These hard corals are the basic building blocks of tropical coral reefs. The polyp of hard corals makes a hard, protective shell like a cup out of calcium carbonate called corallite. The corallite protects the soft, fragile bodies of the polyp. Hard corals grow in colonies. These colonies that give the reef structure lots of calcium carbonate are called reef-building corals. The polyps get calcium carbonate from the sea water.
There are six groups of hard corals: branching & pillar corals; encrusting, mound & boulder corals; brain corals; leaf, plate & sheet corals; fleshy corals; and flower & cup corals.
Let’s take a closer look at the polyp! Polyps live inside the corallites for protection. A polyp is an animal and like all animals it needs to eat. A polyp grabs plankton, microscopic food, using its stinging and feeding tentacles. The food is then digested in its stomach.
The soft body of a coral polyp is about the size of a pencil eraser. Some corals have 8 tentacles, others have multiples of 6. When the polyp dies, the chalky skeleton remains, and another polyp will grow on top of the old one. Coral colonies grow in many shapes and come in many colours.
Many corals have symbiotic algae that live inside them. Symbionts are two organisms that help one another. These algae are called zooxanthellae and they help form the coral's stony exoskeleton and give the coral its colour.
This picture taken through a microscope shows a soft coral polyp. Green shows the polyp tissue, while the red shows the symbiotic zooxanthellae.
In the photo below, a goby hides in a hole in the coral and is surrounded by millions of polyps. Look closely at the photo at those tiny, whitish ‘flower petals’ close to the fish. That’s the delicate flesh of the polyps. In the photo below of brain coral, the polyps have their tentacles extended for feeding. The polyps only come out at night to feed.
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