SummaryAltogether 180 specimens of Perca fluviatilis (L.) from Srebarna Lake, North-eastern Bulgaria, were examined for helminths during spring, summer and autumn of 2005 -2006. Of them, 143 (79.4 %) perch were infected with helminths. Thirteen helminth species were found: metacercariae of Bolbophorus confusus, Diplostomum pseudospathaceum, D. spathaceum, Ichthyocotylurus pileatus, Posthodiplostomum cuticola and Tylodelphys clavata; cestode Proteocephalus percae; acantocephalans Acanthocephalus anguillae and A. lucii and nematode larvae Contracaecum microcephalum, Eustrongilides excisus, E. tubifex and Rhaphidascaris acus. The digenean T. clavata was dominant species of the perch component community. Little seasonal differences were found in the prevalence, abundance and mean infection of helminths. Eustrongilides tubifex reported from the territory of Bulgaria for the first time, represent the new geographical records and new species of Bulgarian helminth fauna. P. fluviatilis was reported as a new host record for two digeneans, namely Diplostomum pseuspathaceum and Posthodiplostomum cuticola, the cestode Proteocephalus percae, the acantocephalan Acantocephalus anguillae and the nematodes Contracaecum microcephalum and E. tubifex from the territory of Bulgaria. P. fluviatilis was reported as a new host record for D. pseudospathaceum, P. cuticola, C. microcephalum and E. tubifex from Balkan peninsula.

Recent morphological and molecular data have shown that one of the most common parasites of freshwater fish in the Palaearctic Region, the cestode Caryophyllaeus laticeps (Pallas, 1781) (Eucestoda: Caryophyllidea), is highly polymorphic. Five distinct morphotypes of C. laticeps, largely corresponding to different fish hosts and representing separate, yet closely related genetic lineages, have been recognised and they are characterised in the present paper. Morphotype 1 from breams, Abramis brama (L.) (type-host) and Ballerus spp., corresponds to the original Taenia laticeps Pallas, 1781 and its neotype (paragenophore ex A. brama in Russia) is designated. This morphotype is characterised by a slender body and flabellate scolex. Morphotype 2 was found in the Macedonian vimba Vimba melanops (Heckel) and the vimba bream V. vimba (L.); it is typified by a more robust body, with most anterior extent of the vitelline follicles near the scolex and the cirrus-sac situated more anteriorly than in other morphotypes. Morphotype 3 is represented by worms from the common carp Cyprinus carpio L. that possess a cuneicrispitate scolex (having the form of a wedge with shallow indentations on anterior margin). Morphotype 4 from the common nase Chondrostoma nasus (L.) has a large, robust body and a wide scolex with numerous superficial grooves (wrinkles) in its anterior part. Morphotype 5 is represented by worms from the white-eye bream Ballerus sapa (Pallas); its typical characteristics are a festoon-like anterior margin of the scolex, the absence of vitelline follicles posterior to the cirrus-sac and the absence of a well-developed internal seminal vesicle. Discriminant analysis of 15 morphometric variables readily separated Morphotypes 3, 4 and 5 and confirmed the key discriminating power of traits related to the reproductive system, especially the terminal reproductive organs. Morphological polymorphism and the genetic divergence of different morphotypes of C. laticeps correspond to its wide spectrum of fish definitive hosts and a large distribution area that includes Europe, most of Palaearctic Asia and northern Africa.

The present study presents the results from examinations of bleak’s endohelminth species and structure of helminth communities from the Bulgarian part of the Lower Danube River. In 2015 and 2016, on a seasonal manner, 91 specimens of Alburnus alburnus (Linnaeus, 1758) (bleak) were examined with standard techniques for parasites. Five species of parasites: Nicolla skrjabini (Ivanitzky, 1928), Ligula intestinalis (Linnaeus, 1758), Acanthocephalus lucii (Müller, 1776), Pomphorhynchus laevis (Müller, 1776) and Contracoecum microcephalum (Stossich, 1890), larvae were identified. The analysis of the dominant structure of the found parasite species is presented to the component and infracommunities levels. All parasite species were accidental for the parasitic communities of examined fish with the exception of P. laevis and N. skrjabini. P. laevis was a core parasite species and N. skrjabini was a component parasite species for the helminth communities of bleak. The parasite communities of A. alburnus were discussed and compared with previous research data on parasite communities of bleak from River Danube in Bulgaria. New data for helminths and helminth communities of A. alburnus from Danube River (biotope Vetren) and their seasonal occurrence are presented.

Ecologoparasitological research was done based on the helminths and helminth communities of the Prussian carp (Carassius gibelio (Bloch, 1782)) from the freshwater ecosystem of the Panicheri Reservoir, Aegean Water Basin, Bulgaria. As a result of the examined nine specimens of the Prussian carp, two species of helminths were found: Ligula intestinalis (Linnaeus, 1758), larvae and Pomphorhynchus laevis (Müller, 1776). The dominant structure of the helminth communities was determined. The Panicheri Reservoir is a new habitat in Bulgaria of P. laevis and L. intestinalis from C. gibelio. C. gibelio is a new host record for L. intestinalis in Bulgaria. The two helminth species are core species for the helminth communities of the examined species of the freshwater fish. P. laevis was distinguished with higher prevalence and mean intensity (P%=33.34; MI=1.34) than L. intestinalis (P%=22.23; MI=1.0). The circulatory pathways of the helminth flow were traced.

A total of 180 individuals of Scardinius erythrophthalmus from Srebarna Lake, north-east Bulgaria, were examined for helminth parasites during spring, summer and autumn of 2005-2006 (30 specimens per season). Helminth parasites were recorded in 166 individuals (92.2%). Ten helminth species were found: trematodes--metacercariae of Posthodiplostomum cuticola, Diplostomum chromatophorum and D. spathaceum; monogeneans--Dactylogyrus difformis, D. difformoides and Diplozoon scardinii; cestodes--metacestodes of Paradilepis scolecina; nematodes--larvae of Spiroxys contortus and Rhabdochona denudata and adults of Schulmanela petruschewskii. Significant seasonal differences in the mean abundance were recorded for D. difformis, D. difformoides and D. scardinii. Significant seasonal differences in prevalence were recorded for P. cuticola, D. difformis, D. difformoides and D. scardinii. Dominant species in the component communities were P. cuticola, D. difformis and D. difformoides. The species composition of the component community essentially differs from that in the adjacent Danube River; a possible reason is the specific hydrobiological situation of the lake, characterized by a low density of benthic molluscs and crustaceans.