Coral biology and the zooxanthellae symbiosis (QCE Marine Science Unit 3)
“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”
A coral is a colony of tiny animals called polyps, each a sac with a ring of stinging tentacles, that secretes a calcium carbonate skeleton. Each polyp houses symbiotic algae called zooxanthellae in a mutualism: the algae photosynthesise and supply the coral with most of its energy, and the coral gives them shelter, nutrients and carbon dioxide. Corals grow by adding skeleton (calcification) and reproduce both asexually by budding and sexually, often in spectacular synchronised mass spawning events on the Great Barrier Reef.
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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.
A mutualism is a close, long-term relationship between two species in which both partners benefit. The coral and zooxanthellae are the classic marine example: the alga gains shelter and nutrients while the coral gains most of its food. Contrast this with commensalism (one benefits, the other is unaffected) and parasitism (one benefits at the other's expense).
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.
- Calling coral a plant
- Coral is a colonial animal; the zooxanthellae living inside it are algae.
- Saying the relationship is parasitic or one-sided
- It is a mutualism; both the coral and the algae benefit.
- Confusing the two reproduction modes
- Budding and fragmentation are asexual (clones); spawning planula larvae is sexual (new gene combinations).
- Forgetting corals also feed
- Corals capture plankton with stinging tentacles as well as relying on their algae.
Exam-style questions
Questions in the style of QCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice3 marksDescribe the relationship between corals and zooxanthellae.Show worked answer →
For 3 marks, name the relationship and give the two-way exchange.
Type of relationship. Corals and zooxanthellae (symbiotic dinoflagellate algae, genus Symbiodinium) live in a mutualistic symbiosis - both partners benefit. The zooxanthellae live inside the gastrodermal cells of the coral polyp.
What the coral gains. The zooxanthellae photosynthesise and pass on up to about 90 per cent of the organic carbon they fix (sugars, glycerol, amino acids) to the coral. This supplies most of the coral's energy and accelerates calcification of the calcium carbonate skeleton.
What the zooxanthellae gain. The coral provides a protected, well lit position in its tissue, plus a supply of carbon dioxide and nitrogen and phosphate nutrients (from the coral's metabolic waste) needed for photosynthesis.
Full marks: state mutualism, then one benefit to each partner.
Practice3 marksThe graph in the exam showed how the number of extended polyps in a coral colony changes over 24 hours: few polyps are extended in daylight, the number rises sharply after sunset to a maximum overnight, and it falls again around sunrise. Explain how the time of day affects the feeding of coral polyps. Support your answer with evidence from the graph.Show worked answer →
For 3 marks: state the pattern, link it to feeding, and quote the graph.
Pattern from the graph. The number of extended polyps is low during daylight and rises sharply after sunset to a maximum overnight, then falls again around sunrise. This shows the polyps are mainly nocturnal feeders, extending their tentacles at night.
Why this aids feeding. At night the polyps extend their tentacles, which carry stinging nematocysts, to capture zooplankton that migrate towards the surface after dark. Feeding at night maximises prey capture when zooplankton are most abundant in the water column.
Why polyps retract by day. During the day the polyps retract, which reduces predation and exposure, and lets light reach the zooxanthellae for photosynthesis. The coral therefore feeds heterotrophically at night and relies on its algal symbionts by day.