One of the major threats to biodiversity involves biological invasions with direct consequences on the stability of ecosystems. In this context, the role of parasites is not negligible as it may enhance the success of invaders. The red-eared slider, Trachemys scripta elegans, has been globally considered among the worst invasive species. Since its introduction through the pet trade, T. s. elegans is now widespread and represents a threat for indigenous species. Because T. s. elegans coexists with Emys orbicularis and Mauremys leprosa in Europe, it has been suggested it may compete with the native turtle species and transmit pathogens. We examined parasite transfer from American captive to the two native species that co-exist in artificial pools of a Turtle Farm in France. As model parasite species we used platyhelminth worms of the family Polystomatidae (Monogenea) because polystomes have been described from American turtles in their native range. Phylogenetic relationships among polystomes parasitizing chelonian host species that are geographically widespread show patterns of diversification more complex than expected. Using DNA barcoding to identify species from adult and/or polystome eggs, several cases of host switching from exotic to indigenous individuals were illustrated, corroborating that parasite transmission is important when considering the pet trade and in reintroduction programmes to reinforce wild populations of indigenous species.

The reproductive biology and ecology of the European pond turtle Emys orbicularis is little known, and available information was previously mainly anecdotal. Morphological data on E. orbicularis are now well documented in most of the known distribution area. Body size in northern populations tends to be larger than in southern populations. There are also indications that clutch size may be larger in northern than in southern European pond turtle populations. Clutch size and reproductive status of a central Italy population were detected by radiography and palpation, respectively, and comparative data were obtained by direct count of intact nest eggs. Reproductive females averaged 13.9 cm and rarely exceeded 15.5 cm carapace length. They produced a clutch size of six, the same number found in intact nests. Nest egg-size averaged 32 mm in length. Egg number was positively associated with female body size, and especially with carapace height.