talking points for nature guides
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Phylum Cnidaria > Class Anthozoa > Subclass Zoantharia/Hexacorallia > Order Sclerectinia
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For nature guides: introducing hard corals
updated Dec 08

Don't step on the hard corals!
It is important to stop and explain hard corals before entering the coral rubble and reef area. Do a good 'first station' to sensitise visitors to the fact that every inch of the shore is alive.

The Zebra hard coral (Oulastrea crispata) is commonly seen even on silty shores in the North on stones and rocks. Look out for them and ensure they are not stepped upon.

What are hard corals?

  • Can you guess? Are they animal, vegetable or mineral?
    • Yes they are animals: each hard coral is a colony of many tiny animals called polyps.

    • Yes, they do appear to be rocks or minerals: each polyp builds a tiny hard skeleton to live in.

    • But each hard skeleton is covered with a thin layer of living tissue. So don't touch them or step on them.

    • Yes, they can be kind of considered vegetables too! Many hard corals contain tiny algae inside their bodies. The microscopic, single-celled algae (called symbiotic zooxanthallae, pronounced 'zoo-zan-tell-ay') undergo photosynthesis to produce food from sunlight. The food produced is shared with the polyp, which in return provides the algae with shelter and minerals.

    • Hard corals grow VERY SLOWLY. Some grow only 1cm a year. So a specimen in that is a metre across can be 100 years old! Even a tiny one (point to a tiny one) only 1cm may be growing for one year already. So please don't step on the hard corals.

Hard coral shapes
Hard corals have different shapes and textures. These are the result of the way the tiny polyps create their tiny skeletons. Use washed up, dead coral pieces to illustrate different corallite shapes.

Hard corals are important to the ecosystem

  • Hard corals are like trees in an underwater rainforest
    • They provide a hard, permanent structure for small animals to shelter and hide.
    • Even when they die, dead corals are a place for animals and plants to grow upon.
    • These small animals in turn are eaten by larger animals.

  • Hard corals NEED a good ecosystem in order to thrive. They need clear water so that sunlight can allow their symbiotic algae to photosynthesise. Sediments not only cloud the water, but also smother the tiny polyps.
Hard corals and you
  • The live fish trade is a major threat to coral reefs. Many 'live' fish are caught either by:
    • Bomb fishing, where a bomb is let off in a reef to stun the fish.
    • Cyanide fishing, where cyanide is released into the water to stun the fish.
    • Both methods destroy reefs. And only a few of the commercially valuable fish are taken. The vast majority of marine life that is stunned or poisoned are left behind in a devastated reef.

  • The aquarium trade also has an impact as wild hard corals are harvested for the trade. Although there is some success in captive breeding of corals, because these grow so slowly, it is cheaper and easier to harvest wild corals for the trade.

  • Coral reefs will also be impacted by global warming.
    • Warmer waters will cause coral bleaching, a result of the polyps losing their symbiotic algae.
    • As the antarctic and arctic ice melts, sea levels are expected to rise faster than corals can grow to keep close enough to the surface for sunlight.
    • Excessive carbon dioxide in the air will also make seawater more acidic which will make it difficult for corals to produce their skeletons.

Hard coral myths to dispel

  • Just because they are hard, doesn't mean hard corals are tough. See above the risks due to human collection and global warming.

Handling tips

Explain corals BEFORE you enter the coral rubble area so that visitors are sensitive to small growing corals and don't step on things that they think are just dead rocks.

Don't touch hard corals: Living hard corals are covered in a thin layer of tissue that is easily bruised!

Use dead corals washed up on the shore to explain internal features, instead of poking living corals.

Don't remove hard corals from their attachment. They will die. Don't remove mushroom corals from the water.

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