The North American mud crab Rhithropanopeus harrisii (Gould, 1841) in newly colonized Northern Baltic Sea: distribution and ecology

Here we present the known distribution and population demography of the most northern known population of the North American whitefingered mud crab, Rhithropanopeus harrisii, from southwest Finland in the Baltic Sea. This species was first reported in Finland in 2009 from the archipelago close to Turku and has been found from 82 locations within a 30 km radius since then. Due to the presence of young of year, juveniles, and gravid females observed at three sites in Finland, R. harrisii has established successful populations that are able to overwinter under ice and can opportunistically occupy diverse habitats, such as shafts of dead marsh plants, self-made burrows in muddy bottoms, and the brown algae Fucus vesiculosus in hard bottoms. In its native range, a rhizocephalan barnacle parasitizes R. harrisii, but this parasite was not found in the introduced Finnish population. While R. harrisii is expected to expand its range along the coast of the northern Baltic Sea, the ultimate impact of this species on food web dynamics, both as a predator and prey item, remains to be seen.