A comparative study of the scolex hook morphology of five species of tapeworms of the genus Triaenophorus Rudolphi, 1793 (Cestoda: Pseudophyllidea), parasites of pikes (Esox lucius L. and E. reichertii Dybowski) in the Palaearctic Region, was carried out. Measurements of scolex hooks of 81 plerocercoids and 492 adults from different hosts and regions were compared using basic statistics and forward stepwise linear discriminant analysis. The shape of the scolex and that of tridental hooks were found to be suitable only for differentiation of the taxa with a similar shape of hooks, i.e. Triaenophorus nodulosus (Pallas, 1781) from T. amurensis Kuperman, 1968, and T. crassus Forel, 1868 from T. meridionalis Kuperman, 1968 and T. orientalis Kuperman, 1968, respectively. In contrast, discriminant analysis did not enable reliable separation of specimens of individual taxa of these two morphological groups due to high intraspecific variability and overlaps between species. This was reflected in low classification efficiencies (average 83%) of all species of the T. crassus group, whereas all T. amurensis specimens were misidentified as T. nodulosus. The new data also considerably enlarge (up to twofold) the size range of the species described by Kuperman in 1968, which invalidates suitability of the most important discriminant characteristic, the width of the basal plate, for delimitation of Triaenophorus species. Based on the present data, all Kuperman's taxa are considered to represent only distinct geographical populations of T. nodulosus and T. crassus. As a result, T. amurensis is synonymized with T. nodulosus, whereas T. orientalis is considered to be a synonym of T. crassus. Previous synonymisation of T. meridionalis with T. crassus, first proposed by Dubinina (1987), is also accepted.
The tegument ultrastructure of the cestode Triaenophorus nodulosus has been studied in the stages of oncosphere, procercoid, plerocercoid and adult. The syncytium of primary tegument has glandular origin and is located in the peripheral areas of the oncosphere. The primary tegument degenerates at the initial stages of the procercoid development and is replaced with secondary tegument persisting throughout all following stages of the worm’s development. Two ways of microthrix formation on the body surface of procercoid were discovered. The formation of the cyst consisting of fibrillar material around the plerocercoid was observed. It fills spaces between numerous finger-like évaginations of plerocercoid’s tegument. The structural differentiation of tegument and microtriches was demonstrated on the scolex and all parts of strabila of T. nodulosus.