Effects of the swimming crab Charybdis japonica on sea urchin Mesocentrotus nudus grazing: cage experiments in barren ground and land-based tanks

Abstract

To examine the effects of the crab Charybdis japonica (72–96 mm in carapace width) on the grazing behavior of the sea urchin Mesocentrotus nudus, cage experiments were conducted in a sea urchin-dominated barren ground and in land-based tanks. In the field, one crab consumed almost all 10 sea urchins (20–40 mm in test diameter: TD) given in an unescapable small-mesh cage within 4–15 days. In a coarse-mesh cage experiment where smaller sea urchins (< 31 mm in TD) could pass through the mesh, a crab prevented intrusion and grazing on kelp blades (100 g per cage), while 4–50 sea urchins intruded and grazed on kelp blades in the control cage (no crabs inside). In the laboratory, different-sized coarse-mesh cage experiments showed that one crab could repel sea urchins and protect kelp blades even in the largest cage (6 m²). Furthermore, a year-round feeding experiment (14 months) revealed that three crabs consumed sea urchins from May to December (> 12 °C in water temperature), and that predation rates varied among the crabs, ranging from 1653 to 4777 g (124 to 312 sea urchins) per crab per year. These results suggest the importance of C. japonica in the control of grazing by M. nudus.