First data on parasites of the invasive brown bullhead Ameiurus nebulosus (Siluriformes: Ictaluridae) in Ukraine

The issue of invasive fish species has come to the forefront of scientific interest in recent decades (Gozlan et al., 2010; Catford et al., 2012; Simberloff et al., 2013; Zenni & Nuñez, 2013; Ricciardi, 2015). One of the reasons is the negative effects that such species may have on all trophic levels of aquatic communities, e.g. by reducing the abundance of native fish, zooplankton and/or macrophytes (Hermoso et al., 2011; Gallardo et al., 2016). A further important aspect of invasive fish introductions is the co-introduction of their natural parasites, which, in some cases, can have deleterious effects on native fish hosts, particularly those with which the parasite does not share evolutionary history (Taraschewski, 2006; Kmentová et al., 2019; Sarabeev et al., 2022). Among the most frequently co-introduced non-indigenous parasites are helminths, arthropods and protozoans, with fish being the most common non-indigenous hosts (Lymbery et al., 2014).

Over the last 50 years or so, our understanding of host-parasite interactions has increased considerably (e.g. see Anderson & May, 1978, 1979, 1982; Brooks & Hoberg, 2007; Gómez-Arreaza et al., 2017; Kennerley et al., 2022). Host specificity implies that co-introduced parasites will primarily infect their natural (i.e. introduced) hosts, rather than species native to the invaded area (Taraschewski, 2006; Lymbery et al., 2014; Goedknegt et al., 2016). Nevertheless, a range of host-parasite interactions have been observed in invaded waters, and several hypotheses have been formulated to explain these, e.g. the enemy release hypothesis, parasite spillback, dilution effect and parasite spillover (summarised in Goedknegt et al., 2016). With regards to the non-native fish introductions, current research is increasingly focused on the possible negative impacts of both non-native hosts and their parasites on local economically-important hosts (Galli et al., 2003; Goedknegt et al., 2016).

The brown bullhead catfish Ameirus nebulosus Lesueur, 1819 (Ictaluridae) is native to Canada and the USA (Schindler, 1957). It was first introduced into Europe as an ornamental pond species in Poland in 1885 (Nowak et al., 2008) and is now common in most European countries (Rutkayová et al., 2013; Rechulicz & Płaska, 2018). The species generally inhabits backwaters and pools of large lowland rivers, nutrient-rich lakes and ponds, reservoirs and irrigation canals (Page & Burr, 1991; Ulikowski et al., 2022). Due to its high tolerance of poor environmental conditions and high temperatures, along with a general lack of native predators, the distribution of brown bullhead (and its parasites) is highly likely to increase throughout Europe (Kutsokon et al., 2018; Bănăduc et al., 2022; Ulikowski et al., 2022).

Despite its present wide European distribution (Rutkayová et al., 2013; Rechulicz & Plaska, 2018), knowledge of brown bullhead parasites and pathogens remains relatively scarce. In 2012, a mass mortality event was reported for a Hungarian brown bullhead population, the deaths being linked to an outbreak of European sheatfish ranavirus (Fehér et al., 2016). Seven ciliate parasite species and the acanthocephalan Acanthocephalus anguillae, all likely of local origin, have been reported for brown bullhead caught in the Serbian River Danube (summarised in Đikanović et al., 2018). There have been several reports of North-American monogeneans co-introduced with brown bullhead in Polish lakes (Prost, 1973), ponds and oxbows of the Czech Republic (Ondračková et al., 2020) and the River Tisza (Galli et al., 2010). The brown bullhead has also been reported to act as an intermediate or paratenic host of the cestode Ophiotaenia europaea, infecting colubrid snakes in Europe (Ondračková et al., 2021). To date, however, there has been no comprehensive study investigating the overall parasite fauna of brown bullhead in Europe.

The brown bullhead was first observed in Ukraine in the Shatsk Lakes (Vistula River drainage) around 1937, having been introduced from the River Prypyat in Belarus (Ivlev & Protasov, 1948). In 2017, the species was found in the city of Lviv (Upper Dniester basin; Kutsokon et al., 2018). The main aim of this study was to provide a comprehensive description of the parasite community of non-native brown bullhead in Ukraine. We also provide a comparison of the metric features of North American monogeneans in their invasive and native areas.

Brown bullhead were sampled with a 80×73×72 cm (10 mm mesh size) dipnet from four localities in Ukraine, three situated in the Vistula basin (Lake Svitiaz, Lake Pisochne, Lake on Plastova) and one in the Dniester basin (Lake on Stryiska) between April and November 2021 (Fig. 1). Two of the lakes, Svitiaz (51.483444, 23.793333) and Pisochne (51.567722, 23.901417), lie adjacent to the Shatsk glacial lakes, a Natural Protected Area. The others, Lake on Stryiska (49.800658, 24.016692), is a natural lake in the southern part of the city (Zubria river drainage), and Lake on Plastova (49.859328, 24.079917), a pond in the north (Poltva river drainage), lie within the city of Lviv itself. The fish were transported live to the laboratory, where they were kept in aerated tanks until dissection. All fish were dissected within three days of sampling to ensure maximum parasite recovery (Kvach et al., 2016, 2018). A total of 102 fish were studied for parasites (Table 1). Prior to dissection, each fish was measured to the nearest 1 mm for standard (SL) and total (TL) length, after which the fins, skin, gills, muscles and internal organs were examined for presence of parasites. Living unicellular and myxozoan microparasites were examined under light microscopy, while monogeneans and mites were preserved in Glycerin-ammonium-picrate (GAP) and prepared as semi-permanent slides (Malmberg, 1970). Digeneans, cestodes and nematodes were preserved in hot 4 % formaldehyde (Cribb & Bray, 2010). Encysted and capsuled parasites were isolated from the cyst/capsule before fixation. Cestodes and digeneans were stained with iron acetic carmine, dehydrated in ethanol of increasing concentration and mounted in Canada balsam as permanent slides (Georgiev et al., 1986). Glochidia and crustaceans were preserved in 4 % formaldehyde and identified under light microscopy. All parasites were identified to species level or, where not possible, to the lowest taxonomic level possible.

Fig. 1.

The morphology of non-native monogenean parasites was examined using an Olympus BX53F2 light microscope equipped with phase contrast optics (Olympus, Japan). Drawings of the hard structures of the haptor and copulatory organs were produced with the aid of a drawing attachment. Measurements (in micrometers) were obtained using cellSens standard digital image analysis software v.3.2 (Olympus, Japan). Bray-Curtis index based on presence/absence data was used for calculation of a qualitative similarity between parasite communities in Ukrainian localities, and between Ukraine (this study), the native American and Canadian range, and Puerto-Rico (non-native range), using published data (see Supplementary material for details). All statistical analyses were performed using Primer v.5.0 and Statistica v.12.0 (StatSoft, Inc.).

This research was undertaken in line with the ethical requirements of the Czech Republic and has been approved by the appropriate ethics committee.

In this study, we provide the first data for parasites of the invasive brown bullhead in Ukraine. Our data also represent the first comprehensive study of the parasite communities of this non-indigenous fish species in Europe. Furthermore, the three monogenean species recorded, G. nebulosus, L. pricei and L. monticellii, are recorded in Ukraine for the first time, widening the European distribution of these North American parasites.

The parasite fauna of brown bullhead in Europe is poorly described compared to its native range (Supplementary Table S7), with most studies focusing on monogenean parasites (Chechyna et al., 1953; Prost, 1973; Šimková et al., 2003; Ondračková et al., 2020). The three monogenean species recorded in this study, i.e. G. nebulosus, L. pricei and L. monticellii, are specific parasites of bullheads (Hanek & Fernando, 1971; de León et al., 2010, Leis et al., 2020) and have been co-introduced to Europe from North America. Though these parasites have already been recorded in other European countries, they are reported in Ukraine for the first time. Gyrodactylus nebulosus was first recorded in Europe in Poland (Prost, 1973; denoted as G. fairporti), with further records from the River Tisza (Galli et al., 2010), ponds, oxbows and side arms of the River Elbe in the Czech Republic (Ondračková et al., 2020) and lakes in Bulgaria, where it was reported to infect related black bullhead Ameiurus melas (Vancheva et al., 2020). In Ukrainian lakes, the species has a lower prevalence (10 %) and abundance (0.1–0.2) than similar localities in the Czech Republic (Elbe river basin), where up to 47 % of fish were infected, with some sites having mean abundances reaching 1.15 (Ondračková et al., 2020). Although the highest prevalence and abundance recorded to date has been reported from Bulgaria (Lake Srebarna, Danube river basin), where up to 72 % of fish were infected with abundances of 5.3±4.9 (Vancheva et al., 2020), the authors, in fact, referred to closely related Gyrodactylus melas. Further morphometrical and molecular study of G. nebulosus by Ondračková et al. (2020) showed that previous records of G. nebulosus include at least two species, i.e. G. nebulosus and G. melas, strictly host specific to brown and black bullhead, respectively. Morphological comparisons indicate that G. nebulosus collected in Ukraine have slightly shorter haptoral hard parts (see Supplementary Table S1); this may be related to different periods of material collection, since the associated changes in water temperature possibly affect the size of the haptoral hard parts (Mo, 1993).

As with G. nebulosus, both North American dactylogyrids L. pricei and L. monticellii were first reported to infect brown bullhead in Poland (Prost, 1973), though an earlier report is known from Italy on Ameiurus catus (Linnaeus, 1758) (Cognetti & Maktiis, 1924). Later European reports (summarised in Nitta & Nagasawa, 2015; Vancheva et al., 2020) are known from Bosnia and Herzegovina in 1988 (Nedić et al., 2021), Hungary (Adamczyk, 1973), Romania (Roman-Chiriac, 1960) and the Czech Republic (Moravec, 2001; Šimková et al., 2003). In contrast, L. monticellii in brown bullhead has only the above-mentioned record from Poland (Prost, 1973). A related non-native species, L. pricei, had been recorded at high prevalence (100 %) and abundance (13.3) levels in Bulgaria (black bullhead, Vancheva et al., 2020) and Ukraine (this study). Morphological comparisons between non-native regions indicate that Ukrainian L. pricei are either similarly sized or even slightly larger compared to other studies (Prost, 1973; Klassen & Beverley-Burton, 1985; Vancheva et al., 2020; Supplementary Table S2). While information on the distribution and infection rate of L. monticellii is scarce, morphological comparisons between specimens from Ukraine and the USA (Klassen & Beverley-Burton, 1985) show no significant difference. In comparison, the measurements of anchor features and copulatory organ provided by Prost (1973) were slightly larger than Ukrainian specimens (see Supplementary Table S3).

Regarding species composition, parasite communities at the four Ukrainian sites consisted mainly of acquired parasites, including the larval trematodes of Diplostomum spp., Diplostomum spathaceum and Hysteromorpha triloba, the larval nematodes A. crassus and Agamonema sp., and unidentified myxozoans (Table 1). In its native range, the brown bullhead is frequently infected with Diplostomum spp. eye flukes (both in the lens and vitreous humour), with high prevalence in some localities (Marcogliese & Compagna, 1999). In Ukraine, fish from Lake Svitiaz and the Lake on Stryiska also exhibited a high prevalence of Diplostomum metacercariae, probably reflecting the frequent occurrence of aquatic molluscs (intermediate hosts) and birds (definitive hosts) in the locality. Indeed, aquatic birds, the most numerous and diverse group of vertebrates in this wetland region (Tsaryk et al., 2002) provide excellent conditions for completion of the trematode life cycle and, particularly, of Diplostomum eye flukes.

Interestingly, brown bullhead in Lakes Svitiaz and Pisochne were infected with a high prevalence and abundance of the Asian nematode, A. crassus. This parasite infects the European eel (Anguilla anguilla) in its adult stage but uses various freshwater fishes as paratenic hosts (Moravec, 2013). Our data would suggest that brown bullhead is a susceptible host for this parasite. The larvae of A. crassus are able to act as ‘immunological hitchhikers’, avoiding the paratenic host’s own immune response (Emde et al., 2014). Consequently, it has been suggested that distribution of larval A. crassus with an invasive paratenic host, e.g., round goby (Neogobius melanostomus), provides support for the ‘invasional meltdown hypothesis’ (Hohenadler et al., 2018; Kvach et al., 2019). In this case, the successful expansion of a non-native parasite is dependent on the spread of its invasive host, even when the parasites are introduced through other vectors (Taraschewski, 2006; Emde et al., 2014). The two lakes in which the brown bullhead infected with A. crassus larvae were found form part of the Shatsk Lakes group, which lie within a Natural Protected Region with the country’s highest abundance of European eel (Vasenko & Starko, 2022). As such, it is possible that brown bullhead could spread the invasive nematode, A. crassus, into other Ukrainian waters, providing further support for ‘invasional meltdown’.

Non-indigenous species often lose some or all of their natural enemies, including parasites, during introduction to a new environment, giving them a potential advantage over local species (Torchin et al., 2003). A comparative analysis of our findings with those published previously showed that the brown bullhead has a wider parasite diversity in its native range than in its non-native areas (Supplementary Table S7). For example, specimens from the USA and Canada had Shannon–Wiener indices of 4.6 and 3.8, respectively, compared with 2.8 in non-native Ukrainian water bodies, while those introduced to Puerto Rico had an index value of 1.8 (Supplementary Table S6, S1). Note, however, that the indices for North America were calculated for a large area and, consequently, may be affected by a lack of publications for some regions. On the other hand, brown bullhead in their native range had a wide spectrum of parasite species through all higher taxonomic groups, while those in non-native areas had a much lower species richness, which may well support the ‘parasite release or reduction hypothesis’ (Goedknegt et al., 2016). At Ukrainian localities, the dominant species recorded in the brown bullhead parasite community were either specific species co-introduced directly from the native region, such as monogeneans, or species for which brown bullhead represent the intermediate or paratenic host, i.e. A. crassus, H. triloba and D. spathaceum. The poorest parasite community was recorded on fish from the Lake on Plastova, which comprised one introduced species (L. pricei) and sporadic occurrence of two local species (Trichodina spp., Diplostomum sp.). The Lake on Plastova is a small artificial pond that lies by the River Poltva (Vistula river basin) in the industrial part of Lviv with high disturbance factor and risk of organic pollution (municipal composting station), which potentially explains such a low parasite species richness. On the other hand, the parasite community of another invasive fish in this same pond, the Chinese sleeper (Perccottus glenii), was similar, or even higher, to that found at other localities in Ukraine (Kvach et al., 2022). Thus, the low infection parameters for brown bullhead at this site may be the result of some other factor, such as the young age of the fish population. In comparison, brown bullhead from the Lake of Stryiska (Dniester river basin) had the same infection parameters as those in the Shatsk Lakes, probably having been translocated from there due to human activity.

Our study confirms that the North American parasites G. nebulosus, L. pricei and L. monticellii were co-introduced into Europe and Ukraine with brown bullhead. However, as the three parasites are specific to North American catfish and have direct life-cycles we do not expect them to switch to local fish fauna. Co-introduced monogenean parasites have previously been used as a bio-indicator for assessing the relatedness of non-indigenous fish host populations (Ondračková et al., 2021). As all three parasite species in this case were previously recorded in brown bullhead from Poland (Prost, 1973), the only other European population for which all three parasites have been reported together, it is quite likely that the more recent Ukrainian populations are derived from fish previously introduced to Poland. A Polish origin is more probable considering that both bullhead populations occur in the same watershed, i.e. the Vistula river basin. The Lake on Stryiska (Dniester river basin) is located in Lviv, which lies within both the Black Sea (Dniester river watershed) and Baltic Sea (Vistula river watershed) river basins (Koinova & Chorna, 2019). Thus, the presence of bullhead at sites inside Lviv is most likely the result of translocation due to human activity (Kutsokon et al., 2018), while differences in fish parasite fauna in lakes inside Lviv most likely reflect temporary local conditions than any difference in river drainage (Kvach et al., 2022). This is also reflected in the brown bullhead’s parasite fauna, which was similar across the entire western Ukrainian region.