Keras were large coral crab Rahi found in the ocean off Ga-Metru.[1, 2] At one time, Matoran hoped that Keras could be as useful to them as Ussal crabs were, but the hostile nature of the Keras made that impossible.
The Keras coral crab was a reclusive reef-dwelling creature, famed for protecting its undersea home with great ferocity. The Gahlok drove a number of Keras onto the beaches of Mata Nui, and volunteers from the Onu-Matoran Ussalry helped the villagers of Ga-Koro tame a few as partners in the struggle against the Bohrok swarms. Outfitted with special riding plates, these fierce and clever crabs could carry riders across both sea and land. They contentedly served as defenders of the village in exchange for shelter from the sun and a meal of fresh fish.