The colossal crab was obviously alive and had clearly been for a very long time, floating on the sea. Its shell and its claws—the twin reefs—were covered with sand and plants and trees, though the deep red color of the crab’s shell dominated along the edge of it.
The crab protected other creatures if they helped him. For example, it offers the squid protection from the eel if the squid brings it food. However, if a creature moves too far away from its claws, the crab cannot protect it.
Karkinos becomes ill with age and is later healed by Henry Haluki who uses some of the seaweed that was given to him by Ishibashi-San on the Island of Shipwrecks.