Enemy release hypothesis: comparison of parasites infection in native and invasive species in Lake Maggiore

In the last decade, in Lake Maggiore (Italy), the dominant native mussel (Unio elongatulus) decreased on average by 75%, and was gradually replaced by the invasive Corbicula fluminea. Could the enemy release hypothesis explain this heavy invader impact? The base-case scenario assumes the introduction of an alien invasive species (AIS) to a new region leads to a decrease in the regulation by its natural enemies (predators, parasites and pathogens), resulting in its rapid increase in distribution and abundance. Did parasites play a powerful role in the shift from a native dominated mussel assemblage to an invasive one? We compared the prevalence and intensity of infestation in the native and in the invasive species in an area colonized since 2010. Data from 2016 showed that C. fluminea is completely parasite-free, whereas U. elongatulus is heavily infested with tissue-dwelling watermites (91-94% prevalence) and sterilizing trematodes (Rhipidocotyle campanula - 72% prevalence). Despite the results are consistent with the enemy release hypothesis, they are not enough to assess whether C. fluminea simply has taken advantage by native mussel disease, or whether it affected the rates of parasitism by weakening the resistance of native bivalves to infections. Since physiologically compromised mussels are more susceptible to parasitism, we hypothesized that the arrival of an alien species caused an increase in the parasite load contributing with other stresses (drought, heat waves) to weaken mussels' resistance to infection. If the hypothesis is true, the rate of parasitism in U. elongatulus should be lower before C. fluminea arrival, than after. By contrast, the comparison with samples collected in 2002-2006 in the same area, surprisingly revealed that the native population was heavily infested even before the invasion. However, before the invasion, the density of Unio was roughly constant with 46 ind/m2, but it declined to about 2-3 ind/m2 after Corbicula's establishment. Hence, in agreement with the enemy release hypothesis: 1) the alien invasive species did not affect the parasite load, 2) AIS was advantaged by being parasite-free, while the native reproductive capacity and physiological status were lowered by heavy infection; 3) the invaders' competition exacerbated the effects of parasites on the native population.

Presentazione orale sul ruolo delle parassitosi, rispetto all'introduzione di specie aliene, nel declino della specie nativa Unio elongatulus nel Lago Maggiore. Confronto di prevalenza e intensità di infestazione in: 1) specie native ed invasive; periodo pre- e post- ingresso di specie aliene; tipologie di habitat e profondità differenti