How a coral reef is born.

01

A larva finds rock

Coral begins as plankton. Colonies release eggs and sperm into the water, and the fertilised eggs develop into larvae that drift with the current for days or weeks. Most are eaten. The survivors are looking for one thing: a hard, clean surface in shallow water where light reaches the bottom.

When a larva finds such a surface it settles and changes form, becoming a small polyp fixed in place. It secretes a cup of calcium carbonate around its body. From that point the animal cannot move, and everything the reef becomes is built by polyps doing the same small thing in the same place.

Bare rock, dead coral skeleton and the hull of a wreck can all serve. Sand and soft mud usually cannot, which is one reason a reef tends to start at the edges of things, on a drop-off or a current-swept ridge rather than in the middle of a lagoon.

• Hard surface: rock, old coral skeleton or wreckage, free of loose sediment.
• Shallow water: enough sunlight for the algae that live inside coral tissue.
• Clean water: low sediment and pollution, so the larva can settle and the polyp can feed.
• Warm water: tropical temperatures, without extremes of heat or cold.

02

Colonies and skeletons

A settled polyp divides, again and again, and the copies stay joined. What began as one animal becomes a colony, and colonies of different species spread across the rock. Each polyp adds calcium carbonate beneath itself, so the colony grows upward and outward on a skeleton of its own making.

Reef-building corals also host microscopic algae in their tissues, which pass sugars to the coral in return for shelter. That partnership is why reefs need light, and why they are found in clear, shallow water rather than in the deep or the murky.

The pace is slow and uneven. Growth rates vary widely between coral species, and between places and seasons, so no single figure describes a reef. A structure large enough to walk on is the work of a very long time, and the living layer on top of it is thin.

03

Fringing, barrier and atoll

Reefs are usually sorted into three shapes. A fringing reef grows directly against the shore of an island, with a shallow flat and a slope dropping away. Many Maldivian island house reefs are of this kind, close enough to swim to from the beach.

A barrier reef lies further offshore, separated from land by a lagoon. Water inside it is sheltered and shallow; outside, the reef edge takes the ocean swell. An atoll is a ring of reef and islands around a central lagoon, and the Maldives is built almost entirely of them.

The difference matters to a visitor because it decides where the drop-off sits. On a fringing reef you can wade in and reach the slope. On an atoll you may cross a wide lagoon first, and the reef edge is where the fish and the currents are.

04

Darwin's subsidence idea

The classic explanation for how a fringing reef becomes a barrier reef and then an atoll is Charles Darwin's subsidence theory, set out in the nineteenth century. The idea is that a volcanic island slowly sinks while the reef around it keeps growing upward, staying near the surface.

As the land goes down, the reef is left further from shore, becoming a barrier, and eventually only the ring remains, with a lagoon where the island was. It is an elegant model and it fits much of what is seen in the Indian and Pacific oceans.

It is not the only model. Other explanations involve changes in sea level, the shape of the underlying rock and the history of the ocean floor, and reefs in different places have different stories. Treat subsidence as the classic account rather than the whole answer.

05

Algae and grazers at work

Coral does not build a reef alone. Coralline algae, which grow as pink and mauve crusts over rock and dead coral, cement the structure together and encourage new coral larvae to settle. Without them the framework would be looser and slower to form.

Grazing fish do the other half of the work. Parrotfish, surgeonfish and others crop the algae that would otherwise smother young coral and crowd the bare surfaces a larva needs. A reef with its grazers intact recovers better and keeps more room for coral to grow.

This is why reef health is a matter of balance rather than of coral alone. Remove the grazers through overfishing and the algae take over. Add too much nutrient from pollution and the algae grow faster still. Either way the slow building of the reef stalls.

• Coralline algae: bind the reef frame and help larvae settle.
• Parrotfish and surgeonfish: graze algae that would cover bare rock and young coral.
• Sea urchins: another grazer, useful where fish numbers are low.

06

Where reefs grow well

Reef-building corals need warm, clear, shallow water. Sunlight has to reach the polyps' algae, so sediment that clouds the water is a problem, and so is anything that settles on the coral and smothers it. Fresh water running off the land after heavy rain can also do damage close to shore.

Heat is the other pressure. Water that stays unusually warm for too long stresses coral and can drive it to lose its algae, which turns the colony white. Pollution and runoff add to the strain. A reef can recover from some of this, given time and clean water, but repeated stress wears it down.

For a visitor the practical points are simple. Do not touch or stand on coral, do not collect shells or coral, and do not feed the fish. Dive within the 30-metre limit that Maldivian regulations set. Small habits, repeated by many people, are what keep a reef building.

The reef you swim over on a Maldivian holiday is the top of a structure built over a very long time by animals the size of a pinhead. It is worth looking at closely, and worth leaving exactly as you found it.

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