Background Leeches (Hirudinida) play a significant role as intermediate hosts in the circulation of trematodes in the aquatic environment. However, species richness and the molecular diversity and phylogeny of larval stages of strigeid trematodes (tetracotyle) occurring in this group of aquatic invertebrates remain poorly understood. Here, we report our use of recently obtained sequences of several molecular markers to analyse some aspects of the ecology, taxonomy and phylogeny of the genera Australapatemon and Cotylurus, which utilise leeches as intermediate hosts. Methods From April 2017 to September 2018, 153 leeches were collected from several sampling stations in small rivers with slow-flowing waters and related drainage canals located in three regions of Poland. The distinctive forms of tetracotyle metacercariae collected from leeches supplemented with adult Strigeidae specimens sampled from a wide range of water birds were analysed using the 28S rDNA partial gene, the second internal transcribed spacer region (ITS2) region and the cytochrome c oxidase (COI) fragment. Results Among investigated leeches, metacercariae of the tetracotyle type were detected in the parenchyma and musculature of 62 specimens (prevalence 40.5%) with a mean intensity reaching 19.9 individuals. The taxonomic generic affiliation of metacercariae derived from the leeches revealed the occurrence of two strigeid genera: Australapatemon Sudarikov, 1959 and Cotylurus Szidat, 1928. Phylogenetic reconstructions based on the partial 28S rRNA gene, ITS2 region and partial COI gene confirmed the separation of the Australapatemon and Cotylurus clades. Taking currently available molecular data and our results into consideration, recently sequenced tetracotyle of Australapatemon represents most probably Au. minor; however, unclear phylogenetic relationships between Au. burti and Au. minor reduce the reliability of this conclusion. On the other hand, on the basis of the obtained sequences, supplemented with previously published data, the metacercariae of Cotylurus detected in leeches were identified as two species: C. strigeoides Dubois, 1958 and C. syrius Dubois, 1934. This is the first record of C. syrius from the intermediate host. Conclusions The results of this study suggest the separation of ecological niches and life cycles between C. cornutus (Rudolphi, 1808) and C. strigeoides/C. syrius, with potential serious evolutionary consequences for a wide range of host–parasite relationships. Moreover, phylogenetic analyses corroborated the polyphyletic character of C. syrius, the unclear status of C. cornutus and the separate position of Cotylurus raabei Bezubik, 1958 within Cotylurus. The data demonstrate the inconsistent taxonomic status of the sequenced tetracotyle of Australapatemon, resulting, in our opinion, from the limited availability of fully reliable, comparative sequences of related taxa in GenBank.

In the present study, we investigate the effect extrinsic (habitat and season) and intrinsic (host’s age and sex) factors on the richness, diversity, and structure of parasite component communities and aggregation patterns in the helminth fauna of the great cormorant Phalacrocorax carbo from northeastern Poland. The helminth fauna of cormorants from the brackish water habitat was far richer (30 species) than in those from freshwater lakes (18 species) and strongly depend on season and age of the host. The values of diversity index strongly varied in relation to habitat and host age with clear seasonal differences in the value of diversity index, i.e., its value increased over time in adults from the brackish water habitat and decreased in those from the freshwater lakes. The number of helminths in adult and immature birds varied, depending on the season and habitat: in the brackish water habitat, the overall percentage of helminths was higher in spring than in summer, while in the freshwater habitat a higher proportion of helminths was recorded in summer. During spring, in the brackish water habitat, we observed a higher level of aggregation (for all groups of helminths) than in autumn. The opposite pattern was found in the freshwater habitat. However, this regularity was typical of adult birds only. In immature hosts, the level of aggregation was not predictable and varied among the higher taxa. Our study clearly showed that processes determinate diversity, structure, richness, and patterns of aggregation in helminth assemblages of avian hosts are multi-origin and highly complex.Electronic supplementary materialThe online version of this article (doi:10.1007/s00436-013-3714-7) contains supplementary material, which is available to authorized users.

Parameters related to the occurrence, aggregation, and population structure of the nematode Contracaecum rudolphii Hartwich, 1964 , in the great cormorant ( Phalacrocorax carbo ) from northeastern Poland were analyzed. A total of 491 birds of different ages (adults, immatures, and nestlings) was examined; the cormorants studied were taken from both fresh- and brackish water habitats. Contracaecum rudolphii were found in stomachs of 454 birds (92.5%); the 46,244 nematodes included third- (L3) and fourth-stage larvae (L4), and sub-adult and adult females and males. The distribution of nematodes in the host population were highly aggregated. The occurrence of C. rudolphii was significantly dependent on the host's age and habitat, as well as on season; the proportion of development stages differed significantly depending on birds' age and season (the latter only in adult birds). The infrapopulations of C. rudolphii in the adult cormorants showed distinct density-dependent correlations: that is, as the infrapopulation size increased, the number of adult females C. rudolphii decreased, and the proportion of larvae and sub-adult females increased. A higher proportion of larvae and sub-adult females characterized the component population structure of the nematodes in the cormorant nestlings, compared with adult birds, probably because of immune system deficiency in the immature birds, coupled with the development of the nematode population. Seasonal changes in the C. rudolphii population, observed in the adult cormorants, were not related to seasonality of the L3 occurrence in food; instead, the changes are believed to have resulted from independent processes of elimination of the oldest nematodes and their replacement by larval stages that subsequently matured.