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Biographical sketch describes the scientific activities of Professor Oleg Bauer, who was among the pioneers in the field of ecological parasitology and fish pathology in Russia.

The analysis of taxonomical and ecological diversity of acanthocephalans in fishes of Asiatic sub-Arctic region freshwaters, summarizing changes in modern views on species composition, life cycles, and ecology of background groups of these parasites is given. A priority role of studies provided by O.N. Bauer and his scientific school in organization and development of these aspects of acanthocephalology is demonstrated. Special attention is paid to the assessment of acanthocephalan biodiversity of the genus Neoechinorhynchus, the background group of freshwater fish parasites of the Asiatic sub-Arctic region, and an original key for their species is given. The distribution of acanthocephalans of the genus Acanthocephalus in northeastern Asia is analyzed and prospective study of this parasite group, evolutionary associated with freshwater isopods of the genus Asellus as intermediate hosts, is outlined. The absence of documented evidences on intermediate hosts of other background parasites of freshwater fishes in the region, acanthocephalans of the genus Metechinorhynchus, is revealed. It is assumed that subsequent taxonomic revisions based both on morphological and molecular genetic studies are necessary for the reliable revealing of species composition in each genus of the background acanthocephalans from freshwater fishes of Northern Asia. Theoretical significance of the study of acanthocephalan life cycles and revealing their natural intermediate hosts for the reliable estimation of structural and functional organization of their host-parasite systems in different parts of the range is substantiated and the possibility of the distribution of taxonomic conclusions in new territories is analyzed. A brief annotated taxonomical list of freshwater acanthocephalans of the Asiatic sub-Arctic region is given.

The spatial structure and seasonal aspects in the number distribution of the cestode Proteocephalus longicollis population, maintained by trophic connections of specialized plankton-feeders (the vendace Coregonus albula L. and the smelt Osmerus eperlanus L.) in the Lake Onego were studied. Differences in the distribution of number of cestode population in host population, size-age structure, and pre-reproductive/reproductive stages ratio of the studied parasites were revealed. It was demonstrated that the spatial structure of P. longicollis was shaped by the specific characteristics of vendace and smelt populations in the Lake Onego. These species possess partially overlapping ecological niches and differences in the use of food resources, significantly determining the infection dynamics of the examined fish species.

The material was collected according to commonly accepted methods in 1992—2013 from the following sites: the Lake Krivoye (Kolguev Island); Volonga, Schapkina, Kolva, More-Yu, Vorkuta, Kara, and Khalmer-Yu Rivers; Lakes Nikaramata (the Kara River basin) and Mercempertato (Yamal Peninsula). A total of 436 of the minnow Phoxinus phoxinus with age 1+—3+ were examined. In the examined water bodies, excluding the Kolva River, the parasite fauna of the minnow is impoverished, and, to a greater extent, possesses an occasional character. In rivers Kolpina and Shapkina Rivers, component communities of minnow parasites are mature (balanced, according to Pugachev, 1999). These minnow parasite communities consist of three groups of species separated according to the ratio of their biomasses. In More-Yu, Vorkuta, Kara, and Khalmer-Yu Rivers, and in Lakes Nikaramata, and probably, Mercempertato, parasite communities are immature (unbalanced). These minnow parasite communities consist of 1—3 groups of species separated according the ratio of their biomass. Characteristics of communities from the Volonga River need clarification. Associations between the state of the described minnow parasite communities and their geological age (Quaternary history) are traced.

The native population of the Arctic char in the Lake Mushtalampi was studied. The dwarf form of the Arctic char inhabiting the lake was revealed. The fish is characterized by extremely low growth rate and by early maturation. Twelve parasite species were found in the arctic char. The parasite fauna is typical of representatives of Salmonidae. Distinguishing features of this fauna include the absence of narrowly specialized char parasite species. High intensity of infection with nematodes Philonema oncorhynchi allows estimating the epizootic state of the studied char population as insufficient.

Развитие метацеркарий большинства видов Diplostomum, включая D. pseudospathaceum, происходит в глазном хрусталике рыб и сопровождается глубокими морфологическими трансформациями, которые часто называют метаморфозом. Одной из структур, которая претерпевает существенные изменения у D. pseudospathaceum, являются шипики тегумента. В настоящем исследовании эти изменения изучались у D. pseudospathaceum при помощи окраски фаллоидином и конфокальной микроскопии на последовательных стадиях развития от трехдневных до инвазионных метацеркарий. Хотя общая организация вооружения покровов оставалась в целом неизменной, большинство личиночных шипиков продолжало расти до самых поздних стадий развития метацеркарий. Начиная с 10-го дня развития у метацеркарий происходила постепенная замена личиночных шипиков на маленькие зачаточные дефинитивные шипики, и инвазионные метацеркарии были покрыты только дефинитивными шипиками. В статье обсуждается возможная адаптивная роль шипиков у развивающихся метацеркарий.