First they destroy fences with their pinchers, making fences useless as protection. Second they have a hard shell, and disgusting taste - so are useless as food for the colonists. The red and green colouration makes them quite distinct. Unlike the Seaside Greble and Crab Greble this creature can only live on land. Fences are no match for its huge sharp teeth.
Colonists placed by the player are a tough breed of human, who can live on the land.
They are able to eat the Spider Greble and the Seaside Greble. They also build fences when they encounter these creatures in order to displace them. They can only live on land however. The only creature that can kill a Colonist is the Dino Greble. The basic resource of this game is food.