Corals crabs provide a symbiotic relationship with some SPS (stony) corals. Just like the symbiotic relationship between a Clownfish and an Anemone, or a Watchman Goby and a Pistol Shrimp, or a Cleaner Shrimp and a Reef Fish, the crab and the coral both benefit greatly from having a relationship with each other.
Coral crabs can be found living.
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