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Biological invasions are frequently studied topics in ecological research. Unfortunately, within invasion ecology parasite-associated aspects such as parasite impacts on new environments and on local host populations are less well-studied. Round gobies migrating from the Ponto-Caspian region into the Rhine River system are heavily infested with the Ponto-Caspian acanthocephalan parasite Pomphorhynchus laevis. As shown by experimental infestations the acanthocephalans occur as pre-adults in host-encapsulated cysts within the internal organs of the migrating gobies, but remain infective for their definitive host chub. Recently, we described the occurrence of larvae of another parasite, the invasive eel swim bladder nematode Anguillicola crassus, in these Pomphorhynchus cysts. In the present study, we could prove the infectivity of the nematode larvae for European eels for the first time. After experimental inoculation of Pomphorhynchus cysts occasionally infested with A. crassus larvae, the nematodes grow to maturity and reproduce whereas all P. laevis were unviable. We therefore postulate that the nematode larvae behave like immunological hitchhikers that follow a “Trojan horse strategy” in order to avoid the paratenic host’s immune response. Accordingly, the interaction between both invasive parasites gives first evidence that the invasional meltdown hypothesis may also apply to parasites.

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Ponto-Caspian gobies became highly abundant in many regions outside their native distribution range (e.g. Rhine River system). In the newly invaded habitats, the parasite communities of the invasive gobies are characterized by a lower species richness compared to their native range. However, acanthocephalans of the genus Pomphorhynchus are highly abundant, whereas in gobies they do not become mature and mostly remain encapsulated in the abdominal cavity as preadults. Thus, gobiids could either represent a dead-end host leading to a decline of the Pomphorhynchus sp. population (dilution effect) or act as a paratenic host that could increase the infection pressure to the original host community (spill back). To determine the importance of gobiids for one or the other process mentioned, we conducted two infection experiments using smaller and larger individuals of the definitive host chub (Squalius cephalus), which were infected with preadults of P. bosniacus collected from the abdominal cavity of Neogobius melanostomus. The results showed that preadults obtained from gobiids can develop and mature in the definitive host with mean recovery rates of 17.9 % in smaller and 27.0 % in larger chubs. No infection was observed in 38.0 % and 20.0 % for smaller and larger chubs respectively. Our study clearly demonstrated that gobies serve as a paratenic host for acanthocephalans of genus Pomphorhynchus and thus spill back the infection into the local fish community. However, comparisons with previous experimental studies conducted with cystacanths from intermediate gammarid hosts showed that the preadults have significantly lower recovery rates than cystacanths.

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