Coral: Animal, Mineral, or Vegetable?

What is coral? When you see pictures in National Geographic of huge rock like things in the ocean with fish swimming all around, is that coral? Well, sort of. That is a coral reef.

Coral is an animal that belongs to the phylum cnidaria. A phylum is a group that scientists place animals in which share certain characteristics. Cnidarians are radially symmetric, which means that they are the same all the way around, 360 degrees! They are built like sacs with a hole in one end that is surrounded by stinging tentacles. Jellyfish are cnidaria. Now, you are probably thinking, jellyfish don't look anything like what I thought coral was! That's because the most common pictures of coral are colonies called reefs.

During the mating season coral polyp release eggs and sperm into the water (picture below) and when an egg and a sperm meet they form a larva known as a planula.

The baby coral looks like a little tiny jellyfish and it floats around in the water until it finds a hard place to attach to, usually a coral reef. Then it lands and starts to build itself a shell. It builds it by combining carbon dioxide (CO2) and calcium (Ca) in the water to make calcium carbonate (CaCO3) also known as limestone. This shell is shaped like a round vase and the coral polyp lives inside.

Mountainous star coral (Montastraea faveolata) in the process of releasing both eggs and sperm. Photo taken by Susan Colley, University of New Orleans in 1990s.
Coral Polyp (Photo courtesy Jeffrey N. Jeffords)

Coral polyps are primarily nocturnal. At night a coral polyp will stick its tentacles out of its vase and let the tentacles wave in the current. Then, when plankton float by, the coral polyp stings them with its tentacles and brings the plankton inside its shell to have for lunch.

A coral reef is about a million of these individual coral polyp shells all stuck one on top of the other. When coral polyps die, new ones land and grow right on top of the old empty shells. There are over 500 different species of coral. Some look like brains and some like fans and some like the antlers of deer, but they are all made up of tiny coral polyps.

Organisms other than coral can form reefs. A reef is simply a structure in the shallow parts of the ocean that serves as a home to animals and plants. Many sunken ships have become reefs and humans even create artificial reefs to replace the coral reefs that we have destroyed. Some artificial reefs are specially constructed for the purpose, but others are made of tires linked together, old appliances linked together and even discarded military equipment like tanks and helicopters.