Crabs are representative of tenacity and perseverance, often to a fault. They are also representative of irritability - being 'crabby.' Crabs are creatures of water, and water is associated with emotion and intuition.
To me, this dream suggests that you are holding on to negative emotions. I believe the large crab was you, and the smaller crabs represent how this negativity slowly 'devours' you. Your subconscious is warning you to let go.
Good luck, and blessed be.