The occurrence of the common European fish cestode Caryophyllaeus laticeps (Pallas, 1781) in the River Irtysh, China: a morphological characterization and molecular data.

The cestode Caryophyllaeus laticeps (Pallas, 1781) is a highly polymorphic species with several morphotypes, and belongs to the most widespread species of monozoic tapeworm (Cestoda: Caryophyllidea) found in the Palaearctic zoogeographic region with a wide range of cyprinid fish hosts. This paper provides the first reliable evidence about the occurrence of C. laticeps in northern China that is based on detailed morphological characteristics and mitochondrial cox1 sequences. C. laticeps was found in freshwater bream Abramis brama (L.), in the River Irtysh, with the prevalence of 40% and mean intensity of infection 8. The tapeworms were rather similar in their morphological characters to the morphotype 1 of C. laticeps which is typified by the possession of a slender body with a flabellate wide scolex, narrow neck, robust, long and clearly visible vas deferens, large, and almost spherical cirrus-sac containing well-developed internal seminal vesicle and the first vitelline follicles and first testes beginning far from anterior margin of the body. Subsequent molecular analysis confirmed the morphological identification of the species. Our results show that the studied specimens were firmly clustered into the C. laticeps clade and formed a monophyletic group with C. laticeps morphotype 1 and 2 from Europe and Russia. In China, C. laticeps is a non-indigenous fish parasite, which has the most probably been introduced into the Irtysh River basin with its typical host inhabiting Russia.