Corals exemplify a common characteristic among animals -- they fight when confronted by "foreign" tissue. Corals are composed of individual yet connected zooids basically constructed much like anemones. Corals of different species will grow very close to one another in competition for space. As they come into contact. the bordering zooids of each species will attack one another. One of the two colonies may overcome the other or it may result in a standoff where neither encroaches any further. Their relative aggressiveness was determined by experiment in the laboratory but has not proved to be predictable in the field.
--David K. Bulloch, "The American Littoral Society Handbook for the Marine Naturalist"