Aims The aim of the study was to investigate the skin microbiota of Prussian carp infested by ectoparasites from the genera Argulus and Lernaea. Methods and Results Associated microbiota of skin of Prussian carp and ectoparasites were investigated by sequencing of the V3, V4 hypervariable regions of 16S rRNA using Illumina MiSeq sequencing platform. Conclusions According to the Spearman rank correlation test, the increasing load of ulcerations of the skin of Prussian carp was weakly negatively correlated with reduction in the abundance of the following taxa: Acrobacter, bacteria C39 (Rhodocyclaceae), Rheinheimera, Comamonadaceae, Helicobacteraceae and Vogesella. In this study, the microbiota of ectoparasites from the genera Lernaea and Argulus were characterized for the first time. The microbiota associated with L. cyprinacea was significantly different from microbial communities of intact skin mucosa of both infested and uninfested fish and skin ulcers (ADONIS, P ≤ 0·05). The microbiota associated with parasitic crustaceans L. cyprinacea were dominated by unclassified bacteria from Comamonadaceae, Aeromonadaceae families and Vogesella. The dominant microbiota of A. foliaceus were represented by Flavobacterium, Corynebacterium and unclassified Comamonadaceae. Significance and Impact of the Study Results from these studies indicate that ectoparasites have the potential to alter skin microbiota, which can play a possible role in the transmission of secondary bacterial infections in fish, caused by pathogenic bacteria.

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Lake Teletskoye (West Siberia, Russia) is inhabited by a sympatric pair of whitefish, with each member of the pair being characterized by different feeding habits. Coregonus lavaretus pidschian (Gmelin, 1789) is a large ‘benthivorous’ form, while C. l. pravdinellus (Dulkeit, 1949) is a small ‘planktivorous’ form. Fish were collected from the end of August to the middle of September in 2017 and 2019–2020 in the north part of Lake Teletskoye. For the ‘benthivorous’ form the prevalence, intensity and abundance of T. crassus ranged from 22.4% to 51.9%, 1.9–2.8 and 0.4–1.3, respectively, whereas the same indices for the ‘planktivorous’ form ranged from 94.7% to 97.5%, 4.2–4.8 and 4.0–4.7, respectively. The level of prevalence of infection and abundance of T. crassus in muscle was relatively stable among studied years for both forms. The level of prevalence was higher in the years 2019 and 2020 than in 2017 for the ‘benthivorous’ form, whereas for the ‘planktivorous’ form this index did not change during the studied years. For the first time, a partial sequencing of the cox1 gene (593 bp) for T. crassus was sequenced. All 15 plerocercoids of T. crassus were represented by four haplotypes.

The impact of parasites on gut microbiota of the host is well documented, but the role of the relationship between the parasite and the host in the formation of the microbiota is poorly understood. Using 16S amplicon sequencing and newly developed methodological approaches, we characterize the gut microbiota of the sympatric pair of whitefish Coregonus lavaretus complex and the associated microbiota of cestodes parasitizing their intestine. The essence of the proposed approaches is, firstly, to use the method of successive washes of the microbiota from the cestode’s surfaces to analyze the degree of bacterial association to the tegument of the parasite. Secondly, to use a method combining the sampling of intestinal content and mucosa with the wash-out procedure from the mucosa to understand the real structure of the fish gut microbiota. Our results demonstrate that the trophic diversification of a sympatric pair of whitefish predetermines a segregation by ecological niches of their respective microbial communities within their intestine. Additional environmental niches for settlement of bacteria in the intestine are formed by the parasitic helminths that caused the restructuring of the bacterial community in infected fish compared to those uninfected. Using the desorption method in Ringer’s solution, we have demonstrated that Proteocephalus sp. cestodes possess their own microbial community which is put together from “surface” bacteria received from the host, bacteria which are weakly and strongly associated with the tegument, and microbiota obtained after removal of the tegument from the cestodes.

Introduction Data on rainbow trout infection with the copepod Caligus lacustris in cage aquaculture on Lake Ladoga is presented. Materials and MethodsCaligus lacustris (n=127 ex.) were collected from a farm in Lake Ladoga housing cage-reared rainbow trout to describe the size-age and sex structure of the copepod population. Morphological features of the copepods were evaluated according to 10 characters with terminology proposed by Kabata, Gusev (1966). To determinate the phylogenetic position of C. lacustris within the genus Caligus, fragments of the cytochrome c oxidase subunit 1 mitochondrial gene (COI, 645 bp) and 18S rRNA gene (1617 bp) were sequenced. ResultsAn increase of parasite prevalence was observed as the lake was warming up from July to September. The morphological features of the crustacean's larval and adult stages, characterized by specific parameters of quantitative traits, are described.Three COI haplotypes and only one 18S rRNA haplotype of C. lacustris were identified among five samples. Based on 18S rRNA analysis (resolution of the COI tree was poor) we can conclude that the clade containing C. lacustris, and the aforementioned sister species, appears as an early radiation of the genus Caligus. ConclusionsThe development of freshwater aquaculture contributes to the transfer of the native parasite Caligus lacustris to farmed rainbow trout.