Detecting an Alien Reptile through Parasitological Analysis of the Widely Distributed Alien Fish Perccottus glenii

The current geographical distribution of alien terrapins is poorly understood because these reptiles are difficult to detect in large natural water bodies, as they don't vocalize, have cryptic coloration, and are typically not numerous in temperate climate. The present study tests the previously unconfirmed oral reports suggesting the presence of terrapins in several urban water bodies in the European part of Russia, which is outside the geographical ranges of native European terrapins. We assessed the presence of the terrapins indirectly, by using a parasitological analysis of a widespread alien fish Perccottus glenii which is numerous in shallow lentic water bodies. This fish can harbour diverse parasitic species. We detected the alien nematode Spiroxys sp. in the wall of the digestive tract of the studied fish. The life cycle of this nematode includes freshwater terrapins as the obligatory definitive host. Thus, detecting Spiroxys sp. in P. glenii indicates the presence of terrapins in three of the studied water bodies. This approach will be particularly valuable when the direct detection of alien terrapins is limited by the seasonality, weather, macrophyte densities, landscape features, or other factors. The discussed indirect method is an additional tool for confirming the presence of alien terrapins in water bodies in the European regions, where native terrapins are absent.