Why Carcinus maenas cannot get a grip on South Africa's wave-exposed coastline

Abstract

The European green crab Carcinus maenas has established considerable breeding populations in harbours and sheltered bays in the South-Western Cape, South Africa, but appears unable to flourish on the wave-exposed coastline. This study compares the abilities of C. maenas and those of an indigenous rocky-shore crab, Plagusia chabrus, to resist hydrodynamic forces. C. maenas had less than half the vertical tenacity of P. chabrus (371.5g and 780.5g respectively) and was unable to grip against as fast a unidirectional flow (0.23m s⁻¹ vs 0.53m s⁻¹) as P. chabrus. C. maenas also has significantly shorter and lighter limbs than P. chabrus and the dactyls of its walking legs are poorly adapted to grip onto rocky substrata. We conclude that C. maenas is poorly adapted to survive in wave-swept conditions and hence unlikely to displace indigenous crab species along the open wave-exposed coastline of South Africa. However, it may invade other sheltered locations, particularly Saldanha Bay and False Bay.