Invertebrates
Coral
Anthozoa · also called corals, coral polyp
A coral is not a thing but a crowd. Each of the hundreds to millions of polyps in a coral colony is a separate animal, a soft cousin of the jellyfish sitting in a cup of limestone it built itself. The reef-building kinds farm algae inside their own tissue for food, and over thousands of years their skeletons pile up into the largest structures any living thing has ever made.
Overview
Corals are members of the class Anthozoa — cnidarians, related to jellyfish and sea anemones. A coral is a colony of polyps, each a tiny animal with a ring of tentacles around a mouth, connected by shared tissue. Stony corals secrete a limestone skeleton beneath themselves; over millennia these accumulate into reefs.
Farming the sun
Reef-building corals house single-celled algae (zooxanthellae) in their tissue. The algae photosynthesise and pass most of the resulting sugar to the coral — often 90 per cent of its food — which is how corals grow fast enough to build reefs in clear, nutrient-poor tropical water.
The largest living structures
The Great Barrier Reef runs over 2,300 km and is visible from space, the largest structure built by life. On many reefs, most coral species spawn on the same few nights a year, cued by temperature and moon, turning the sea into an underwater snowstorm.
Bleaching
When water stays too warm for weeks, corals expel their algae — bleaching — losing colour and their food supply. A bleached coral is starving, not yet dead, and dies if the heat does not ease. Mass bleaching now hits reefs every few years.
Status
Corals as a class are Not Evaluated, but reef-building corals are in a global crisis, trend decreasing — the world has lost roughly half its shallow coral since the 1950s, from marine heatwaves, ocean acidification, land pollution, destructive fishing, disease, and crown-of-thorns starfish outbreaks. Reefs cover under 1% of the sea floor but support a quarter to a third of all marine species.
Coral: common questions
Is coral an animal, a plant, or a rock?
Primarily an animal, with elements of the other two. Each coral polyp is a true animal, a cnidarian closely related to sea anemones and jellyfish, that catches food with stinging tentacles and has muscle and nerve tissue. The plant part is the symbiotic algae living inside the polyps of reef-building corals, which do the photosynthesis that feeds the colony and give it its colour. The rock part is the skeleton, a framework of calcium carbonate that stony corals secrete beneath and around themselves and that remains, reef-building, long after the polyps die. So a piece of living coral is an animal colony farming algae on top of the limestone tombs of its ancestors.
What causes coral bleaching, and can a reef recover?
Bleaching happens when corals are stressed enough to expel the symbiotic algae in their tissue, most often by water that stays a degree or two above the normal summer maximum for several weeks, sometimes made worse by strong sunlight. Without the algae the coral loses its colour, turning white, and loses the sugars that supply most of its food, so it begins to starve. If the heat passes quickly, corals can take the algae back up and survive, though often with reduced growth and reproduction for a year or more. If the stress is prolonged or repeated, the corals die, and a reef that dies back can take a decade or longer to rebuild even under good conditions, which is why the rising frequency of marine heatwaves is so damaging.
Why are coral reefs so important if they cover so little of the ocean?
Because they are the ocean's richest habitat by far. Reefs occupy less than one per cent of the sea floor but provide the food, shelter, nursery grounds, and breeding sites for something like a quarter to a third of all marine species at some stage of life, including a large fraction of commercial fish. For people, reefs support fisheries that feed hundreds of millions, draw tourism worth tens of billions of dollars a year, and act as breakwaters that absorb most of the energy of waves before they reach the shore, protecting coastlines and the communities on them from erosion and storm surge. Losing reefs means losing all of that at once.
Can damaged coral reefs be restored?
In a limited way, and it is an active field. Techniques include growing coral fragments on underwater frames or ropes in nurseries and transplanting them onto degraded reef, spreading coral larvae from mass spawning onto damaged areas, and stabilising broken reef structure. There is also research into breeding or conditioning corals to tolerate warmer water, and into protecting the algal symbionts. These methods can rebuild small areas and are valuable for buying time and keeping species going, but they operate at the scale of hectares while reefs are being lost at the scale of thousands of square kilometres. Most scientists stress that restoration cannot substitute for cutting greenhouse gas emissions and reducing local pollution and overfishing.