Good question. I looked this up on corals. It said that corals usually grow 0.5 cm to 2 cm per year. But under very good conditions, they can grow up to 4.5 cm per year. What are good conditions for coral?

It will help to understand what a coral reef is. As you probably know the reef is a sort of apartment building for little animals that are related to jellyfish. Each little animal forms its own little cup of hard material that is sort of similar to what’s in your bones. As the animals build these little cups, and as new little animals settle in to start building new cups, the reef grows.

These tiny animals eat smaller animals or food bits that are in the water around them. They grab the food with their tentacles and may sting it with their poison. If there are a lot of waves, the little animals that build the reef pull in their tentacles. This means they can’t feed. Without energy and raw material coming in, there’s not much growth. So when it is calm, there’s more reef-building. Big waves can also break pieces off the coral.

Coral also have microscopic helpers that do photosynthesis. These are a kind of algae. Like plants, they use energy from the sun to put carbon dioxide molecules and water molecules together to make sugar. The coral use the sugar and give off the raw materials that the algae need. This is an example of mutualism, where both kinds of species get something good from each other. So for good coral growth, there has to be plenty of light for their algae.

The corals also do best if the temperature does not get too high or low. Different species of coral have different “favorite” temperatures, but they do best if the temperature doesn’t change a lot.

Why do you think reefs only grow in shallow water?

You may want to study marine biology if these kinds of questions interest you.