Population structure and parasite fauna of stone moroko, Pseudorasbora parva (Temminck et Schlegel, 1846) in a watercourse of the Oder catchment area (‘Central Plains’ European Ecoregion)

Abstract

The stone moroko (Pseudorasbora parva) is an invasive species which spreads rapidly in European water bodies and occupies free ecological niches. In the years 2019–2020, monthly analyses of its population structure were carried out in the Wardynka River (western Poland), determining the age, size (TL and SL), body weight, growth, Fulton’s condition factor, and the parameters of the length–weight relationship. In addition, quantitative descriptions of the parasite communities were made. End-point PCR was used to verify the presence of Sphaerothecum destruens in the fish. The age structure of the population was dominated by fish at the age of 1+ to 3+, and the maximum age was 5 + . The TL of the fish ranged from 2.50 to 10.60 cm (average 6.24 cm), SL from 2.10 to 9.50 cm (average 5.20 cm), and body weight from 0.15 to 11.43 g (average 2.74 g). The average body weight and length of males were higher than in females. The average Fulton’s condition factor for the whole sample was 1.44 ± 0.24 and was similar for both sexes. The slope of the regression line (b > 3) indicates an allometric relationship between the length and weight of fish of both sexes. Back-calculated estimates of standard length fitted the von Bertalanffy growth function, although Taylor’s criterion showed that the asymptotic length (Linf) was overestimated. Comparison of the von Bertalanffy growth function parameters revealed differences between sexes. The presence of parasites not previously recorded in stone moroko in Poland was confirmed: Dactylogyrus squameus, Phyllodistomum elongatum, P. folium, Posthodiplostomum cuticola (metacercaria), and Bivalvia gen. sp. (glochidia). Electrophoresis of the end-point PCR product did not reveal any signs of amplification for either of the primer sets used. S. destruens was not detected in any of the analysed samples of stone moroko.

Keywords:

• Topmouth gudgeon
• invasive species
• sex
• length
• age structure
• Fulton’s condition factor
• parasite fauna
• small river

Introduction

Invasion by expansive alien species is one of the most important threats to biological diversity, due to the unpredictable effects of the appearance of a new species in the environment (IUCN 2000). Newly introduced (alien) species can become invasive and pose serious economic and ecological threats (Pimentel et al. 2000; Gozlan et al. 2005 2010b; Rybczyk et al. 2020), causing changes in the habitats of native species (Rosecchi et al. 1993; Czerniejewski et al. 2020). Moreover, by competing with indigenous species they can affect density and biodiversity within the ecosystem (Pimentel et al. 2005). Pimentel et al. (2000) estimated that the total losses in the global economy resulting from the presence of alien species amount to about 5% of annual production. According to Gozlan et al. (2010a), as many as 625 species of freshwater fish have been introduced to water bodies outside their natural range. One way to fight invasive species is to eliminate them from the aquatic environment and prevent their return (Britton et al. 2008). However, in water bodies in which they cannot be eliminated, control and containment methods can be used to minimize their negative impact and further spread (Sorensen & Stacey 2004). Therefore it is essential to monitor the populations of these species and their habitats (Britton & Brazier 2006; Gozlan et al. 2010a).

The stone moroko Pseudorasbora parva (Temminck & Schlegel 1846) is one of the smallest invasive species in European water bodies. This fish has been recorded in standing water bodies, i.e. lakes, reservoirs, and fish ponds (Kapusta et al. 2008; Benzer & Benzer 2020; Piria et al. 2020; Davies & Britton 2021), as well as in watercourses (Carosi et al. 2016; Baek et al. 2022). Its body size makes its populations difficult to locate (Rosecchi et al. 1993; Gozlan et al. 2010b). It was introduced to Europe unintentionally together with stocking material for herbivorous fish species (grass carp Ctenopharyngodon idella, bighead carp Hypophthalmichthys nobilis, and silver carp Hypophthalmichthys molitrix). It was recorded for the first time in Europe in 1961, in southern Romania (in the Nucet fish farm in the Dambovita basin) and in Albania (Knezevič 1981; Bănărescu 1999), from which it spread to nearly all of Europe (Gozlan et al. 2010b). In Poland it was recorded for the first time in 1990, in fish ponds in the Barycz Valley near Milicz (Witkowski 1991), from which it spread together with stocking material not only to other fish ponds but also to natural surface waters (Witkowski 2002; Witkowski & Wiśniewolski 2005). The species is currently widespread in Poland, mainly in central and lowland regions. A few sites of this species are also known in northern Poland, where it colonizes fish ponds, small water bodies, lakes, and rivers, including small watercourses and drainage ditches (Czerniejewski et al. 2019a). Due to its very rapid spread, its negative impact on native hydrobionts, and the spread of infectious disease (Sphaerothecum destruens Arkush, Mendoza, Adkison & Hedrick, 2003 (Protozoa: Ichthyosporea), it is considered an “international pest species” in Europe (Welcomme 1992; Rosecchi et al. 1993; Gozlan et al. 2005; Pinder et al. 2005).

The state of infection of stone moroko by parasites and the pathogenic species Sphaerothecum destruens in Europe is not fully known. There have been reports of the occurrence of S. destruens in stone moroko in France (Charrier et al. 2016) and the Netherlands (Spikmans et al. 2020). As a host of S. destruens, stone moroko poses a potential threat to indigenous species of fish, both wild and farmed (Gozlan et al. 2005; Sana et al. 2018, 2020; Spikmans et al. 2020). Stone moroko populations in European countries are infected by parasites introduced from the natural sites of these fish, e.g. Dactylogyrus obscurus (Monogenea), as well as by parasites with a wide geographic range, acquired in ecosystems new to stone moroko, such as Trichodinella epizootica (Yuryshynets & Zaichenko 2015). Thus far eight parasite species have been recorded in stone moroko, four of which are new to this host: Trichodinella subtilis, Diplozoon paradoxum, Apatemon gracilis and Caryophyllaeus laticeps (Czerniejewski et al. 2019b). These species are recorded in Poland in indigenous fish species (Niewiadomska 2003; Pojmańska et al. 2007). Since the stone moroko appeared in Europe and Poland, it has played the role of a new, “additional” host in aquatic ecosystems, contributing to the development of populations of the above-mentioned parasite species. The occurrence of parasites may be associated with the growth and body condition of fish, which has been demonstrated in stone moroko in Poland (Czerniejewski et al. 2019b). Very little is known of the impact of parasites of stone moroko on its growth and body condition in European countries.

The aim of the study was to analyse and assess the population structure of stone moroko in the Wardynka River (Oder catchment area, north-western Poland) in an annual cycle, in terms of age, length, weight, Fulton’s condition factor, the length–weight relationship, growth, and infection by external and internal parasites. This study is part of a larger series on the ecology of P. parva populations in small rivers of the “Central Plains” European ecoregion, the results of which will be used to devise a management plan aimed at reducing the potential environmental damage caused by rapidly spreading populations of stone moroko.

Material and methods

Study site

The stone moroko population was monitored in the Wardynka River (Oder catchment area, north-western Poland, GPS: 53°08ʹ43.49”N, 15°34ʹ09.54”E) on the basis of monthly catches from May 2019 to October 2020 (Figure 1). The mean depth of the river at the site of the catches was 0.1–0.4 m. The bottom at this location was covered with a layer of stones (15%), gravel (45%), sand (35%) and a small amount of mud (5%), diversified by depressions providing hiding places for fish. The hydrochemical parameters of the water at this site are presented in Kirczuk et al. (2021). Fish were caught using an engine-powered electrofisher (Hans Grassl Gmbh IG, ELT60IIHI, Germany) on a 100 m stretch of the river, by wading across its entire width.

Figure 1. Location of the research site in the Wardynka River (GPS: 53°08ʹ47.0”N, 15°34ʹ00.8”E).

Length, weight and condition

A total of 683 stone moroko individuals were caught and transported to the laboratory, where they were measured (total length, TL, mm and standard length, SL, mm) to within 0.1 mm using an electronic calliper (115/B, Toolpack, Poland) and weighed to within 0.1 g using a digital balance (WLY 1/D, Radwag, Poland). Sex was determined on the basis of visual analysis of the gonads. The sex of the smallest fish, at the age of 0+, was not determined. Fulton’s condition factor was calculated according to the formula CF = 100*W/TL³ (Le Cren 1951), and parameters of the weight–length relationship (WLR) were determined. This relationship is calculated using the exponential equation W = a TL^b, in which both variables are log-transformed to linearize the regression (log W = a + b × log TL), where a is the intercept of the regression line with the y axis, and b is the slope of the regression line. The slope (b) of the WLR relationship indicates whether it is isometric (b = 3) or allometric (b ≠ 3) (Froese 2006).

Age and growth

The age structure of the fish was determined by collecting 10 scales between the dorsal and caudal fins from above the lateral line of the fish (Rosecchi et al. 1993). The scales were cleaned by the alkaline immersion method (Huang et al. 2015), and age was determined by counting the annuli. Age and yearly increases in length were read using a Nikon Eclipse E600 stereomicroscope with an HD video camera and Lucia image analysis software (Nikon). The growth of fish was described using the von Bertalanffy equation (VBGR) (Ogle & Isermann 2017):

$L_t=L_{\mathrm{i}\mathrm{n}\mathrm{f}}\left(1-e^{-K\left(t-t_0\right)}\right)$

where: L_t is the length (cm) at time t (age in years), L_inf is the standard length (cm) at time infinity (the predicted mean maximum length for the population), K is a growth constant which describes the rate at which L_inf is attained (cm, year ⁻¹), t is age (years), and t_o is the time at which length = 0. The parameters of the equation were calculated in the R programming environment with the FSA, nlstools, magrittr, dplyr, and nnnet packages (Ogle 2016). To assess the accuracy of the von Bertalanffy growth rate (VBGR) we used Taylor’s criterion (Taylor 1962), which states that the asymptotic length is satisfactorily estimated when the maximum observed length represents approximately 95% of L_inf. Because the parameters L_inf and k are inversely correlated (Moreau et al. 1985), the index of growth performance φ’ (Munro & Pauly 1983) was calculated as follows:

${\phi }^{\mathrm{\prime }}={log}_{10}(k)+2{log}_{10}(L_{inf})$

where: L_inf is the asymptotic standard length (in mm), and k is the rate at which the asymptotic length is approached.

Molecular detection of Sphaerothecum destruens

Samples of stone moroko (n = 58) were delivered to the laboratory and dissected, and pooled internal organs (kidney, liver, and spleen) were used for DNA extraction. The High Pure PCR Template Preparation Kit (Roche) was used to extract DNA, according to the manufacturer’s instructions. Spectrophotometric measurements using NanoDrop 2000 (Thermo Scientific) and agarose gel electrophoresis (1.5% agarose gel) were used to assess the quantity and quality of DNA extracts. DNA isolates were tested using end-point PCR to amplify the ITS1 and ITS2 or only ITS1 regions of Sphaerothecum destruens, as described by Gozlan et al. (2009). Briefly, amplifications were conducted on the T100™ Thermal Cycler (Bio-Rad, USA) using the GoTaq PCR kit (Promega) according to the manufacturer recommendations, under the following conditions: 1 step of 5 min at 94°C followed by 35 cycles at 94°C for 30s, 56°C for 45s, 72°C for 90s, and final extension at 72°C for 7 min. The PCR results were assessed by 2% agarose gel electrophoresis.

Parasite communities of stone moroko

Parasitological examination of 124 stone moroko individuals was carried out in accordance with recommendations given by Klimpel et al. (2019), Sepulveda and Kinsella (2013), Pojmańska and Cielecka (2001), Niewiadomska (2010), Dzika (2008), Grabda-Kazubska and Okulewicz (2005), Prost (1994), Moravec (1994), and Lonc and Złotorzycka (1994). The skin, fins, oral and nasal cavities, lens and vitreous body of the eye, gills, coelom, peritoneum, heart, swim bladder, gonads, intestine, gall bladder, urinary bladder and ureters, kidneys, and muscles were examined. Some of the fish were examined directly upon transport to the laboratory, while others (about 1/3) were frozen for further analysis. Parasites were fixed in 70–75% ethyl alcohol or identified to genus and species without fixation. Nematode (Nematoda) specimens were cleared in glycerine and Faure’s medium (Grabda-Kazubska & Okulewicz 2005), as were monogenetic trematodes (Monogenea) (Dzika 1987, 2008). Digenetic trematodes were stained with carmine alum, cleared in clove oil, and mounted in Canada balsam (Niewiadomska 2003). The presence of glochidia was noted in fresh specimens. Morphological examination of parasites was performed using the Zeiss StereoLumar v.12 stereoscopic microscope (5‒40×) and the ZEISS AxioScope.A1 biological light microscope (10‒400×) equipped with a Canon video camera and AxioVision Rel.4.8.2 image analysis software. Species identification was based on keys and original works (Bauer 1985, 1987; Moravec 1994; Kent et al. 2002; Niewiadomska 2003; Grabda-Kazubska & Okulewicz 2005; Niewiadomska & Pojmańska 2018).

Parasitological indices, i.e. prevalence, intensity of infection, and mean abundance, were calculated in accordance with terminology described by Bush et al. 1997. Parasitological indices were calculated from data obtained in spring (April–May), summer (June–August), and autumn (September–October). Parasitological examination was not performed on material obtained from November to March.

To characterize the parasite communities of stone moroko, i.e. the component community and infracommunity (terminology according to Bush et al. 1997), ecological indices were calculated for the entire study period: 1) the biodiversity index of the component community of parasites (Simpson’s diversity index), 2) the Berger–Parker dominance index defining the share of the dominant species in the component community, and the 3) Brillouin biodiversity index of infracommunities of parasites (Magurran 2004).

Ecological indices were calculated using Past v.2.11 software (Hammer et al. 2001).

Statistical analysis

Prior to the comparative statistical analyses, the normality of distributions of variables was tested using the Shapiro–Wilk test and homogeneity of variance by Levene’s test. The Mann–Whitney U test was used to compare data on the length (TL and SL), weight, and condition of fish (Sokal & Rohlf 2012). In addition, the significance of differences in numbers of males and females in each month (sex ratio) was estimated by the chi-square test with Yates’ correction.

Differences in the prevalence of parasites in different phenological seasons were compared using the chi-square test, and when the assumptions of the test were not met, Fisher’s exact test was applied. The non-parametric Mann–Whitney U test was used to compare differences in intensity of infection by parasites in phenological seasons. The relationship between the number of parasites and the length of fish (TL and SL) was examined by linear regression. A significance level of p ≤ 0.05 was adopted for the statistical analyses. Statistical computations were performed using the STATISTICA 12.0 PL software package (StatSoft, Polska).

Results

Sex structure

Among the fish caught (683 in total), a statistically significant disproportion was noted in the sex structure of the entire sample (χ² = 24.8066, p < 0.01) and for individual months (Table I). The proportion of males was statistically significantly higher in February, June, August, and December. The sex ratio for the entire sample of fish was 1:0.64.

Table I. Hydrochemical parameters of the Wardynka River at the site of the catches.

Month	Males (M)	Females (F)	Juveniles (age 0+)^1	Sex ratio (M:F)	χ^2	p^2
January	14	8	15	1:0.57	2.45	*
February	43	13	-	1:0.30	24.11	****
March	20	10	-	1:0.50	5.00	*
April	16	14	-	1:0.88	0.20	*
May	54	43	2	1:0.80	1.87	*
June	57	46	3	1:0.81	1.76	*
July	38	15	21	1:0.39	14.97	****
August	48	21	-	1:0.44	15.85	****
September	45	38	13	1:0.84	0.89	*
October	12	13	5	1:1.08	0.06	*
November	13	12	1	1:0.92	0.06	*
December	21	9	-	1:0.43	7.20	**

¹undetermined sex

²*ns, **p < 0.05, ***p < 0.01, ****p < 0.001

Length structure

The total length (TL) of the fish specimens ranged from 2.50 to 10.60 cm (average 6.24 cm), and the standard length (SL) ranged from 2.10 to 9.50 cm (average 5.20 cm). A statistically significant linear correlation was noted between TL and SL. The equation for the whole sample was TL = 0.1807 + 1.1645× SL (R² = 0.98744, p < 0.001), and the equations for individual sexes were TL_Male = 0.2292 + 1.1562× SL (R² = 0.98, p < 0.001) for males and TL_Female = 0.1854 + 1.1642× SL (R² = 0.98, p < 0.001) for females. In the case of juvenile fish (0+) of undetermined sex, the relationship is represented by the equation TL = 0.0214 + 1.1969× SL (R² = 0.99, p < 0.001).

Males had statistically significantly greater total length (Mann–Whitney U test, Z = −6.9513, p < 0.001), standard body length (Z = −6.9048, p < 0.001), and individual weight (Z = −6.4122, p < 0.001) than females (Table II). There were also significant differences in these parameters between sexes in individual months (Figure 2).

Table II. Mean (± SD) total length (TL), standard length (SL) (in cm), and body weight (in g) of male, female and juvenile stone moroko individuals in the Wardynka River.

Group	Total length (TL)	Standard length (SL)	Weight (g)
Females	5.71±1.57	4.75±1.33	2.02±1.84
Males	6.76±1.19	5.64±1.56	3.32±2.73
Juveniles*	5.01±2.07	4.17±1.73	1.88±2.56

*undetermined sex

Figure 2. Standard length (SL, cm) (A) and individual body weight (W, g) of stone moroko caught in individual months. Values marked with different letters (a, b) indicate statistically significant differences between females (○) and males (∆) in a given month (Mann–Whitney U test, p < 0.05); mean ± SD.

Statistically significant differences were also noted in individual months for the standard length (ANOVA, F = 27.0216, p < 0.001) and weight (ANOVA, F = 22.5189, p < 0.001) of the fish (Figure 2).

Age structure

Six age groups were noted (0+ to 5+) among the 683 fish caught, with clear dominance of individuals in age groups 1+ (45.83%) and 2+ (23.78%). Fish in these groups accounted for 76.05% of females and 68.67% of males (Figure 3).

Figure 3. Age structure of male and female stone moroko fish from the Wardynka River.

High monthly variation was observed in the percentages of individual age groups in the population structure (Figure 4), which influenced the average length and individual weight of the fish in different months of the year (Figure 2).

Figure 4. Percentage share of age groups in each month of the year in stone moroko from the Wardynka River.

Table III presents the growth parameters of stone moroko based on the von Bertalanffy equation. Males of the species attained greater lengths than females in individual years of the study, as well as greater asymptotic maximum length. Greater growth in males was confirmed by the index of growth performance φ. Back-calculated standard length showed a good fit to the von Bertalanffy growth model. According to the Taylor criterion, however, the von Bertalanffy growth function describes growth well if the maximum observed length is about 95% of L_inf. In both sexes the maximum observed lengths ranged from 89.85% to 92.23% of L_inf (Table III), which suggests that the asymptotic lengths determined by the von Bertalanffy model were slightly overestimated.

Table III. Estimation of von Bertalanffy growth function (VBGF) parameters for stone moroko in the Wardynka River.

Parameter	Total				Males				Females
	Mean	Std. error	T value	p	Mean	Std. error	T value	p	Mean	Std. error	T value	p
Linf K To	9.3872 0.3331 −0.5812	0.252 0.019 0.041	37.19 16.72 −14.00	<0.001 <0.001 <0.001	9.5254 0.3245 −0.5904	0.272 0.020 0.043	35.04 15.95 −13.63	<0.001 <0.001 <0.001	8.6948 0.3725 −0.5396	0.453 0.045 0.084	19.195 8.258 −6.416	<0.001 <0.001 <0.001
n φ max SLobs Lifespan Taylors (%)	683* 3.467 8.658 5 92.23				381 3.469 8.701 5 91.34				242 3.449 7.812 5 89.85

Body condition

The mean value of Fulton’s condition factor for the entire sample of fish was 1.44 ± 0.24. Females (CF_F = 1.44 ± 0.23) and males (CF_M = 1.43 ± 0.22) had similar body condition (Mann–Whitney U test, Z = 0.0872, p = 0.93). The body condition of juveniles was CF_J = 1.47 ± 0.34. No significant differences in the condition factor were found between males and females in individual months, except for February (Figure 5). It should be noted that the condition factors were highest (CF > 1.45) during the period from May to September. This was most likely because the water temperature was highest during this period, at > 16°C, close to the optimum temperature for the growth of the species.

Figure 5. Monthly fluctuations in the condition factor (CF) values of stone moroko from the Wardynka River. Values marked with different letters (a,b) indicate statistically significant differences in CF between females (○) and males (∆) in a given month (Mann–Whitney U test, p < 0.05); mean ± SD.

Analysis of correlations between the total length and individual weight of fish (Table IV) revealed that the growth of stone moroko was allometric (b > 3.0), with a statistically significant (p < 0.001) and high correlation (R > 0.97).

Table IV. Parameters of the length–weight relationship (LogW = a + b logTL) of stone moroko from the Wardynka River.

	a	b	R	p
Whole sample	−2.2679	3.2335	0.9800	< 0.001
Females	−2.2776	3.2547	0.9804	< 0.001
Males	−2.3127	3.2835	0.9802	< 0.001
Juveniles	−2.2377	3.2293	0.9798	< 0.001

The wide variation in the habitats of stone moroko populations contributes to disproportions in the size, growth, condition and population structures of the species, which suggests that it has a high capacity to adapt to environmental conditions.

Parasite fauna

The parasite community of stone moroko is represented by 9 taxa, including 5 identified to species. The parasites found were Chromista, Cilophora – 1 taxon determined to genus; Animalia: Platyhelminthes: Monogenea - 1 species, Trematoda – 3 species and 1 taxon identified as Subclass Digenea; Nematoda – 1 species and 1 taxon identified to family, and Mollusca – 1 taxon identified to family. Larvae of nematodes Anisakidae gen. sp., whose genus and species were not determined, were curled in a spiral, encysted and free (Table V).

Table V. Parasites of stone moroko from the Wardynka River (2019–2020).

	Spring (Apr–May) n = 33				Summer (Jun–Aug) n = 50				Autumn (Sep–Oct) n = 41
Parasite	N	P [%]	MI (R)	MA	N	P [%]	MI (R)	MA	N	P [%]	MI (R)	MA	Infection site
Trichodinella sp.	1	3.03	1.00 (-)	0.03	0	0.00	-	-	0	0.00	-	-	nasal cavity
Dactylogyrus squameus Gussev, 1955	1	3.03	1.00 (-)	0.03	2	4.00	1.00 (1–1)	0.04	4	9.76	2.25 (1–5)	0.22	gills
Phyllodistomum elongatum Nybelin, 1926	0	0.00	-	-	2	4.00	2.00 (1–3)	0.08	1	2.44	5.00 (-)	0.12	ureters
Phyllodistomum folium (Olfers, 1816)	1	3.03	2.00 (-)	0.06	0	0.00	-		0	0.00	-	-	ureters
Posthodiplostomum cuticola (von Nordmann, 1832), metacercaria	1	3.03	1.00 (-)	0.03	0	0.00	-	-	0	0.00	-	-	coelom
Digenea gen. sp., metacercaria	0	0.00	-	-	1	2.00	1.00 (-)	0.02	0	0.00	-	-	gills
Pseudocapillaria (Pseudocapillaria) tomentosa Dujardin, 1845) Moravec, 1987	12	36.36	1.83 (1–5)	0.67	16	32.00	3.25 (1–11)**	1.04	19	46.34	1.74 (1–4)**	0.80	intestine
Anisakidae gen. sp., larvae	1	3.03	30.00 (-)	0.91	0	0.00	-	-	3	7.32	4.00 (1–10)	0.29	intestinal wall
Unionidae gen. sp. (glochidia)	6	18.18*	2.00 (1–4)	0.36	13	26.00**	3.54 (1–12)	0.92	0	0.00*^,**	-	-	gills
Total:	21	63.64	3.29 (1–30)	2.09	29	58.00	3.62 (1–21)	2.10	23	56.10	2.57 (1–10)	1.44

N, number of infected stone moroko; n, number of examined stone moroko; P, prevalence; MI (R), mean intensity (range); MA, mean abundance *statistically significant differences between spring and autumn at p ≤ 0.05; ** statistically significant differences between summer and autumn at p ≤ 0.05

Prevalence for the entire study period was 58.87% (73 infected fish), while the average intensity of infection was 3, with a range of 1‒30 parasites per fish. Nematodes Pseudocapillaria (Pseudocapillaria) tomentosa had the highest prevalence and the highest intensity of infection (prevalence 37.90%, mean intensity of infection 2, range 1‒11). Prevalence of glochidia was lower (15.32%, mean intensity 3, range 1‒12). The lowest parameters of infection were found for Dactylogyrus squameus (prevalence 5.65%, mean intensity 2, range 1‒5), Phyllodistomum elongatum (prevalence 2.42%, mean intensity: 3, range 1‒5), and nematode larvae (Anisakidae gen. sp.) (prevalence 4%, mean intensity 11, range 1‒30). The remaining parasites, i.e. Trichodinella sp., Phyllodistomum folium and metacercariae of trematodes (Digenea gen. sp.), were noted in single fish, and the intensity of infection ranged from 1 to 2 parasites per fish. The most fish were infected in the spring, and the fewest in autumn (Table V).

The prevalence of glochidia differed significantly between the spring and autumn (chi-square test = 8.112, Fisher exact p = 0.006) and between the summer and autumn (chi-square test = 12.437, p = 0.0004). The difference in the intensity of infection of stone moroko by P. tomentosa between summer and autumn was statistically significant (Mann–Whitney U test, p = 0.035) (Table V).

The Simpson diversity index was 0.690, and the Berger–Parker dominance index was 0.459 and indicated the dominance of Pseudocapillaria (Pseudocapillaria) tomentosa in the parasite community of stone moroko. The average value of the Brillouin index was 0.0613 ± 0.152.

The number of parasite species and intensity of infection by parasites was shown to be weakly positively correlated with the body length of stone moroko. The correlation coefficient between the number of parasite species and standard length (SL) was 0.268, while the coefficient for the number of parasite species and total length (TL) was 0.237. The correlation coefficient for intensity of infection and standard length (SL) was 0.176, and the coefficient for intensity of infection and total length (TL) was 0.173.

End-point PCR showed no amplification signals for either assay used to detect S. destruens in any of the analysed samples of stone moroko.

Discussion

Stone moroko is an invasive species which is resistant to unfavourable environmental conditions and whose range is spreading in Europe. It is characterized by small body length, short longevity, early maturation, relatively low fecundity, multiple spawning and extended reproductive seasons (Gozlan et al. 2010a, 2010b; Kirczuk et al. 2021). These life-history traits, along with broad diet breath and environmental tolerance, appear to facilitate the invasion of freshwater fish in the Central European bioregion. The total length of the fish from the Wardynka River ranged from 2.50 to 10.60 cm (average 6.24 cm), and their body weight ranged from 0.15 to 11.43 g (average 2.74 g). The mean length, weight and growth parameter K were greater than those of some native and long-established populations. Alien species generally exhibit superior growth in water bodies to which they have been introduced than in their native water bodies (Grabowska et al. 2011). According to a study by Brandner et al. (2013) on gobies, specimens from recently colonized areas were on average larger and heavier and had higher body condition than their conspecifics from long-established areas, where intraspecific competition is stronger and thus food choice is more limited.

Stone moroko should be continuously monitored at every new site of its occurrence not only for density and biological traits, but for parasitological characteristics as well. This is because its presence enables the spread of native and newly introduced parasites for which it is a vector. In the population inhabiting the Wardynka River, whose biological and population characteristics are described in this paper, parasites were recorded that had not previously been found in stone moroko in Poland: Dactylogyrus squameus, Phyllodistomum elongatum, P. folium, Posthodiplostomum cuticola (metacercaria), and Unionidae gen. sp. (glochidia). The trematode D. squameus was recorded in fish in Poland for the first time. Although the PCR analysis did not reveal the presence of S. destruens, parasites of stone moroko can have a negative effect on the local biodiversity and species richness of fish. The helminth fauna of invasive fish species reported from new territories is generally less rich than in their native areas (Torchin et al. 2003). However, transmission of parasites from invading to native species can occur, aiding the invasion process (Ondračková et al. 2004; Torchin & Mitchell 2004).

In fresh water stone moroko prefers shallow habitats, rich in food, with substantial cover by aquatic plants (Benzer 2019). According to literature data on the types of water bodies in which stone moroko is found, the highest average lengths (SL) and individual weights of fish have been noted in lakes, rivers, and large canals, while much lower values for these parameters have been recorded in ponds and wetlands with a smaller area (Figure 6). A similar effect of water body size has been observed in other fish species (Pyrzanowski et al. 2020), but other authors have noted that the impact of the environmental conditions of the water body and the density of fish can be greater (Eckmann 2017; Teubner et al. 2019). The size (SL, W) of the stone moroko individuals from the Wardynka River was greater than in other non-native populations (Stavrescu-Bedivan et al. 2017; Piria et al. 2020; Rakauskas et al. 2021), but similar to that of some indigenous populations. For example, Piria et al. (2020) reported that in the Hong (Hong Hu) and Huangpi Lakes in China, the average length (SL) of this species was 4.82 cm and 5.28 cm, respectively, and the individual weight was 2.25 g and 3.25 g. It should be noted that among Chinese populations, fish from the Yiluo River and Wujiang River stand out in this respect, with an average length of 6.07 cm and 6.60 cm, respectively, and individual weight of 9.95 g and 9.00 g. These differences in average SL and W between the population from the Wardynka River and other water bodies may be due to different age structure. In the Wardynka, fish at the age of 1+ and 2+ were dominant, as in many European water bodies (Stavrescu-Bedivan et al. 2017; Rakauskas et al. 2021), while older age groups predominated in the Yiluo River and Wujiang River.

Figure 6. Mean results for standard length (SL) and weight (W) of stone moroko (topmouth gudgeon) inhabiting various types of inland water bodies according to literature data (Kapusta et al. 2008; Carosi et al. 2016; Benzer & Benzer 2020; Piria et al. 2020; Davies & Britton 2021; Baek et al. 2022).

Literature data on the growth of various stone moroko populations indicate that the growth rate of the species varies significantly within its geographic range, as indicated by the growth parameters defined by the von Bertalanffy equation (Table VI). High variability in the growth of fish is a common phenomenon in species with a wide range of occurrence (Villeneuve et al. 2005; Grabowska & Przybylski 2015). The growth of stone moroko may be influenced by the thermal conditions of the water (Gaspar et al. 1999; Ding et al. 2019) and also depends on density, access to food and its abundance (Britton et al. 2007, 2008).

Table VI. Comparison of growth (in mm) characters of stone moroko in native and non-native areas.

Region	Native/Non-native area	n	Sex	Max age	Growth parameters			References
					$L_{\mathrm{i}\mathrm{n}\mathrm{f}}$	K	to
Baiyangdian Lake	Native	51	♀♂	3+	64.82	1.142	0.250	Han and Li (1995)
Nanwan Lake	Native	532	♀ ♂	3+ 3+	107.01 145.25	0.246 0.181	−0.76 –0.66	Li et al. (2017)
Ushize River	Native	1927	♀♂	1+	49.71	0.97	−0.60	Onikura and Nakajima (2013)
“Top pond”, England	Non-native	163	♀ ♂	4+ 4+	70.50 72.70	0.51 0.77	-- --	Britton et al. (2007)
“Bottom pond”, England	Non-native	149	♀ ♂	2+ 2+	79.6 80.9	0.81 0.77	-- --
Sur pond	Non-native	143	♀ ♂	4+ 4+	59.35 49.82	0.210 0.212	−1.071 –1.646	Záhorská et al. (2010)
Chabalang Wetland	Native	256	♀ ♂	5+ 5+	112.19 123.12	0.179 0.150	−0.801 –0.713	Ding et al. (2019)
Mogan Lake	Non-native	-	♀ ♂	-- --	100.64 107.64	0.375 0.316	−0.57 –0.74	Benzer et al. (2016)
Wardynka River	Non-native	683	♀ ♂	5+ 5+	86.95 95.25	0.3725 0.3245	−0.5396 –0.5904	Our own data

The K parameter is responsible for the shape of the growth curve in fish defined by the von Bertalanffy equation (Pauly 1980). Analysis of the data in Table VI reveals that the highest values for the K parameter in stone moroko were obtained for short-lived populations with a lower L_inf value. For example, the value of this parameter was higher for stone moroko in the Ushize River (Japan) and in a pond in England, in which the age of fish was 1+ and 2+ (Britton et al. 2007; Onikura & Nakajima 2013), than in the population from the Chabalang Wetland and the Wardynka River, in which the maximum length was 5+ (Ding et al. 2019; our own analysis).

The species structure and quantitative structure of parasites of stone moroko analysed in 2019–2020 differs in part from the structure determined in 2015–2016 (Czerniejewski et al. 2019a), except for the presence of protists of the genus Trichodinella and the nematode P. tomentosa. The trematode D. squameus had not previously been recorded in fish in Poland. It was originally described by Gusev (1955), and was recorded for the first time in Central Europe in stone moroko in the Czech Republic and Slovak Republic by Ondračková et al. (2004). According to Ondračková et al. (2004), D. squameus was probably introduced by stone moroko to non-native water bodies during its expansion from native areas. Digenetic trematodes Phyllodistomum elongatum and P. folium were recorded in stone moroko for the first time in European water bodies. These trematodes have also not been recorded in stone moroko in their native water bodies. The hosts of P. elongatum are pea clams (Pisidium) or fingernail clams (Sphaerium), in which larval forms of these trematodes develop (Zhokhov 1987, 1991). The role of host for larval stages of P. folium can be played by zebra mussels Dreisena polymorpha (Peribáñez et al. 2011). The detection of only one metacercaria of P. cuticola and its atypical location in the body of the fish may indicate that stone moroko is an incidental host. The nematode larvae in the intestinal wall of stone moroko (Anisakidae gen. sp., larvae) were most similar in morphology and morphometry to third-stage larvae (L3) of Raphidascaris acus (Bloch, 1779). However, due to the absence of clearly visible morphological characters, it is not certain that the larvae belonged to this species. Several nematode species can be present in fish in larval form, located in the tissues of the internal organs, e.g. R. acus, Anguillicola crassus (Moravec 1994; Grabda-Kazubska & Okulewicz 2005), and Schulmanela petruschewskii (Schulman, 1948) (Moravec 1994). Unionidae gen. sp. (glochidia) have not previously been recorded in stone moroko in Poland. Stone moroko is described in the literature as a “poor host” for glochidia of the swan mussel Anodonta cygnea, in contrast with “good hosts” (e.g. Perca fluviatilis) (Huber & Geist 2017).

Invasive species of hosts, as a new element of the ecosystem, may be sensitive to parasites new to them, due to the lack of developed immune mechanisms against these parasites. The filter theory presented by Combes (1995), which talks about the encounter filter and the match filter, may be important. In a new environment, the spectrum of parasites that can colonize a new host may be narrower than that of parasites that infect native species of hosts. On the other hand, parasites of invasive species may die in a new environment if the conditions of development outside the host are not optimal for the survival of their dispersion forms or if no intermediate hosts are present. Another aspect – parasites of an invasive host species that are alien to new geographic areas can acclimate and become invasive parasite species for native hosts (Pojmańska et al. 2007).

One example illustrating the introduction of parasites is the results of a study of Monkey goby parasites (Neogobius fluviatilis) in the lower Volga basin, during which, out of 19 species of parasites, two were considered to be introduced from other geographic regions (Bothriocephalus opsariichthydis and Nicolla skrjabini). The parasitic infestation parameters of the non-native species of goby in this study area differed from those of the native species of gobies by having a much lower number of parasitic species but a higher level of infection than the native species (Kvach et al. 2015).

Sphaerothecum destruens has not been recorded in stone moroko from the Wardynka River. The absence of this pathogen may be explained by two main factors. First, S. destruens may not yet have been transferred to the Wardynka River system, although its main host, stone moroko, is present in numerous water bodies across Poland and Europe (Czerniejewski et al. 2019a; Spikmans et al. 2020). Secondly, few P. parva were infected with the rosette agent in the catchment area. A large-scale study showed similar results in watercourses in France. Additionally, low water temperatures may restrict the proliferation, transmission and thus detection of S. destruens in fish samples and therefore lead to underestimation of its prevalence (Marine et al. 2022). To our knowledge, this is the first attempt to identify S. destruens in Poland. Further genetic analysis is needed to assess its potential spread and virulence in other native and invasive fish populations, as the pathogen represents a major ecological, health and economic threat to aquatic biodiversity and fish conservation (Combe & Gozlan 2018).

Acknowledgements

The authors kindly thank Professor Ewa Dzika for identifying the parasite species Dactylogyrus squameus

Disclosure statement

No potential conflict of interest was reported by the author(s).