphidae) and Theodoxus (Gastropoda: Neritidae), did not occur in the Volga until recently. At present, these snails, trematodes, and some other parasites have successfully naturalized in the Volga and actively expand their ranges in its basin (Zhokhov and Pugacheva, 2001). Here, we present new data on the dispersal of these invaders in the Volga basin. The distribution of the trematode Nicolla skrjabini Iwanitzki, 1928 is limited to the rivers of the Azov‐ Black Sea and Baltic basins. That of the trematode Plagioporus skrjabini Kowal, 1951 is confined to rivers of the Azov‐Black Sea basin alone. The life cycles of these parasites are known. For N. skrjabini , the first intermediate host is the snail Lithoglyphus natocoides Pfeiffer, and the second intermediate host (according to experimental data) is Gammarus balcanicus Schaferna (Sten’ko, 1976). This trematode has a wide range of definitive hosts that comprises 27 fish species. In P. skrjabini , the snail Theodoxus fluviatilis and six gammarid species are involved in the life cycle (Chernogorenko et al., 1978). The definitive hosts of P. skrjabini are Cobitis taenia L., Neogobius fluviatilis Pall., and Mesogobius batrachocephalus Pall. In 1952, the Volga‐Don connected the two river basins, and the snails with associated trematodes found an opportunity to invade the Volga. The snails of the genus Lithoglyphus appeared in the lower Volga in the late 1960s (Belyavslaya and Vyushkova, 1971). In 1971, they were already numerous in the Volga delta (Pirogov, 1972). At present, these snails occur almost all over the Volga delta and form very dense aggregations (up to 11 180 ind./m 2 ) in the upper part of the delta and in arms of its western part (Biserova, 1990, 1996