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ELI5 Yahaaa

Both, in a way

Is Coral a Rock or an Animal?

Coral is millions of tiny animals living in a stone skeleton they built themselves.

A coral reef Millions of tinyanimals, each in a stonecup
The rock is the house, not the animal.

  1. Step 1 of 5

    Each polyp is a tiny animal.

    The reef One polyp: a mouthsurrounded by tentacles

    A coral polyp is a small sac with a mouth ringed by tentacles — a relative of jellyfish and anemones. Most are a few millimetres across.

  2. Step 2 of 5

    They build stone houses around themselves.

    1 Minerals inseawater2 The polypbuilds a stonecup3 Skeletons pileup into a reef

    Each polyp pulls calcium and carbonate from seawater and builds a hard cup. The reef is those skeletons, accumulated over centuries.

  3. Step 3 of 5

    Algae live inside them and do the cooking.

    The polyp gives Shelter and nutrientsThe algae give Most of the food

    Tiny algae live in the polyp's tissue, photosynthesise, and hand over most of the sugar. In return they get shelter and raw materials.

  4. Step 4 of 5

    The algae are the colour.

    Healthy Algae inside — colouredand fedBleachedAlgae expelled — whiteand starving

    Coral's brilliant colours come mostly from those algae. Stressed by heat, the polyp expels them and turns white — that is bleaching.

  5. Step 5 of 5

    Reefs support a huge share of ocean life.

    Share of ocean floorunder 1%Share of marine speciesabout 25%

    Coral covers well under one percent of the seafloor but shelters around a quarter of all marine species — because it creates structure.

    A reef the size of a house may have taken a thousand years to build, one millimetre at a time.

The short version

Coral is a colony of tiny animals living in stone cups they build, fed by algae inside their own tissue.

Try it yourself

Look at a piece of coral skeleton closely. The tiny cups are the individual apartments each polyp lived in.

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