All life on the tropical coral reefs depends on corals. Either directly or indirectly, corals make possible all the life in our shallow tropical seas. Corals remove minerals from ocean water to build enormous colonial skeletons. Millions of coral animals, called polyps (say "PAH-lips"), build entire islands, and produce food and shelter for millions of fishes and invertebrates.

Even though all these other animals depend on corals, the corals' life is not peaceful. The polyps require a place to live on the surface of the coral skeleton, and neighboring colonies will fight with each other for space on the reef. Other animals (especially colonies of sponges) also fight with the corals for living space on the sea floor.

In this picture we see a colony of Star Corals that has built a towering skeleton that rises 12 feet above the sand. The top of this tower has a healthy colony of Star Coral polyps. Each yellow-brown dot at the top of this tower is a single Star Coral polyp. The colony grows very slowly, building 1/4 of an inch of skeleton each year. As this coral tower is 12 feet tall, this colony is about 600 years old. But the base of this colony has been taken over by other animals. Most obvious are three different kinds of sponges. The purple branches on the left side of the tower are a colony of animals that is called a rope sponge. This colony is made of thousands of microscopic animals that pump water through the branches so they can filter food out of the water. At the base of the coral is an orange sponge. That colony also pumps water to filter food out of the water, and you can see the several holes in the colony that are the "exhaust pipes" for the pumped water. Below the orange sponge is a brown encrusting sponge. Other animals living on this coral tower include hydroids and bryozoans, and a few species of green algae are also visible.

Learn more about how sponges filter water at the ReefNews Yellow Tube Sponge webpage.

See a close-up of Star Coral polyps at the ReefNews Christmas-Tree Worm webpage.