Well, we talked about this breifly two posts ago...What we recognize as coral is actually both plant and animal, living symbiotically. A "plantanimal," as a friend of mine likes to call them. The fleshy animal polyp part lives with the photosynthesizing algae part in the hard stony home the polyp builds for them both. When conditions are bad and the relationship is no longer mutually beneficial, the algae peace out and the coral will turn white and die. We call this bleaching.
Researchers from the Georgia Institute of Technology have found that several species of seaweed contain poisonous chemicals that are lethal upon contact to corals. Contact with these seaweeds will cause tissue death and coral bleaching. What's more is that once a part of the coral is dead, seaweeds will often colonize that area and begin spreading, causing more coral tissue death as they go. The study has found that in protected areas, there is a large enough herbivorous fish population to keep the seaweeds in check and the corals healthy. In areas that are overfished, however, the seaweed is eaten more slowly or not at all and the damage is allowed to spread.
photo credit RTSea via Mark Hay
Full article here (RTSea)