Coral Polyps: The Animals That Build Reefs

The Polyp

A coral polyp is an animal — a member of the class Anthozoa, phylum Cnidaria, related to jellyfish and sea anemones. Each polyp is a tiny, cylindrical structure with a ring of stinging tentacles surrounding a central mouth. Most reef-building coral polyps are 1-10 mm in diameter; some large species have polyps reaching 2-3 cm.

The polyp's relationship with the reef is straightforward: it extracts calcium and carbonate ions from seawater and combines them into calcium carbonate (CaCO3), secreting this as a rigid skeleton beneath its body. Generation after generation of polyps building on the skeletons of previous generations over centuries and millennia creates the reef structure.

A reef 10 metres thick has taken thousands of years to accumulate. The fastest-growing corals (branching Acropora species) grow at 5-25 cm per year. Massive corals (Porites species, the boulder-like formations common on Pacific reefs) grow at 0.5-2 cm per year. A Porites head 3 metres across is approximately 500 years old.

Zooxanthellae: The Hidden Partner

The reef-building ability of coral polyps depends almost entirely on zooxanthellae — single-celled dinoflagellate algae (Symbiodiniaceae) that live within the polyp's tissue in concentrations of 1-2 million cells per cm2. These algae photosynthesize sunlight and transfer 80-90% of the resulting organic carbon to the polyp as sugars and fatty acids — the primary energy source for most reef-building corals.

In return, the coral provides the zooxanthellae with shelter, nutrients (principally the CO2 and ammonia the algae need), and physical position in the light zone. The relationship is obligate mutualism: remove the algae (which happens during bleaching events when temperature stress causes expulsion) and the coral begins to starve.

The presence of zooxanthellae explains why reef-building corals are restricted to the photic zone (depths accessible to significant sunlight, typically above 40m).

Reproduction

Corals reproduce both asexually (clonal budding) and sexually. The sexual reproduction event most remarkable to observe is mass spawning — coordinated broadcast spawning where entire reef sections of multiple species release eggs and sperm simultaneously, triggered by a combination of lunar phase, water temperature, and daylength signals.

On the Great Barrier Reef, mass spawning typically occurs in October or November, one to several nights after the full moon. The surface of the reef becomes covered in a 'blizzard' of small pink, white, and orange bundles — an event described by GBR dive operators as 'the snow' and lasting 1-2 nights.

Hard Coral vs Soft Coral

Scleractinian (stony/hard) corals are the reef builders — they produce the calcium carbonate skeleton. Octocorals (soft corals), including sea fans, sea whips, and the soft corals that give Komodo and Fiji their famous colouring, produce a flexible proteinaceous skeleton and do not contribute significantly to reef-building. Soft corals often grow over hard coral framework but do not build primary structure.