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