Host specificity of parasites is defined by various factors. For instance, the parasite’s gene- ralism or specialism, that determines infectivity and resistance for different host-parasite combinations. Host resistance depends on the reaction of the hosts’ immune system towards the parasite. And host-parasite interactions in general are determined by genetic coevolutionary adaptations but also non genetic transgenerational components like mater- nal effects. The coevolutionary dynamics are strongly affected by how long species have been interacting, making invasive parasites an interesting study subject due to their short coevolutionary interactions with newly infected hosts. The purpose of this study is to investi- gate host specificity and in particular maternal effects in infections with invasive parasitic copepods of the genus Mytilicola. And how specificity and maternal effects feed back on immune system activity of the hosts and the resulting changes in hemolymph microbiota like Vibrio bacteria. All experiments were based on experimental infections of the principal hosts in the German Wadden Sea, Mytilus edulis and Crassostrea gigas, with the specialist Mytili- cola intestinalis or the generalist Mytilicola orientalis. Infection rates and growth rates were investigated as traits of the parasites, while hemocyte composition, phagocytosis rates and Vibrio loads were measured in the hosts. Infection rates first confirmed that M. intestinalis is a specialist, whereas M. orientalis was found in both host species. M. orientalis parasites stemming from C. gigas as hosts furthermore showed higher infection success than parasi- tes stemming from M. edulis. Thus, infection rates confirmed the presence of maternal ef- fects. In the hosts, granulocyte concentrations increased in response to Mytilicola infections, representing a potential defence mechanism against the parasites. Phagocytosis and Vibrio load, on the other hand, showed no correlation with Mytilicola infections. These results not only underline the close relationship of parasite infection and host immune system activities, but also the intricacy of host specificity especially in connection with coevolutionary dyna- mics in parasite invasions and non genetic maternal effects.