Corals form the foundation of coral reefs. Corals are animals that create calcium carbonate (CaCO3) -- or limestone -- skeletons, which form the matrix of the reef. At first glance, these organisms may appear to be rocks on the bottom, but they are alive. A thin skin of tissue overlays the limestone “rock,” and continues to deposit more CaCO3 so the coral can increase size. If the animal dies, the reef no longer grows, and over time, the remaining limestone skeletons will erode, eventually destroying the reef and the animals that rely on it.

Reef-building corals live in a symbiotic relationship with algae called zooxanthelle. This relationship is critical to the survival of the coral and the coral reef. The algae use sunlight to photosynthesize, and provide nutrition to the corals. Zooxanthelle also assist with depositing CaCO3. Without the zooxanthelle, reef-building corals could not survive.

While well adapted to surviving in the marine environment, certain conditions can put stress on the coral animal and reduce its ability to survive. Corals generally need clear, clean water. The temperature of the water is also very important, as most species can only survive in a narrow temperature range. Even though they are made of rock, many coral skeletons are brittle and fragile, and can be easily broken by waves, anchors or feet. Primary threats to Guam’s coral reefs include sedimentation, overfishing, pollution, and typhoons. In nearly every case, threats to Guam’s reef are linked to human activities on the island. The most significant threat to Guam’s coral reefs is sedimentation.