If you like to buy coral as a colony rather than a frag, you'll often times also inherit acro crabs. These little guys rarely stray out of the coral head, because they remain safe within the branches. They catch food that drifts through the coral, and keep the coral clean of detritus, nuisance algae, and other waste. Their symbiotic relationship is extremely beneficial to both the coral and the crab, because the branches keep the crab safe from becoming fish food. Reef-safe.