Massive reef structures are formed when each stony coral polyp secretes a skeleton of CaCO3. Most stony corals have very small polyps, averaging 1 to 3 mm in diameter, but entire colonies can grow very large and weigh several tons. Although all corals secrete CaCO3, not all are reef builders. Some corals, such as Fungia sp., are solitary and have single polyps that can grow as large as 25 cm in diameter. Other coral species are incapable of producing sufficient quantities of CaCO3 to form reefs. Many of these corals do not rely on the algal metabolites produced by zooxanthellae, and live in deeper and/or colder waters beyond the geographic range of most reef systems (Barnes, R.D., 1987; Sumich, 1996).
Two images of the same solitary coral Fungia sp. On the left is a living organism showing its outer covering of living tissue, and clearly visible central mouth. The image on the right shows the underlying skeletal structure of the same organism with the outer covering of living tissue stripped away.