Ponto-Caspian gobies have a high invasion potential, which allow them to spread into areas distant from their native range. In recent years, they successfully established in the Baltic, Aegean and North Sea basins (Skóra and Stolarski 1993; Kakareko et al. 2009; Mierzejewska et al. 2011; Herlevi et al. 2017) and even in the North American Great Lakes (Corkum et al. 2004; Kornis et al. 2013). After the inauguration of the Maine-Danube canal in 1992 several Ponto-Caspian gobiids have already invaded the Rhine River system (Stemmer 2008; van Kessel et al. 2009; Borcherding et al. 2011; Kalchhauser et al. 2013), with the round goby Neogobius melanostomus and bighead goby (Ponticola kessleri) being the most abundant and widespread species among them (Kottelat and Freyhof 2007; Borcherding et al. 2011). Although some ecological parameters such as density, fecundity, growth, predation, and parasitism were already studied in gobies from several non-native regions of the Danube River and Rhine River (Kvach 2002; Jurajda et al. 2005; Adámek et al. 2007; L’avrinčíková and Kováč 2007; Kvach and Stepien 2008; Kováč et al. 2009; Ondračková et al. 2009; Mühlegger et al. 2010; Kalchhauser et al. 2013; David et al. 2018), their impact on species assemblages in the ecosystems throughout Europe and the River Rhine in particular still remains largely unknown. After their immigration, the free-living non-native species may harm endemic organisms and may change the species composition within ecosystems. Direct effects of introduced species on local fauna are often a result of passive or active competition for resources (space and/or food), predation, and/or hybridization with native species (Mooney and Cleland 2001; Dextrase and Mandrak 2006; Shochat et al. 2010). Ponto-Caspian gobies for example were found to negatively affect the population densities of some native fish species (Dubs and Corkum 1996; Mooney and Cleland 2001; Balshine et al. 2005; Karlson et al. 2007; Jakšić et al. 2016; van Kessel et al. 2016). Apart of this, invasive free-living species may also indirectly influence the biodiversity by changing the composition of the parasite communities in the new area (Torchin et al. 2003; Calhoun et al. 2018; Hohenadler et al. 2019), as parasites can reduce host density (Anderson and May 1978; May and Anderson 1978; Kuris and Lafferty 1992; Hudson et al. 1998) or decrease host body size (Torchin et al. 2001). They can for example co-introduce their own endemic parasites, which can “spill over” to native populations (Torchin et al. 2003; Prenter et al. 2004; Kelly et al. 2009; David et al. 2018; Hohenadler et al. 2018b). Hohenadler et al. (2018a) provided examples of how the acanthocephalan Pomphorhynchus laevis (which, according to a recent molecular characterization should now most likely be considered as Pomphorhynchus bosniacus, see Reier et al., 2019), which was introduced by Ponto-Caspian invaders can outcompete a local acanthocephalan species (Pomphorhynchus tereticollis) and thus change the species composition of the parasite communities in the Rhine system. As the evidence for the occurrence of P. bosniacus in Central Europe (Reier et al. 2019) was published after the study by Hohenadler et al. (2018a), it cannot be decided with certainty in retrospect whether they found P. laevis or P. bosniacus in the Rhine system although the latter seems to be more likely.
Additionally, invasive species can contribute to the life cycle of native parasites. If an invasive species can serve as a suitable (e.g. intermediate, paratenic or final) host for local parasites, these parasites may “spill back” to other local hosts which will consequently increase their infection rates within the native host populations (Kelly et al. 2009; Šlapanský et al. 2016). For example, Hohenadler et al. (2018b) demonstrated that gobies in the Rhine River can contribute to the transmission of the eel swim bladder nematode Anguillicola crassus by serving as a paratenic host, thereby leading to higher infection levels in its definitive hosts, the European eel. In contrast, invasive species might also be responsible for a decrease of the infection risk in the native host populations (e.g. Gagne et al. 2016; Šlapanský et al. 2016). The so-called “dilution effect” occurs if the invaders serve as inappropriate hosts for local parasites, in which the parasites cannot develop further or if they are not favored as food item by a predatory definitive host (Ostfeld and Keesing 2000; Johnson et al. 2012).
According to previous studies more than 20 different parasite species are known to infest gobiids in the native range of their distribution (e.g. Lower Danube and Black Sea area; see Kvach 2005; Ondračková et al. 2006), while a significantly lower number of species is usually reported for non-native areas. Within the well documented and studied Rhine River for example, only eight different species have been reported for N. melanosomus (Emde et al. 2012, 2014; Ondračková et al. 2015) and seven for P. kessleri (Ondračková et al. 2015). In general, Ponto-Caspian gobies show high infestation rates with acanthocephalans of the genus Pomphorhynchus in both, their native and non-native range of distribution, with a prevalence often exceeding 90% and accordingly high intensities (Kvach and Skóra 2006; Francová et al. 2011; Emde et al. 2014; Ondračková et al. 2015). However, individuals of Pomphorhynchus sp. cannot complete their life cycle in gobies and therefore remain encysted in their abdominal cavity as larval or preadult stages. Mainly cyprinids serve as appropriate definitive hosts, whereas in the Rhine River fish species such as barbel (Barbus barbus), chub (Squalius cephalus) or idle (Leuciscus idus) play the major roles (David et al. 2018; Hohenadler et al. 2018a). Therefore, until now the relevance of Ponto-Caspian gobiids for the transmission of Pomphorhynchus spp. remains unclear. On the one hand, they can reduce the risk of infection for the native fish populations (dilution effect) if the acanthocephalans cannot be transmitted successfully from gobiids to other fish hosts. On the other hand, the gobiids might increase the infection risk, if they serve as an appropriate paratenic host (spill back), which is preyed by local piscivorous definitive host.
The aim of this study was to understand the role of invasive gobies for the transmission of acanthocephalans from the genus Pomphorhynchus. In order to evaluate the infection potential of preadult (larval) stages obtained from the abdominal cavity of gobiids, a laboratory infection experiment with chub (S. cephalus) was performed to determine infection success (i.e. recovery rate) and development of the preadult acanthocephalans. The resulting data were compared with results from previous infection experiments in which cystacanths of Pomphorhynchus sp. were administered to chub.