The Amazing Coral Polyp

Just what is coral?

The simple answer: coral is an animal. These tiny animals, while seemingly simple, have built the greatest living structure on earth, the Great Barrier Reef!

But a more fascinating and slightly complex answer: coral is animal, vegetable, and mineral.

The coral animal, or polyp, is quite tiny, only a few millimetres although some species can be a few centimetres in size. The biology of a typical polyp consists of a stalk, a stomach, and a mouth surrounded by tentacles covered in stinging cells.

Now the vegetable or plant side of coral. Many of the shallow water corals we encounter while snorkelling have a symbiotic relationship with microscopic single-celled algae, commonly known as zooxanthellae. Zooxanthellae live within the corals’ tissues and give them their colour. The coral provides a protected environment and the compounds zooxanthellae need for photosynthesis. In return, the algae produce carbohydrates that the coral uses for food, as well as oxygen. The algae also help the coral remove waste. Together, the coral and the algae form a very efficient recycling machine with very little waste. Amazing.

When the sun is not shining, corals also feed on tiny animals and plants floating past. This often occurs at night, when the stinging tentacles of the coral extend out of their skeleton, and sting passing morsels, feeding it into their centralised mouth.

Now the mineral side of coral. Hard corals produce a skeleton made of calcium carbonate, or limestone. Coral skeletons grow in many different shapes and form the hard surface of the reef which provides food and shelter to many reef animals and plants.

Soft corals lack a hard limestone skeleton but are incredibly colourful and often live in places protected from waves and currents

The reproductive side of coral is also quite fascinating. Corals grow by asexually reproducing polyps, which means a polyp will ‘bud’ and form another polyp, which is an exact replica of itself. This process grows the coral into what is known as a colony. Coral colonies or formations, take on many weird and wonderful shapes depending on the species. The common names generally describe the shape; branching, boulder, plate, table, vase and so on.

Many corals also breed sexually via ‘broadcast’ spawning where eggs and sperm are released into the water to fertilise. The fertilised egg changes into a free-swimming larva before settling onto the reef’s surface and growing into a single coral polyp.

Mass coral spawning is a spectacular annual phenomenon where colonies from multiple species synchronise release of sperm and eggs over several nights following a full moon. The resulting coral larvae (known as a planula) travel with the currents and eventually settle on the surface of a coral reef. Here, they metamorphose into a coral polyp, then grow through budding, creating new coral colonies. Spawning is a key event in creating future coral generations and replenishing coral ecosystems.

Contributed with thanks to Whitsunday Conservation Council.

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