On the distribution, diet and possible impact of the invasive European shore crab Carcinus maenas (L.) along the South African coast

Abstract

The crab Carcinus maenas (L.) has an endemic distribution along the European Atlantic coastline but has been accidentally introduced into intertidal habitats in several parts of the world. Its discovery at the Cape Peninsula in South Africa is recorded here. The crab is well established in Table Bay harbour and the population is at least seven years old. From here it has spread 117 km north and 10 km south. Populations on the open coast are more recently established. The crab feeds on a wide range of intertidal animals, particularly the isopod Paridotea ungulata, limpets and polychaetes, but it is too early to make quantitative assessments of its impact on local intertidal communities. Initial predation experiments suggest that the barnacle Notomegabalanus algicola and the whelks Oxystele variegata and O. tigrina have no refuge in size or zonation from the crab. O. sinensis, of similar size to O. tigrina, is less vulnerable by virtue of its stronger shell. Its critical size (above which it is too large to be attacked) is correlated with crab size. Larger species, such as the mussels Choromytilus meridionalis and Aulacomya ater and the periwinkle Turbo sarmaticus, clearly achieve a refuge in size, but predation on their recruits may influence their abundance and size structure. C. maenas rejected only the urchin Parechinus angulosus as a source of food. While it is conceivable that shifts in community structure, selection for thicker shells in molluscs and localized extinction of certain species may occur at high crab densities, it is predicted that C. maenas will only have a significant impact in areas protected from wave action.