Acta Herpetologica 11(2): 101-109, 2016 ISSN 1827-9635 (print) © Firenze University Press ISSN 1827-9643 (online) www.fupress.com/ah DOI: 10.13128/Acta_Herpetol-18261 Predator-prey interactions between a recent invader, the Chinese sleeper (Perccottus glenii) and the European pond turtle (Emys orbicularis): a case study from Lithuania Vytautas Rakauskas1,*, Rūta Masiulytė1, Alma Pikūnienė2 1 Life Sciences Centre, Vilnius University, Saulėtekio Ave. 7, LT-10221 Vilnius, Lithuania. *Corresponding author. E-mail: vytucio@ gmail.com 2 Lithuanian Zoo, Radvilėnų str. 21, LT-50299 Kaunas, Lithuania Submitted on May, 5th 2016; revised on June, 11th 2016; accepted on June, 12th 2016 Editor: Uwe Fritz Abstract. The European pond turtle, Emys orbicularis, is a critically endangered species in most European countries. Habitat degradation and fragmentation are considered the main reasons for the decline of E. orbicularis. However, the spread of invasive species may also contribute to the disappearance of E. orbicularis populations. We examined the range overlap and predator-prey interactions between the invasive Chinese sleeper, Perccottus glenii, and E. orbicularis through controlled experiments and in field studies. Field surveys showed that both species occupied similar habitats. Predator-prey experiments suggested that newly hatched turtles are resistant to P. glenii predation. Conversely, adults of E. orbicularis consumed juvenile P. glenii even when other food sources were available. Overall, these findings sug- gested that E. orbicularis is not among the potential prey organisms in the diet of the invasive P. glenii, and that this fish does not directly contribute to the decline of E. orbicularis in Europe. Keywords. Turtle, aquatic invasion, endangered species, inland waters, Lithuania. INTRODUCTION At present, there is no doubt that one of the main causes of the loss of biodiversity is the spread of intro- duced, invasive species. These species reduce local bio- diversity through both indirect competitive and direct predatory impacts on resident native populations. The spread of invasive species is now considered to be an international problem, and local governments are actively working to reduce their spread and impacts (EU Regu- lation No 1143/2014). However, aquatic invasive species spread is on-going, and its impacts on local communities cannot be predicted (Sundseth, 2014). Understanding the pattern of invasive species interactions with native spe- cies can help us to predict those effects and build better strategies for protection of endangered species. From a biodiversity management point of view, the impact of the invasive versatile predator Chinese sleeper, Perccottus gle- nii (Dybowski, 1877) on populations of the endangered native European pond turtle, Emys orbicularis (Linnaeus, 1758) is one such case in point. Perccottus glenii, a fish native to the Amur River basin, eastern Asia (Mori, 1936; Berg, 1949). This is one of the most widespread alien invasive freshwater fish in Eurasia (Reshetnikov, 2010; Reshetnikov and Ficetola, 2011). It is also a recent invader in the Baltic Sea region (Aleksejevs and Birzaks, 2011; Witkowski and Grabows- ka, 2012; Reshetnikov and Karyagina, 2015; Pupina et al., 2015; Rakauskas et al., 2016). In Europe P. glenii occurs mostly in water bodies that either have weak current or are stagnant, with well developed aquatic vegetation. Such habitats include river flood plains, littoral zones 102 Vytautas Rakauskas et alii of lakes, and swampy water bodies (Reshetnikov, 2010; Reshetnikov and Ficetola, 2011; Pupina et al., 2015). Shallow, well-vegetated and isolated lakes, ponds, drainage ditches and oxbows are important reserves of biological diversity for many groups of aquatic and semi-aquatic animals, including pond turtles. Colonisa- tion of such water bodies by P. glenii leads to a dramatic simplification of ecosystem taxonomic structure. P. gle- nii is a versatile predatory fish and represents a specific threat for macroinvertebrates and small fish and amphib- ians (Shlyapkin and Tikhonov, 2001; Reshetnikov, 2003; Reshetnikov, 2004; Koščo et al., 2008; Grabowska et al., 2009; Reshetnikov, 2013). Due to its generalist predatory habit, it has been assumed that P. glenii preys upon hatchling E. orbicula- ris turtles (Pupina et al., 2015). Adult turtles can deter predation by their size and bony shell, but hatchlings lack the adult’s size and shell strength. Populations of E. orbicularis hatchlings were assumed to be under P. gle- nii predation pressure until their first or second years. This paper addresses the predator-prey interactions between P. glenii and locally endangered E. orbicularis. Concerns about P. glenii are based on observations that this fish (1) inhabits similar habitats as E. orbicularis (Mitrus, 2004; Najbar, 2007; Briggs and Reshetylo, 2009; Reshetnikov, 2010; Reshetnikov and Ficetola, 2011); (2) is an opportunistic, competitive predator that rises to the top of the food chain in invaded ecosystems (Reshetnik- ov 2003; Reshetnikov, 2004; Reshetnikov, 2013); and (3) grows large enough to prey on newly hatched E. orbicu- laris. This scenario, however, has not been well substanti- ated for interactions in European waters. To fill this knowledge gap, we performed predator- prey experiments between P. glenii and two aquatic turtle species. In laboratory experiments we tested the capacity of P. glenii to consume newly hatched Chinese pond tur- tles, Mauremys reevesii (Gray, 1831), when it is the only available prey. It was assumed that there would be no dif- ferences in P. glenii feeding preferences for same-sized E. orbicularis and M. reevesii juveniles. Additionally, we tested the capacity of the adult E. orbicularis to consume 0+ age P. glenii when several prey sources were avail- able. As a complement to the laboratory studies, we ana- lyzed the overlap of recent distributions of P. glenii and E. orbicularis species in natural Lithuanian freshwaters. MATERIAL AND METHODS Study area Natural habitat overlap between P. glenii and E. orbicularis species was assessed for the inland waters of Lithuania. Lithua- nia stands within the Baltic Sea drainage basin, along the south- eastern shore of the Baltic Sea. The country has a territory of 64,800 km2, which is divided by seven main river basins (Kažys, 2013) (Fig. 1A). There are 2,850 lakes that have surface areas larger than 0.005 km2, and 3,150 smaller lakes, with a combined area of 913.6 km2. In addition, there are 1,132 reservoirs and more than 3,000 ponds in Lithuania (Kažys, 2013). Distribution of P. glenii and E. orbicularis The recent distribution of P. glenii and E. orbicularis in Lithuanian inland waters was calculated from widely published (Rakauskas et al., 2016) and specialty (Bastytė, 2015) publica- tions. Additional data on the presence of P. glenii within the local range of E. orbicularis was provided by our own (to be published) surveys. The results of our surveys include data from 32 lentic water bodies, each with a surface area smaller than 0.3 km2 and each known to have been occupied by E. orbicularis (Fig. 1B). P. gle- nii populations were investigated by dip net (25 × 25 cm open- ing, 1.5 mm mesh size) sampling in water depths of 0.3-1.3 m in areas of the vegetated littoral zone (Pupina et al., 2015). From five to ten study sites were examined on each water body wav- ing with a dip net for ten minutes at each study site. In gen- eral, if P. glenii was to be found in a body of water, it was caught within the first 5 minutes (Rakauskas pers. obs.). Experiment 1 (P. glenii vs. M. reevesii) Predator-prey experiments (P. glenii vs. M. reevesii) were conducted to determine if the invasive predator P. glenii recog- nize newly hatched aquatic turtles as potential prey, and if they consume them under experimental conditions. M. reevesii was chosen as a model for E. orbicularis as it is easy available, not protected, and is of similar size and coloration as E. orbicularis (Mitrus and Zemanek, 2000; Stephens and Wiens, 2003; Lovich et al., 2011; Lin et al., 2015). Previous studies have shown that E. orbicularis juveniles are similar length (approximate 26 mm) (Drobenkov, 2000; Mitrus and Zemanek, 2000; Zinenko, 2004; Najbar and Mitrus, 2013) when they first reach the water, as were the M. reevesii juveniles used in these experiments (3.0 ± 0.1 mm). E. orbicularis is less available and is protected by European and local laws (Council Directive 92/43/EEC; Rašamavičius, 2007). Newly hatched (two week old) M. reevesii were transport- ed to the laboratory one week before the experiments from the local zoo-shop. The experiment turtles were maintained in twelve 10.6-L aquaria (22.5 cm deep and long × 21 cm wide) filled with tap water forming a closed circulation system with an ammonia filter (38.5 cm deep and wide × 49.0 cm long aquarium filled with expanded clay and JBL filter start (GmbH & Co, Germany)). During this period, daily rations of frozen midge larvae were provided. Adults of P. glenii were collected from small lakes using electric fishing gear (Samus-725mp) in September 2015. For one week before conducting the experiments, all fish were accli- 103Predator-prey interactions of Perccottus glenii and Emys orbicularis mated separately in twelve 92.4-L rectangular tanks (49.0 cm deep and long × 38.5 cm wide) filled with tap water forming a closed circulation system with an ammonia filter. During this period, daily rations of live Rutilus rutilus (Linnaeus, 1758) fry were provided at ~4% of the fish’s body weight. The fish were starved for 48 h before the predation experiment. In total, 12 P. glenii and M. reevesii individuals were used in these experi- ments (Tables 1 and 2). Predator-prey experiments were performed in the same experimental aquaria where fish were acclimatised. Each experi- mental aquarium was filled with tap water to 44 cm depth resulting in an actual water volume of 83 L per aquarium, with water flow through each experimental aquarium of 3.0 L min-1. Animals were kept under a 15 hours per day photoperiod and at a temperature of 19 ºC throughout these studies. No sub- strate was added into the experimental aquaria. After accli- mation period, a single individual of M. reevesii was added to each aquarium. During the experiments, turtles were fed frozen midge larvae every second day. Overall, the fish were allowed to forage for one week, after which turtles were removed back to their acclimation aquaria and the conditions of all turtles in each experimental aquarium were recorded. Assessment of turtle condition was based on appearance and movement. Sur- viving turtles were crawling and no injuries were seen on their skin or carapace. These turtles were kept for three months after the experiments to confirm that they retained their normal via- bility and activity levels. Experiment 2 (E. orbicularis vs. P. glenii) Predation experiments (E. orbicularis vs. P. glenii) were conducted to determine if E. orbicularis recognize P. glenii juve- niles as prey and consumed them. Approximately 150 P. glenii juveniles were collected from a small lake using a standard dip net (25 × 25 cm) in August 2015. All fish were small enough to be preyed upon by turtles and were selected for size simi- larity (body length of 3.3 ± 0.2 cm) to avoid cannibalism. For one week before conducting the experiments, all specimens were acclimated in six 92.4-L rectangular tanks (49.0 cm deep and long × 38.5 cm wide) filled with tap water forming a closed circulation system with an ammonia filter. Daily rations of live midge larvae were provided. Predator-prey experiments (E. orbicularis vs. P. glenii) were performed in August 2015. Three concrete water reservoirs were used for the experiments. Reservoirs were in cages under the natural outdoor conditions. Cages were made from stain- less steel to protect the experiment from wild birds and animals. The first cage contained two identical reservoirs of approximate 5 m2 area (2.8 m long × 1.8 m wide, with a range of 5−45º slope and 0.1−0.6 m of water depth). The second cage contained one large rectangular reservoir of approximate 10 m2 area (4.5 m long × 2.3 m wide, with a range of 5-45º slope and 0.1-0.7 m of water depth). All reservoirs were filled with tap water. Woody shelters, stones and muddy gravel substrate were present in all experimental reservoirs. An average noonday temperature of Table 1. Percottus glenii specimens used in predator-prey experiments. Values are means ± standard deviation. Experiment I Experiment II first cage Experiment II second cage Number of used specimens 12 27 33 Range of total body length, cm 17.0-24.7 2.9-3.2 3.3-3.7 Average of total body length, cm 20.3 ± 3.0 3.0 ± 0.2 3.5 ± 0.1 Range of total weight, g 90.3-280.0 0.9-1.5 1.1-1.8 Average of total weight, g 150.8 ± 62.1 1.2 ± 0.2 1.4 ± 0.2 Range of P. glenii mouth diameter, cm 3.0-4.8 – – Average of P. glenii mouth diameter, cm 3.7 ± 0.6 – – Table 2. Mauremys reevesii and Emys orbicularis specimens used in predator-prey experiments. Values are means ± standard deviation. Experiment I M. reevesii Experiment II (1) E. orbicularis Experiment II (2) E. orbicularis Number of specimens 12 10 8 Range of carapace length, cm 2.9-3.1 9.5-12 15.2-18.1 Average of carapace length, cm 3.0 ± 0.1 10.8 ± 0.8 16.9 ± 1.1 Range of carapace width, cm 2.3-2.0 7.6-10.1 13.6-16.0 Average of carapace width, cm 2.2 ± 0.1 8.7 ± 0.7 14.8 ± 0.9 Range of plastron length, cm 2.7-2.9 7.8-11.5 12.0-18.5 Average of plastron length, cm 2.8 ± 0.1 9.8 ± 1.6 15.5 ± 2.5 Range of total weight, g 6.8-7.2 10.6-37.1 39.0-167.0 Average of total weight, g 7.0 ± 0.1 22.2 ± 9.8 89.5 ± 42.6 104 Vytautas Rakauskas et alii 20.6 ± 3.2 ºC was observed. 18 E. orbicularis adults big enough to prey on these juveniles were divided, with ten smaller indi- viduals closed in the first experimental cage and eight larger ones in the second (Table 2). After an acclimation period, 27 and 33 fish were transferred into the experimental reservoirs of the first and second cages respectively. In the first cage released individuals were equally divided (14 + 13) per each reservoir. Fish in the second cage were slightly bigger compare to the fish in the first one (Table 1). 60 fish were left as a control group in six acclimation tanks (ten individuals per tank) for the whole experiment period. Overall, E. orbicularis were kept with P. glenii juveniles for two weeks. The turtles and the fish were fed once a day with dried or live insects, earth worms, snails, and small pieces of meat during the experiments. Daily turtle food ratios are presented in Table 3. Moreover, there was lots of naturally breeding Culex pipi- ens (Linnaeus, 1758) larvae inside the reservoirs which were an alternative food source for the fish. The reservoirs were surveyed for dead fish every day during the experiments. After the experi- ment termination, reservoirs were pumped out, all remained fish were removed back to their acclimation aquaria and the condi- tion of all fish in each reservoir was recorded. Assessment of fish condition was based on appearance and movement. Surviving fish were swimming and no external injuries were seen. RESULTS Distribution of P. glenii and E. orbicularis In Lithuania less than 400 E. orbicularis survive, mostly in waters of southern part of the country (Bastytė, 2015). Meanwhile P. glenii has been recorded in all of the country’s river basins (Rakauskas et al., 2016). Analysis of the recent distribution range of P. glenii and E. orbicularis in the inland waters showed these species overlapping on a regional scale. Both species were recently reported from the Nemunas River basin in southern Lithuania (Fig. 1A). Additionally, our surveys of P. glenii presence in well known E. orbicularis habitats revealed the small-scale overlap of these two species. Both were found inhabiting one shallow water body (Fig. 1B). Four P. glenii individu- als of body lengths ranging from 43 to 61 mm were cap- tured in a water body inhabited by E. orbicularis. Seven water bodies with well-known E. orbicularis populations were occupied by Carassius carassius (Linnaeus, 1758). Experiment 1 (P. glenii vs. M. reevesii) Experiments showed that none of the tested P. glenii adults were capable of ingesting or even injuring newly hatched M. reevesii turtles, though tested fish appeared big enough to do so. The mouth diameter of tested fish was significantly larger than the carapace lengths of the turtles (Mann-Whitney U test: Z = 3.2, P < 0.002; Tables 1 and 2). However, only a few predation signs were observed during the first ten minutes after newly hatched turtles were offered for P. glenii predation. The largest tested fish specimens (TL > 22.0 cm and mouth diame- ter > 4.0 cm) repeatedly attacked turtles for the first few minutes. In these cases the prey was completely sucked in the fish mouth cavity and jaws were fully closed. Howev- er, after a few seconds the prey was expelled undamaged. Overall, none of the tested turtles were injured; all were alive and healthy three months after the experiments. Experiment 2 (E. orbicularis vs. P. glenii) Adult E. orbicularis were found to consume P. gle- nii juveniles in caged experiments. 15 individual P. glenii (55.6% of the cohort) were missing after 14 days in the first cage where smaller turtles were foraging. Similarly, 13 P. glenii (39.4%) were missing in a cage with larger turtles. No dead or injured fishes were found during or after the experiment. Predatory behaviour was frequently observed. Turtles stalked and struck at P. glenii juveniles. Cannibalism was not observed among the fish, including in control tanks, where all 60 specimens left in tanks sur- vived the experimental period. DISCUSSION The European pond turtle, E. orbicularis is the most widely distributed freshwater turtle species in Europe (Fritz, 2001, 2003). The geographic range of E. orbicularis extends from North Africa over most of Europe to Latvia and to Lake Aral in the Middle East (Fritz, 1998; Fritz, 2001, 2003). However, the turtle is critically endangered in Lithuanian, and all other European waters (Council Directive 92/43/EEC; Fritz, 2001, 2003; Rašamavičius, Table 3. Daily turtle diet rations of various food types per speci- mens during the experiments. Food type quantity Flesh fish 10 (g) Flesh meat 10 (g) Dried crustacean 5 (ind.) Beetle larvae (live) 5 (ind.) insects (cricket, cockroach) (live) 3 – 4 (ind.) Earthworm (live) 5 – 10 (ind.) Snail (live) 3 – 4 (ind.) Midge larvae (live) 10 (ind.) Cabbage 1 (g) 105Predator-prey interactions of Perccottus glenii and Emys orbicularis 2007). In Lithuania reproductive E. orbicularis popula- tions number less than 400 turtles and are found mostly in waters of the southern part of a country (Fritz and Günther, 1996; Meeske, 2008; Bastytė, 2015). Habitat degradation and fragmentation have been identified as the main reasons for the decline of E. orbicu- laris populations (Kovacs et al., 2004; Fritz and Chiari, 2013; Bastytė, 2015). However, the recent spread of inva- sive aquatic species may also negatively affect E. orbicu- laris populations. The introduction of exotic Trachemys scripta (Schoepff, 1792) has been reported to have indi- rect effects on E. orbicularis populations. Introduced T. scripta competes against E. orbicularis for habitat resources, and reduces the survival of the turtle (Cadi and Joly, 2004; Lacomba and Sancho, 2004). The invasive predatory fish Micropterus salmoides (Lacépède, 1802), is assumed to prey on E. orbicularis hatchlings and juveniles (Lacomba and Sancho, 2004; Ayres and Cordero, 2007). Correspondingly, direct and/ or indirect interactions between invasive predatory fish P. glenii and E. orbicula- ris could be expected. Both species prefer similar habitats and diets. However, the impact of P. glenii on the natural populations of E. orbicularis in inland European waters remains largely undocumented. Habitat overlap Our review of the literature on P. glenii and E. orbicu- laris habitat preferences indicated that both species are commonly found in shallow waters with weak or with no current, and with well-developed aquatic vegetation (Mitrus, 2004; Meeske et al. 2006; Najbar, 2007; Briggs and Reshetylo, 2009; Reshetnikov, 2010; Reshetnikov and Ficetola, 2011; Pupina et al., 2015). Both species usually forage in littoral zones of small shallow lakes or swampy water bodies. Both species are particularly abundant in small water bodies unsuitable for most ichthyopha- gous fishes. In these situations, they are the top preda- tors (Meeske et al., 2006; Briggs and Reshetylo, 2009; Bastytė, 2015; Reshetnikov, 2003; Grabowska et al., 2009). Therefore these two species could be expected to com- pete against, and even attack each other in these environ- ments. Our field surveys confirmed that P. glenii and E. orbicularis may settle in the same water bodies, although both species are relatively rare in the inland waters of Lithuania. The presence of both species in the same water body was also reported from Latvian inland waters (Pupina et al., 2015). However, we found P. glenii only in one (3.1%) from all investigated water bodies set- tled by E. orbicularis. An increase in conflicts between these two species is expected in a future where both species expand their ranges within the inland waters of Lithuania. The rapid natural spread of P. glenii is ongoing not only in Lithu- anian (Rakauskas et al. 2016) but also in other European waters (Alexandrov et al., 2007; Grabowska et al., 2010; Mastitsky et al., 2010; Wolter and Röhr, 2010; Reshet- nikov and Ficetola, 2011; Semenchenko et al., 2011; Movchan 2015). The fish pursues a profligate reproduc- tive strategy, and exhibits resistance to adverse conditions Fig. 1. (A) Distribution of the European pond turtle, Emys orbicularis (squares) and the Chinese sleeper, Perccottus glenii (triangles) in the inland waters of Lithuania (followed by Bastytė, 2015; Rakauskas et al., 2016). Different river basins are marked by different shading. (B) P. glenii presence in water bodies inhabited by E. orbicularis: presence (opened circles), absence (closed circles). 106 Vytautas Rakauskas et alii (Reshetnikov, 2004). On the other hand, an intensive reestablishment program of E. orbicularis populations is ongoing in Lithuanian waters. Up to 100 individu- als are released annually to areas they formerly occupied (Pikūnienė unpub. data). Therefore, there is little doubt that direct and indirect interactions between these two species will increase. Predator-prey interactions In small lakes unsuitable for most ichthyopha- gous fish, P. glenii can grow up to 25 cm in body length (Reshetnikov, 2003; Pupina et al., 2015) and become a top predator (Reshetnikov, 2003; Koščo et al., 2008; Grabowska et al., 2009). In such waters almost all links of the trophic network converge to P. glenii. Although adult P. glenii feed primarily on fish and large insects, they occasionally prey on newts, frogs or other larger ani- mals (Shlyapkin and Tikhonov, 2001; Reshetnikov, 2003; Reshetnikov, 2004; Koščo et al., 2008; Grabowska et al., 2009; Reshetnikov, 2013). Due to this dietary breadth, it has been assumed that large P. glenii individuals are capa- ble of ingesting and eating E. orbicularis hatchlings (Pupi- na et al., 2015). Newly hatched pond turtles are less than 3 cm in length (Najbar and Mitrus, 2013) and could be attacked by P. glenii, which has a wide gape (up to 5 cm) and forages in shallow waters. However, our experiments did not support the hypothesis that P. glenii may directly, through the pred- ator-prey interaction, threaten E. orbicularis populations. None of the tested specimens of P. glenii consumed or injured newly hatched turtles, even though they were big enough to prey on them and even though no other food was available to them. Thus it was concluded that the presence of P. glenii would not directly threaten wild E. orbicularis populations. We used a different pond turtle species M. reevesii instead of E. orbicularis during the experiments. The applicability of our conclusions to E. orbicularis is sup- ported by the similarity of the behaviour of the two turtle species in the water and the similarities in their sizes and their camouflage (Mitrus and Zemanek, 2000; Stephens and Wiens, 2003; Lovich et al., 2011; Lin et al., 2015). Our results showed that only the largest P. glenii specimens showed interest in preying on turtle juveniles. P. glenii specimens shorter than 22 cm length did not even approach newly hatched turtles. We used P. glenii specimens in a range of sizes, up to the largest found in European waters, 24.7 cm of a total body length (the big- gest recorded specimens have been 25 cm in total body length; Reshetnikov, 2003; Pupina et al., 2015). Specimens over 20 cm were found only in 11 (14.6%) of a total 75 lentic waters bodies inhabited by P. glenii in Lithuania (Rakauskas unpub. data). Similar results were obtained from other countries where P. glenii was usually reported to live for about 5-7 years and reach up to 15 cm length (Grabowska et al., 2011; Nowak et al., 2008; Grabowska et al., 2009; Terlecki and Palka, 2012; Kutsokon et al., 2014). Our findings suggest that E. orbicularis consump- tion by predatory P. glenii in natural environments where many of other food sources are available is not likely. Similar results were obtained with other pond turtle and predatory fish species. Britson and Gutzke (1993) revealed that although turtle hatchlings of T. scripta and Chrysemys picta (Schneider, 1783) were attacked by pred- atory fish M. salmoides they were subsequently rejected unharmed. It was concluded that hatchling behaviour such as clawing or biting may be harmful to the gill appa- ratus or digestive tract of fish and thus provides defence against predation (Britson and Gutzke, 1993). Our experiments revealed that adults of E. orbicularis preyed on P. glenii juveniles even when other food sourc- es were available. Fish fry consumption by E. orbicu- laris has been shown in other studies (Kotenko, 2000; Briggs and Reshetylo, 2009). No cannibalism cases were observed within a control fish group during our experi- ments, suggesting that the fish were consumed by the tur- tles. Our findings suggest that not only will P. glenii not directly endanger European pond turtle populations, but that E. orbicularis may even control P. glenii populations where their ranges overlap. Concluding remarks The decline in European pond turtle populations likely results from a complex set of factors, linked to the modern decline in biodiversity worldwide. However, this study demonstrates that one factor is probably not the threat posed to these turtles by the invasive predatory fish P. glenii. Conversely, we found that mature adults of E. orbicularis can prey on P. glenii juveniles. The turtle could possible come to control the invasive fish with the increase of the habitat overlap between the two species. P. glenii may impact local E. orbicularis populations indirectly, through depletion of available macroinverte- brate food sources in invaded water bodies (Reshetnikov, 2001; Reshetnikov, 2003). Benthic invertebrates usually dominate in both species diet (Lebboroni and Chelazzi, 1991; Kotenko, 2000; Ottonello et al., 2005; Ficetola and De Bernardi, 2006; Koščo et al., 2008; Grabowska et al., 2009). P. glenii may also serve as a vector for E. orbicu- laris parasite transfer (Pupina et al., 2015). The fish is a host for the E. orbicularis parasitic nematode Spiroxys 107Predator-prey interactions of Perccottus glenii and Emys orbicularis contortus (Rudolphi), whose paratenic hosts also include small fish, insect larvae, tadpoles and adult Anura (Hedrick, 1935, Moravec, 1994). Further investigation of the growth and physiology conditions of E. orbicula- ris during the pre- and post-invasion periods of P. glenii in various water bodies will be needed to sort out these multiple effects. ACKNOWLEDGEMENTS We thank the Environmental Protection Agency of Lithuania, under the Ministry of Environment of the Republic of Lithuania, for the permits used to collect, breed and grow E. orbicularis hatchlings and adults at the Lithuanian Zoo: permits No. (6)-A4-2084; LGF-14- 40; 032/2014. We also thank the Lithuanian Zoo for the permit to conduct predator-prey experiments within the Zoo: permit No. LZS-96/2015. The zoo shop “Zuvytes. com” is greatly appreciated for providing us with newly hatched M. reevesii turtles. For the permission to catch P. glenii we are also grateful to the Environmental Pro- tection Agency under the Ministry of Environment of the Republic of Lithuania, permit No. 056/2015. 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