For me, crabs are still light creatures, they just know how to use darkness.
Crabs cover their shells with particles of darkness and at the same time keep the light in their core intact (you can see it in their tiny bright eyes), while not allowing darkness to completely corrupt or immobilize them, as would happen to other light creatures.
Actually, it's very impressive how crabs were able to use darkness and adapt to constant contact with it. They don't even use darkness to consume light, only to protect themselves and their kin!













