In brief
Reef-building corals are animals that live in partnership with tiny algae. The algae feed the coral, and the coral builds a limestone skeleton that grows into a reef. If the water stays too warm for too long, the partnership breaks down and the coral bleaches. Reefs shelter a large range of marine life and protect coasts from waves.
What it is
A coral colony is made of many small animals called polyps, each a soft, tube-shaped body with a ring of stinging tentacles. Polyps are related to sea anemones and jellyfish. A reef-building polyp takes calcium and carbonate from seawater and makes a hard cup of calcium carbonate around its body. Generation after generation of polyps build on the skeletons of earlier ones. The reef is mostly old, dead limestone with a thin living skin.
The polyps host microscopic algae called zooxanthellae in their tissues. The algae use sunlight to make sugars and share much of them with the coral, and the coral provides shelter and some of the nutrients the algae need. This partnership lets reefs flourish in clear tropical water that holds very few nutrients.
| Kind | Where it lies | How it forms (Darwin’s idea) |
|---|---|---|
| Fringing reef | Right beside the shore of an island or coast | Coral grows out from a young volcanic island |
| Barrier reef | Separated from land by a lagoon | As the island slowly sinks, the reef keeps growing upward and the gap becomes a lagoon |
| Atoll | A ring of reef around a lagoon, with no island in the middle | When the island has sunk completely, the reef ring remains |
Charles Darwin put forward this idea of sinking islands in 1842. Deep drilling on an atoll in the Pacific in the 1950s reached volcanic rock beneath more than a kilometre of reef limestone, which supported the idea.
Why reefs matter
- Life. Reefs cover well under one per cent of the sea floor, yet a very large share of marine species lives on or near them. A figure of about a quarter of all marine species is widely quoted, but it is hard to verify.
- Coast protection. Reefs break waves. A global study (Ferrario and others, 2014) found that reefs reduce wave energy by an average of around 97 per cent, which lowers the force that reaches the shore.
- Food and work. Millions of people depend on reef fish and on tourism.
- Not only tropical. Cold-water corals live in deep, dark water, with no algae partners.
Bleaching
Reef corals live close to the upper limit of temperature they can bear. If water stays more than about one degree Celsius above the usual summer maximum for several weeks, the coral loses its algae and turns white. This is bleaching. A bleached coral is not dead, but it is starving. If the water cools soon enough, the algae can return and the coral recovers. If the heat lasts, the coral dies.
| Stage | What happens |
|---|---|
| Heat stress | Water stays above the local summer maximum for days to weeks |
| Loss of algae | Coral expels or loses its zooxanthellae and its colour fades |
| Bleached coral | White skeleton visible through clear tissue; coral is starving but alive |
| Recovery or death | If the water cools in time, colour returns over weeks to months; otherwise the coral dies and algae may cover the skeleton |
Mass bleaching has hit reefs around the world in several years, including 1998, 2010 and 2014 to 2017, years in which unusually warm water spread across ocean basins.
Other pressures
- Ocean acidification. The sea absorbs carbon dioxide from the air, which lowers its pH and the amount of carbonate available. Surface ocean acidity has risen by roughly 30 per cent since the start of the industrial era. Corals have to work harder to build skeletons, and reefs may grow more slowly or erode faster.
- Runoff. Soil and nutrients from land cloud the water and feed algae that compete with corals.
- Overfishing. Removing grazing fish can let seaweed overgrow reefs.
- Damage. Anchors, trampling, destructive fishing and some outbreaks of coral-eating crown-of-thorns starfish damage reefs.
- Disease. Several coral diseases can spread through weakened reefs.
Reefs that are not stressed by local pollution and fishing tend to recover better after heat waves. Reducing greenhouse gas emissions is the only way to address warming and acidification at their source.
Field exercise
If you can visit a reef, float quietly at the surface, never stand on coral, and sketch three shapes of colony (branching, plate-like, boulder-like). If you cannot, use the table in this unit to explain, in your own words and with a drawing, why a barrier reef has a lagoon.
Check yourself
Read the question, think of your answer, then open it with a tap or the Enter key.
What is a coral polyp?
A small, soft animal with stinging tentacles; reef-building polyps make a hard limestone cup around themselves.
What do zooxanthellae do for the coral?
They use sunlight to make sugars and share much of them with the coral.
What is coral bleaching?
The loss of the coral’s algae, usually because the water is too warm for too long; the coral turns white and starves.
Is a bleached coral always dead?
No. If the water cools in time, the algae can return and the coral can recover.
In Darwin’s idea, how does a barrier reef become an atoll?
As the volcanic island sinks, the reef grows upward; when the island is gone, only a ring of reef is left.
What is ocean acidification?
A fall in the sea’s pH as it absorbs carbon dioxide from the air, which makes it harder for corals and shellfish to build skeletons and shells.
Basis of the facts
Where these facts come from. The biology of polyps and zooxanthellae and the three reef types follow Veron (2000) and standard texts; the sinking-island idea is from Darwin (1842), and the atoll drilling result is described in broad terms (“more than a kilometre”). The 97 per cent wave-energy figure is from Ferrario and others (2014). The link between heat, bleaching and acidification follows Hoegh-Guldberg and others (2007). The quarter-of-all-marine-species figure is marked as widely quoted but hard to verify. The years of global bleaching events are those commonly cited by reef monitoring agencies. See the sources list.