Another important example of mu-tualism on the reef is the relationship be-tween corals and the crabs, shrimps, andfishes that help protect them from thecrown-of-thorns sea star and other pred-ators. Most corals host a number of sym-bionts, especially crustaceans. Some of these are only casual symbionts and canlive as well off the coral as on it. Othersare much more specialized and are foundonly on their host coral. These are called
obligate symbionts.
Some of the obligatesymbionts are parasites and harm thecoral, some are commensals and appar-ently do not affect the coral one way orthe other, and some are mutualists thatbenefit the coral. It is often hard to tell which is which, because for most sym-bionts the nature of the relationship be-tween coral and symbiont is not wellunderstood. Those who study such or-ganisms must frequently revise their ideasas more information is obtained. Thecrabs that protect their coral from preda-tors, for instance, were once thought tobe parasites