How Coral Reefs Grow and Shape Coasts

A coral polyp is a small animal related to jellyfish and sea anemones. Many reef-building species live in colonies and secrete cups of calcium carbonate beneath their soft bodies. New generations grow on the remains of earlier skeletons, gradually expanding the reef.

Most tropical reef-building corals contain microscopic algae living inside their tissues. The algae use sunlight to photosynthesise and share part of the resulting energy with the coral. In return, they receive shelter and access to carbon dioxide and nutrients.

This partnership helps reefs grow quickly in bright water, but it also sets strict environmental limits. Muddy water blocks light, while heavy sediment can cover living coral. Reefs often thrive in clear, shallow tropical seas where sunlight reaches the algae and temperatures remain within a suitable range.

A reef forms a rough barrier that causes incoming waves to break before they reach the shore. This reduces some wave energy and can protect lagoons, beaches and coastal settlements. The amount of protection depends on reef width, depth, shape and condition.

Waves and organisms break pieces from coral skeletons and shells. Currents move this carbonate sediment into beaches, sand bars and islands. Some low tropical islands are therefore built largely from material produced by reef organisms and rearranged by waves.

Reefs also create sheltered water behind them. Fine sediment may settle in lagoons, while seagrass beds and mangroves develop in nearby habitats. These linked environments exchange sediment, nutrients and living organisms. Geography, naturally, has turned one coastline into a group project.

Fringing reefs grow close to a mainland or island shore. Barrier reefs lie farther offshore with a lagoon between reef and land. Atolls form rings around central lagoons and are often connected with volcanic islands that slowly sink or erode while coral growth continues near sea level.

Reefs face stress from unusually warm water, pollution, destructive fishing, disease and changing ocean chemistry. During bleaching, corals lose many of their algae and turn pale. They can recover if conditions improve, but long or repeated stress may cause death.

When studying a reef, check:

• •Water depth and clarity.
• •Temperature and sunlight.
• •Distance from the shoreline.
• •Wave direction and energy.
• •Nearby lagoons, islands and sediment.

Atolls puzzled early naturalists. Why would reefs form neat rings in the open ocean, rising from deep water where coral cannot grow? During the voyage of the Beagle in the 1830s, Charles Darwin proposed an answer. A reef first forms as a fringe around a volcanic island. As the island slowly sinks, the coral keeps growing upward to stay near the sunlit surface, turning the fringe into a barrier reef with a lagoon behind it. Eventually the island disappears beneath the lagoon, leaving only the ring of reef.

Darwin could not check his idea directly, and it remained debated for about a century. The test came in the early 1950s, when deep boreholes were drilled on Enewetak Atoll in the Marshall Islands. The drills passed through more than a kilometre of reef limestone before reaching volcanic rock underneath, exactly what the sinking-island explanation predicted. Plate tectonics later explained why such islands sink: ocean crust cools and becomes denser as it moves away from where it formed, and the heavy volcano also presses down on the crust beneath it.

Many large corals grow steadily for centuries, and they lay down their skeletons in a way that resembles tree rings. When a slice of a massive coral is X-rayed, it shows alternating bands of higher and lower density, usually one pair for each year, because growth changes with the seasons. Scientists drill long cores from living coral heads without killing them and count the bands back from the present, giving a timeline that can stretch back hundreds of years in a single colony.

The chemistry of each band adds more detail. As a coral builds its skeleton, it takes in small amounts of other elements, such as strontium alongside calcium, and the ratio between them changes with water temperature. Oxygen isotopes in the skeleton also respond to temperature and rainfall. By measuring these signals band by band, researchers can reconstruct past sea surface temperatures and climate events such as El Nino episodes in regions where no thermometer records exist, and compare past conditions with the warming seen today.

Coral colonies grow by budding, as each polyp divides to make new copies of itself, but reaching new places depends on reproduction that spreads further. On the Great Barrier Reef, many coral species spawn on the same few nights each year, usually a few days after a full moon in late spring or early summer. Colonies release bundles of eggs and sperm that float to the surface, where fertilisation happens in open water. Releasing everything at once improves the chances that eggs and sperm from different colonies meet, and overwhelms predators that cannot possibly eat them all.

The resulting larvae drift for days or weeks before settling. They need a hard, clean surface, and many respond to chemical cues from crusty pink algae called crustose coralline algae, which signal a good place to attach. Once settled, a larva becomes a single polyp and begins building a skeleton. Corals can also spread when storms snap off branches that land elsewhere, reattach and keep growing. Reef restoration projects copy this by raising coral fragments in underwater nurseries before planting them out, although such work is slow and cannot replace healthy water conditions.

A reef is never simply growing. Its height reflects a budget, with living organisms adding calcium carbonate on one side and other organisms grinding it away on the other. Fast branching corals can extend by ten centimetres or more in a year, while heavy, rounded colonies often add only about a centimetre. Crusts of coralline algae cement loose pieces together. Yet the reef surface as a whole rises far more slowly than any single colony grows, because a large share of each year's new skeleton is broken down, scattered or dissolved before it can be locked into the framework.

The grinding is done by a surprising crowd. Parrotfish scrape algae from dead coral with beak-like teeth, swallowing rock along the way and passing it out as fine sand. Studies in the Maldives have identified parrotfish as major producers of the sediment that builds some reef islands. Boring sponges, worms and sea urchins also tunnel into or rasp away the limestone. On a healthy reef, building outpaces this erosion. After heavy coral death, though, the erosion carries on while new growth stalls, so the reef can begin to lose height. A flatter, lower reef then lets more wave energy reach the shore behind it.

Coral reefs grow as colonies of polyps add calcium carbonate skeletons in warm, clear and shallow water. The resulting structures reduce wave energy, produce sediment and help shape lagoons, beaches and islands. Follow the link from living coral to physical landform, and reefs become a vivid example of biology building geography.