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1Native fauna in species poor communities, such as those of the Baltic Sea, may be particularly 2 vulnerable to the effects of species invasions. However, the interspecific interactions that 3 result in the negative impacts on native species tend to be poorly understood. One 4 contributing factor to this knowledge gap may be that the vulnerability of native species can 5 vary between different life-history stages. For example, the parental care phase is often risky 6 both for the developing offspring and care-giving parents. Accordingly, we investigated the 7 interactions between invasive mud crabs, Rhithropanopeus harrissii, and native nest-building 8 littoral fish, with a special focus on the sand goby, Pomatoschistus minutus. Firstly, our field 9 survey demonstrated that while the recently established mud crab and native nest-building 10 fish have different habitat and depth optima, their distributions also have a considerable 11 overlap, with a high potential for interactions between them. Secondly, our laboratory 12 experiments indicate that the crabs are likely to impact the fish negatively, especially by 13 taking over occupied nests, as well as pre-occupying nesting resources that are of very limited 14 supply in the studied population. We did not find evidence for successful predation on eggs 15 guarded by male sand gobies naive to the mud crabs. Collectively, the results suggest that the 16 invasive crabs have high potential to negatively impact native fish. Furthermore, the results 17 support the scenario that the parental phase can be a particularly vulnerable life-history stage 18 in face of novel selection pressures, such as species invasions. 19 20

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