PT Unknown
AU Honka, K
TI Interaction of non-indigenous endoparasites of the  European eel Anguilla anguilla
PD 02
PY 2024
DI 10.17185/duepublico/81476
LA en
DE parasite; eel; Anguillicola; invasive; stress; cortisol; hybridization
AB Biotic interaction is an essential feature of ecosystems, expressing their stable and dynamic structure. The present thesis is focusing on the interaction of different non-indigenous endoparasites of the European eel (Anguilla anguilla). One of them, Anguillicola crassus, is topic of research since decades, however very few is known about its interaction with other parasites. After this invasive parasite was found inside an acanthocephalan cyst (Pomphorhynchus spp.) – the second protagonist of the present thesis – it was indicated that there might be some beneficial interaction between both species. This work proves that hidden nematode larvae are still infectious to their final host, revealing previously unidentified but effective ways to complete their life cycle. It is already known that invasive parasites influence their new habitats to some extent. The here applied invasional meltdown hypothesis describes the beneficial influence of two non-indigenous species by accelerating the distribution of each other. Because this hypothesis was only used to describe interactions of free-living species, Pomphorhynchus spp. and A. crassus are the first parasites it is applied to. Furthermore, the co-evolutionary adaptation of host-parasite systems is considered in this thesis, as both parasites are native to different eel species. By analyzing plasma cortisol levels after inoculation, the co-evolutionary well adapted system of A. crassus with Japanese eels (A. japonica) revealed the lowest stress response, whereas the same eel species displayed the highest response to Pomphorhynchus spp. as a naïve parasite. Cortisol levels of the European eels ranged between those of the Japanese eel, but likewise with lower levels to the familiar parasite. The results underline the clear dependence of the cortisol response on mutual adaptation occurring in host-parasite systems, from high cortisol levels in naïve systems and low cortisol levels in adapted systems. When considering the invasional success of A. crassus, the focus of this thesis is not only on its interaction with the phylogenetically distinct parasites Pomphorhynchus spp. but equally with the phylogenetically close nematode A. novaezelandiae. This species, native to the short-finned eel A. australis, was established in Lago Bracciano, Italy, even before the introduction of A. crassus to Europe. Only couple of years later it got displaced by its Asian relative. Since then, it is unknown how A. crassus was able to outcompete A. novaezelandiae in such a short period of time. Some determinants might be a more efficient life cycle, for instance by releasing eggs over a longer period of time; or consequences of unidirectional hybridization between these two species, meaning that only A. novaezelandiae females fertilized by A. crassus males are able to produce viable offspring, but not the other way around. As only rudimental information is known about this hybridization hypothesis, the here presented thesis is also researching the viability of hybrid offspring for both hybridization patterns. By microsatellite analysis, it is detected that both crossbreeding produce hybrid eggs, but only those of the already described pattern A. novaezelandiae female x A. crassusmale develop further into adult nematodes. This discovery not only underlines the understanding of what has happened at Lago Bracciano in the early 1980s, but further conduces to a better understanding of the global invasion success of A. crassus. The interaction of invasive parasites in newly conquered ecosystems is a critical component for understanding successful invasion processes. The phylogenetic relation of the parasites studied in this work highlights the extensive variance with which organisms such as A. crassus can establish and spread in new areas.
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