Animal · Australia, Indonesia, Philippines
Stony coral
Scleractinia — the name this animal has in the language scientists everywhere share.
A stony coral is not a plant but a colony of tiny animals, each one a soft polyp sitting in its own cup of hard white limestone. Inside every polyp live even smaller algae that hand over food in return for a safe place to stay.
Picture credit: Jstuby (Public domain) · Nhobgood (CC BY-SA 3.0) · Wikimedia Commons
Where does the stony coral live?
Home: warm, clear, sunlit shallow seas.
In the wild it lives in these countries: Australia, Indonesia, the Philippines, the Maldives, Egypt, Mexico, Belize, Fiji.
The short profile
Fully grown: polyps a few millimetres, colonies many metres. It weighs about 0 001 kilograms. On the menu: meat.
The story behind it
Each polyp is a soft tube with a ring of tentacles around its mouth, and at night it stretches them out to catch drifting plankton. By day it lets its algae work instead: they use sunlight to make sugar, and a large share of the polyp's food comes straight from them. Under the living skin the polyp builds a cup of limestone, and as new polyps bud off, the cups stack into branches, plates and boulders that we call a reef. Branching corals can grow about ten centimetres in a year, while heavy dome-shaped ones add barely one centimetre, so an old reef head may be older than your grandparents' house. When the sea gets too warm for too long, the polyps throw out their algae and the white skeleton shows through the clear skin; that is coral bleaching, and a bleached coral is starving, not yet dead. If the water cools again soon enough, algae move back in and the colour returns. On many reefs whole colonies spawn on the same few nights a year, releasing clouds of eggs and sperm that turn the water milky.
Find it on the globe
Stony coral is there as a tiny animated figure that breathes and looks around. From far away the planet is only a blue ball; detail arrives as you zoom in, and this one appears once you are close enough. Open it on the globe.
It sits at its real coordinates, not near them — the same numbers listed in the facts above. Switch off the layer for this kind of place and it disappears again; the buttons for that are down the right-hand edge.
Questions children ask
How big is a single coral polyp?
Most polyps in a colony are only a few millimetres across, about the width of a pencil lead. Millions of them together build the branches and boulders that make a reef look like a whole landscape.
Where do stony corals grow?
Reef-building stony corals need warm, clear water with plenty of light, so they grow in shallow tropical seas such as the Great Barrier Reef, the Red Sea and the Caribbean. Other stony corals live cold and dark, hundreds of metres down, and those ones have no algae at all.
What exactly is coral bleaching?
The colour of a coral comes mostly from the algae inside it, so when heat stress makes the polyps push those algae out, the bare white skeleton shows through. The coral is still alive at that point but has lost most of its food supply, and it only recovers if cooler water lets the algae return.
Where these numbers come from
Every figure on this page was checked against public reference sources before publication. Where sources genuinely disagree, the text says so.
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