Describe the structure and feeding of the coral polyp, explain the mutualistic symbiosis between coral and zooxanthellae, and describe how corals grow and reproduce to build reefs

What this dot point is asking

QCAA wants you to explain how a coral polyp is built and fed, why its partnership with zooxanthellae is a mutualism, how corals lay down their calcium carbonate skeletons, and how they reproduce to build and replenish reefs. This biology underpins both the reef-structure dot point and the bleaching mechanism in Unit 4.

The coral polyp

A reef-building coral is a colony of thousands of genetically identical polyps. Each polyp is a small cnidarian animal: a hollow sac topped by a mouth surrounded by a ring of tentacles armed with stinging cells (nematocysts). The polyp sits in a cup of calcium carbonate that it has secreted. At night many corals extend their tentacles to catch drifting plankton, which they sting, capture and pass to the central mouth. So a coral is both a predator that feeds and a host for photosynthetic algae.

The mutualism with zooxanthellae

Inside the tissue of each polyp live single-celled algae called zooxanthellae (symbiotic dinoflagellates). The relationship is a mutualism, meaning both partners benefit.

• The zooxanthellae photosynthesise and pass organic compounds (sugars) to the coral, supplying up to about 90 per cent of its energy. They also give the coral its colour.
• The coral provides the algae with a protected, sunlit position, a supply of carbon dioxide and nitrogen waste they can use as nutrients, and shelter from grazers.

Because the algae need light, this partnership is the single biggest reason reef-building corals are confined to clear, shallow, sunlit water.

How corals build skeletons

Reef-building (hermatypic) corals lay down skeletons of calcium carbonate in the form of aragonite. The polyp takes up calcium and carbonate ions from the surrounding seawater and deposits them beneath its tissue, a process called calcification. The energy supplied by the zooxanthellae greatly speeds calcification, which is why corals in good light grow far faster than corals in the dark. Over thousands of years, the accumulated skeletons of countless colonies build the physical structure of a reef. This dependence on aragonite-saturated water is exactly why ocean acidification (Unit 4) threatens reefs.

How corals reproduce

Corals reproduce in two ways.

• Asexual reproduction. A polyp buds off a copy of itself, growing the colony, and broken fragments can settle and form new colonies. This produces genetically identical clones and lets a colony expand quickly.
• Sexual reproduction. Polyps release eggs and sperm that fuse to form larvae called planulae. The larvae drift in the plankton, then settle on hard substrate and grow into new colonies, spreading coral genes to new reefs. This is the basis of reef connectivity.

On the Great Barrier Reef, many coral species release their eggs and sperm together in a mass spawning event over a few nights in spring, cued by water temperature, day length and the lunar cycle. Synchronising release improves fertilisation success and overwhelms predators.

Why this matters

Coral biology ties the whole of Unit 3 together. The polyp-zooxanthellae mutualism explains why reefs need light, warmth and clear water; calcification explains why they need aragonite; and larval dispersal explains the connectivity between reefs. It also sets up Unit 4: bleaching is the breakdown of this mutualism under heat stress, and acidification is the chemistry of calcification running in reverse.