Habitat expansion of the Harris mud crab Rhithropanopeus harrisii (Gould, 1841) in the northern Baltic Sea: potential consequences for the eelgrass food web

Since at least 2009, the Harris mud crab Rhithropanopeus harrisii (Gould, 1841) has been spreading in the northern Baltic Sea, an area with no native crab species. Thus far, this species has invaded muddy and rocky coastal areas, especially shallow habitats dominated by reeds and the brown macroalga Fucus vesiculosus Linnaeus, 1753. Here, we document the first sighting and recruitment of mud crabs in an eelgrass Zostera marina Linnaeus, 1753 meadow - a critically important habitat for fish and invertebrate species. We found both adult and juvenile mud crabs in repeated sampling over the summer, suggesting continuous use of eelgrass as habitat for mud crabs and recruitment into the meadow. Given the simple food web structure of the Baltic Sea, this novel predator could lead to new energy pathways with unexpected consequences and potentially important effects on the functioning of the native eelgrass community.