An introduced species meets the local fauna: predatory behavior of the crab Rhithropanopeus harrisii in the Northern Baltic Sea

Crabs are some of the most successful introduced species among marine organisms, and they can be an important structuring force in marine communities. Recently, the North American white-fingered mud crab, Rhithropanopeus harrisii, has invaded the Northern Baltic Sea. This is an area where no native crab species exist, and the addition of a novel functional species to the low species diversity of the Baltic Sea could have large community-level impacts i.e. modifying biotic interactions and/or altering ecosystem functioning. We examined the predatory behavior of introduced R. harrisii both in the laboratory and field focusing in shallow, hard bottom habitats dominated by the alga Fucus vesiculosus. In the laboratory environment, R. harrisii was an effective predator of littoral grazers, readily consuming both sessile fauna (Mytilus trossulus) and also mobile species such as isopods (Idotea balthica) and gammarid amphipods (Gammarus sp.). When studying the predation of different sized prey items, R. harrisii preyed upon small and medium sized prey of both mobile and sessile species. However, in the field experiment with the native faunal community associated with F. vesiculosus, R. harrisii negatively impacted only the abundance of the snail Theodoxus fluviatilis, possibly through indirect effects. Nevertheless, R. harrisii significantly decreased both the prey species richness and diversity but not the total number of potential prey individuals associated with F. vesiculosus. In conclusion, predatory behavior of this novel crab has the potential to impact the native macroinvertebrate littoral community, but the realized predation pressure in the field is lower than could be expected from laboratory experiments.