How do corals grow?

Staghorn Acropora can grow rapidly Photo: Vicki Harriott

Most newly-settled corals contain zooxanthellae (pronounced zo-zan-thel-y), the single-celled algae that live inside the coral animal and help to supply it with food. Providing enough light for the zooxanthellae is one reason why most corals are found only in clear, shallow water.

As the coral grows, the polyp divides repeatedly and produces more skeleton. The way the polyp grows and divides will determine the shape of the new coral colony. Growth is initially slow, so that after one year, the coral will still be quite small, perhaps 1 cm in diameter. After the first year, growth is more rapid, and a staghorn coral (Acropora) can increase in diameter at more than 15 cm per year.

The shape of a particular coral species is not always fixed. Corals are able to grow in different shapes, depending on the environmental conditions in which they find themselves. Where there is strong wave action and currents, corals will grow in a stunted and robust form able to withstand the harsh wave conditions. If the same species of coral was to find itself in a deep sheltered environment, it could form finer, more delicate branches or structures, and grow more rapidly. This variability contributes to making coral identification very difficult.

Some excellent reference materials on coral identification are:

Veron JEN. 1986. Corals of Australia and the Indo-Pacific. Australian Institute of Marine Science.

Veron JEN. 2000. Corals of the World. Australian Institute of Marine Science. (AIMS website)

Wallace CC. 1999. Staghorn corals of the World. CSIRO, Melbourne. (Museum Tropical North Qld website)

Some coral species which form massive growth forms lay down a record of growth in their skeletons similar to the annual growth rings in trees. This has proven to be a useful tool for research, as the age of corals can be calculated from counting back from the outer-most recent ring (or growth band). The coral skeleton have also been found to contain information on the environmental conditions at the time the skeleton was laid down, and a whole new research discipline has evolved in the last 30 years. Deriving environmental records from very large, old corals and from fossil corals is helpful when examining climate change.