What are corals?

Welcome to an underwater world!

Welcome to an underwater world.
Source: NOAA.

When corals are mentioned, most people immediately think about clear, warm tropical seas, shallow lagoons, beaches of sparkling white sand lined with coconut palms.

In fact, the stony shallow-water corals - the kind that builds reefs - are only one type of coral. There are also soft corals and deep water corals that live in dark cold waters.

In this tutorial, however, we will only deal with the shallow depth tropical corals.

Orange cup coral (Tubastraea faulkneri) open at night to feed in East Timor (Indonesia).
Photo: Wikimedia.

Zoo-what !?!

• What are coral reefs?
  1. plants
  2. animals
  3. rocks
Answer ↓ ↑

2. Corals are animals that are made of tiny polyps, which look like upside-down jellyfish.

• Corals cannot survive without certain organisms. Which?
  1. fishes, octopuses, squids, scallops
  2. worms, shrimps, crabs, lobsters
  3. algae
Answer ↓ ↑

3. Corals live in symbiosis with algae. This means they live together and help each other. These algae are called zooxanthellae.



Single-celled zooxanthellae live in the tissue of many corals. In this picture, the orange-brown spots are zooxanthellae in coral polyps.
Photo: LSA University of Michigan.

Zooxanthellae make oxygen, remove the polyp's wastes, and make food for the polyp by means of photosynthesis.
Coral polyps protect the zooxanthellae, release CO₂, and provide it with necessary nutrients from their own waste.

• How do corals protect themselves?
  1. they make a limestone cup to hide in
  2. a tiny poisonous layer that covers the coral
  3. tentacles release stinging cells when something brushes by them
Answer ↓ ↑

Both 1 and 3 are correct. They have tentacles which release stinging cells and they make a limestone cup to hide in during the day. At night, polyps come out to catch plankton floating by.

What does a polyp look like?

Cross section of a coral polyp. Source: NOAA

Each individual polyp has a stomach that opens at only one end. This opening, called the mouth, is surrounded by a circle of tentacles. The polyp uses these tentacles for defense, to capture small animals for food, and to clear away debris. Food enters the stomach through the mouth. After the food is consumed, waste products are expelled through the same opening.