The parasitofauna of the Siberian sterlet Acipenser ruthenus marsiglii of the Lower

Center for Parasitology of the A. N. Severtsov Institute of Ecology and Evolution of the RAS, Leninsky Prospect, 33, Moscow, 119071, Russian Federation. Tel.: +7-925-509-40-96 E-mail: kts2@yandex.ru Liberman, E. L., & Voropaeva, E. L. (2018). The parasitofauna of the Siberian sterlet Acipenser ruthenus marsiglii of the Lower Irtysh. Regulatory Mechanisms in Biosystems, 9(3), 329–334. doi:10.15421/021848


Introduction
Sturgeons are one of the oldest groups of fish, the biology and cultivation of which is a subject of interest all around the world (Guénette et al., 1992;Israel & May, 2010;Wuertz et al., 2011;Akbulut et al., 2013;Vasil'ev et al., 2014). The smallest representative of sturgeons is the sterlet (Acipenser ruthenus (Linnaeus, 1758)). It is a nonmigratory species, inhabits the rivers of the following basins: Azov, Caspian, Black, Adriatic and Baltic seas, and also the Northern Dvina River and its tributaries -Suhona and Vychegda. It is common in the basins of the Siberian rivers: Ob and Yenisei, where it forms a subspecies -Acipenser ruthenus marsiglii (Brandt, 1833) (Zhuravlev, 2000). Sterlet quite like warm water, winter in river pockets, and are typical benhtophages -the basis of their food is chironomid larvae, Simuliidae, mayflies, dragonflies, molluscs, and other representatives of macrozoobenthos (Strel'nikova, 2012). Because of the high industrial value of sterlet, and therefore their intensive fishing, and also change in hydrological and hydrochemical regimes of water bodies (places of spawning and fattening), by the end of the 1970s, the reserves of sterlet in the Ob-Irtysh basin had become critical (Liberman, 2017). Currently, there is no industrial fishing of sterlet in this basin, catching takes place only for scientific purposes.
The decrease in the number of sterlet over the last half-century could have led to change in qualitative and quantitative composition of their parasitofauna. Therefore, due to absence of the corresponding studies over many years, the presentation of new data on the species composition of parasites of sterlet is relevant.
The objective of this research was to study the current condition of the parasitofauna of the Siberian sterlet in the Irtysh and Tobol rivers.

Materials and methods
The material was collected during the periods from 5th to 21st June and from 14th to 22st July of 2017 from the Tobol and Irtysh rivers, located within the Tobol and Uvatsky districts of the Tyumen oblast.
Full parasitological analysis, standard methods of fixation and staining of parasites were conducted according to the methods of I. E. Byhovskaya-Pavlovskaya (1985). Blood was drawn from the tail vein, a smear was prepared, dried in the air, and then fixated in a mixture of 95% ethyl spirit and diethyl ether (1 : 1) during 30 minutes, the fixated smears were then dried in the air at room temperature, stained using azure-eosine in a 1 : 10 solution during 40 minutes. For identifying the nematodes, we used the work of Moravec (2013), for the remaining groups of parasites -identification guides to parasites of fresh water fish of the fauna of the USSR (Opredelitel' …, 1984(Opredelitel' …, , 1987. We calculated the extensity of the invasion (EI, %), intensity of the invasion (II, minimum -maximum, specimens), abundance index (AI, specimens per fish), for intra-erythrocytic parasites -parasitemia index (X) (Bush et al., 1997;Woo et al., 2006).
where Е -invasion extensivity, n -number of host individuals infested with parasites, N -number of studied host individuals. where A -number of affected erythrocytes, B -number of erythrocytes in one microscope field, C -number of examined microscope fields. No less than 50 microscopic fields were examined for each preparation.
For identifying the dominant species in the communities, we used the Berger-Parker dominance index (d) (Mehgarran, 1992): where N max -number of the most abundant species, N -total number of parasites in the community.
H. acipenseris has an oval body shape, is 6.5-8.2 ˟ 2.2-3.0 µm in size, with two rounded ends or one rounded and one sharpened end (Fig. 2). The nucleus is composed of a few chromatin granules. It is positioned both in the middle of the body and in one of its ends. Most often, one parasite is found in an erythrocyte, more rarely two, and we also found parasites outside erythrocytes due to their decomposition. The highest EI of this parasite was recorded in a selection of fish caught in July in the Irtysh River in the area of Tobolsk -31.2% at average value of parasitemia of 0.06 ± 0.02%. The lowest EI was in fish caught in the same month in the Irtysh in the area of Gornoslinkino village (8.3%) at average value of parasitemia equaling 0.12 ± 0.11%. The highest average values of parasitemia were observed in sterlet from the Tobol River -0.19 ± 0.13%. It is interesting that these parasites were not found at all in the selection of fish from the Irtysh, collected in the same month as the fish from the Tobol (June). Note: EI -extensity of invasion, II -intensity of invasion min-max, AIabundance index, mtc -metacercariae, pl -plerocercoid, ⃰ -larvae stage.
Crepidostomum auriculatum trematode (Fig. 3) and Capillospirura ovotrichura nematode (Fig. 4) were found in all examined selections of fish. All specimens found of C. auriculatum and C. ovotrichura were mature. The nematode parasitized mostly in the stomach, more rarelyin the intestine, trematodes were found throughout the length of intestine. The highest infestation with C. auriculatum was recorded in fish from the Tobol River in June (Table 1). In the Irtysh, maximum EI equaled 37.5%, with infestation intensity of 1-36 specimens per fish and AI -3.9 (area of Tobolsk, July). Maximum extensivity of infestation with Capillospirura ovotrichura was observed in fish from the Irtysh River near Gornoslinkino (July) -25% at II of 1-21 specimens per fish and AI -1.7, whereas maximum intensity of invasion -93 specimens per fish at 5.9 abundance index was recorded near Tobolsk (EI -12.5%). On the whole, Capillospirura ovotrichura is the most abundant parasite of sterlet in the Irtysh (d = 0.680), whereas in the Tobol River, the dominant was Crepidostomum auriculatum trematode (d = 0.690).
In the Irtysh river, in the lenses of the studied fish, we found Diplostomum chromatophorum (Вrown, 1931) metacercariae. Maximum EI with this parasite was recorded in the fish selection from the area of Tobolsk -12.5% and II -1 and AI -0.12. In the area of Gornoslinkino in June and July, the infestation was low (EI -4.3% and 8.3% respectively). In the June selection of fish from the Tobol River, no metacercariae of this species were found. (Joff, Lewashow, Boschenko, 1926) in blood of Siberian starlet Among the rest of the recorded species, parasites of sterlet which belong to Trichodina sp. were frequently found, parasitizing the body surface, gills and fins. The identification to species level was impossible due to the poor condition of the material. All fish species were infested with this parasite only in the June selection from the Tobol (Table 1). In each preparation of slime scrape, we found 1-3 trichodinids. At the same time, fish from the Irtysh (June) was found to be freer from this parasite. In July, in the Irtysh River, the extensity of invasion, depending on the area of the study, ranged from 8.3% (Gornoslinkino) to 25.0% (Tobolsk). Rare parasite species were Proteocephalus sp. cestode (plerocercoid), Diplostomum chromatophorum (Вrown, 1931) trematode (metacercaria), Echinorhynchus cinctulus (Porta, 1905) acanthocephala (immature female), Ergasilus sieboldi (Nordmann, 1832) copepod crustacean, Piscicola geometra (Linnaeus, 1761) leech and Unionidae gen. sp. mollusc (glochidium).
Cryptobia acipenseri were recorded in different sturgeon in the basins of Don, Volga, Donau and Yenisei (Dubinin, 1952;Bauer et al., 2002). Most often, this parasite was recorded in sterlet (Shul'man, 1954;Lyubarskaya & Lavrent'eva, 1985;Baska, 1990;Bauer et al., 2002). According to Shul'man (1954), the range of C. acipenseris should coincide with the range of sterlet. However, the absence of data on the records of this parasite in the Ob-Irtysh basin could not prove this supposition. Our data verify the hypothesis of this author. The studies by Pazooki and Masoumian (2004), conducted on mature Acipenser gueldenstaedtii and A. persicus from the Caspian Sea, demonstrated a significantly different occurrence of Cryptobia acipenseri during examination of humid and stained blood smears (EI -45% and 55% compared to 15% and 25%). These authors recommend assessing the infestation of fish with kinetoplastids and flagellates by humid blood smears. The evidence presented by Pazooki and Masoumian (2004) should be checked by referring to additional material. The life cycle of C. acipenseri has not been studied completely. According to the literature data, transmission of Cryptobia to fish is possible by direct transmission from leeches (Woo, 2003).

Fig. 3. Crepidostomum auriculatum (Wedl, 1858)
Perekropov (1930) thought that the entire life cycle of H. acipenseris takes place in the blood of sterlet. Shul'man (1954) doubted the reliability of this study, for he assumed that transmission of haemogregarines from one fish to another is not possible without participation of blood-sucking invertebrates. It seems that as a result of experimental studies, it was determined that haemogregarines of fish are transmitted by leeches of the Piscicolidae family (Khan, 1980;Woo, 2006). Carriers of H. acipenseris in the Lower Irtysh are Piscicola geometra leeches, which were found on sterlets` pharyngeal arches and fins.
The most abundant helmiths of Siberian sterlet in the Lower Irtysh basin were C. auriculatum trematode and C. ovotrichura nematode, both specific to sturgeon. It should be mentioned that C. ovotrichura in the Irtysh basin were recorded earlier only in the Zaysan Lake (Zahvatkin, 1938;Dobrohotova, 1960). The distribution of these parasites coincides with the range of sturgeon (Skryabina, 1974;. Molecular-genetic data do not prove that C. auriculatum belongs to the Crepidostomum (Braun, 1900) genus (Atopkin & Shedko, 2014;Soldanova et al., 2017). The genus status of this parasite requires confirmation. Their life cycle is still incompletely studied. Presumably, the intermediary hosts for these species (the second for C. auriculatum and the only one for C. ovotrichura) are benthos invertebrates, particularly Gammaridae and mayfly larvae (Caira, 1989;Moravec, 1994). The Tobol River is a tributary of the Irtysh, there are both artificial and natural barriers between them, which limits the freedom of movement of fish between the water currents. The differences in the dominance of Crepidostomum auriculatum and Capillospirura ovotrichura in the Tobol and Irtysh are, perhaps, related to the intensity of consumption of the abovementioned components of the zoobenthos.
It is interesting that without an account of the abovementioned local differences, the extent of sterlet infestation with C. auriculatum in the Lower Irtysh was practically similar to that recorded by Petrushevskij et al. (1948) for this host in the region over 70 years ago (Table 2).  demonstrated that a high number of the intermediary host when there is a decrease in the number of the final host leads to higher resistance of the "host -parasite" system than at increase in the number of the latter. And, if a higher number of parasite individuals concentrate in a small number of host individuals, then it is enough for survival of the parasites` population due to the larva forms which live in intermediary hosts .
Metacercariae of Diplostomum chromatophorum trematodes were recorded in Siberian sterlet for the first time. Infestation with this common and low-specific parasite was insignificant and causes no great damage to fish (Marcogliese et al., 2001;Voutilainen et al., 2008).
The species composition of non-specific parasites which occur in sturgeon depends on the local ecological conditions and differs in each particular water body. In water bodies of Siberia (Ob, Yenisei, Lena), sturgeon become infested with species typical for northern fish: Salmonidae, Coregonus and burbot (Shul'man, 1954). This was proved by our discovery of the acanthocephalan E. cinctulus. This parasite was recorded for the first time in sterlet of the Ob-Irtysh basin.

Table 2
Species composition of parasites of sterlet in the rivers of Siberia and the extensivity of invasion (%) Parasitofauna Yenisei Basin (Bauer, 1948;Skryabina, 1974 R. Irtush (Petrushevskij et al., 1948) Lake Zaysan and the Black Irtush (Zahvatkin, 1938;Dobrohotova, 1960) R. Ob (Titova, 1965;Razmashkin, 1976  Note: * -single specimens of the parasite found. The total number of species recorded for the Siberian sterlet in the Ob-Irtysh basin was higher than that for sterlet from the Yenisei: 18 and 11 respectively ( Table 2). The species found in fish of two basins were the following: Cryptobia acipenseris, Diclybothrium armatum, Crepidostomum auriculatum, Capillospirura ovotrichuria, Truttaedacnitis clitellarius, Echinorhynchus cinctulus, Piscicola geometra, Unionidae gen. sp. To a large extent, the interbasin parasitological differences are related to the lower extent of study of parasitofauna of sterlet in the Yenisei. However, the absence of Amphilina foliacea, which is parasite specific to sturgeon, in the Ob-Irtysh basin, and the high infestation with this species in the Yenisei, and also its presence in Siberian sturgeon in the Ob-Irtysh basin indicates that there are other factors.
In the water bodies of Siberia, the species composition of parasites of the Siberian sterlet is significantly poor compared to that of sterlet in the Volga-Caspian basin, where the species is recorded as hosting 44 species of parasites, 20 of which are specific to sturgeon (Kazarnikova & Shestakovskaya, 2006).

Conclusion
During the summer of 2017, Acipenser ruthenus marsiglii in the Lower Irtysh basin was found to host 11 species of parasites, the most abundant of which are specific parasites of sturgeon: the trematode Crepidostomum auriculatum and the nematode Capillospirura ovotrichura. Compared to the data obtained for this region over 70 years ago (Petrushevskij et al., 1948), infestation of sterlet with Crepidostomum auriculatum almost has not changed, and blood parasites Cryptobia acipenseris and Haemogregarina acipenseris were recorded in the Ob-Irtysh basin for the first time.