Mochi Isle
Coral bleaching · Coral UnderseaScience for kidsWhy does coral turn white?
In short: Stress such as unusually warm water can make coral push out the algae living inside it, so its white skeleton shows through. This is coral bleaching. Bleached coral is not always dead, but recovery depends on how long the stress lasts.
How it works
Healthy coral gets much of its colour from the algae living inside it. When the sea stays warmer than normal for weeks, the partnership breaks down and the coral expels the algae. Coral tissue is almost see-through, so the white limestone skeleton underneath shows through.
Without its algae, the coral also loses its main source of food, a bit like suddenly losing most of your meals. It is still alive and can catch some plankton with its tentacles, but it becomes hungry, weak and more likely to get sick.
If the water cools back to normal soon enough, algae can move back in and the coral slowly regains its colour. If the heat lasts too long, the coral may starve and die. Pollution, muddy water and very low tides can also cause bleaching.
Where to see it in New Zealand
New Zealand has no large coral reefs, but across the Tasman Sea, Australia's Great Barrier Reef has suffered several mass bleaching events. Scientists in New Zealand also watch for marine heatwaves, because unusually warm seas can harm local sea life too.
Try it together
Fill two identical cups with the same amount of water. Put one in the sun and one in the shade, then measure them with a thermometer after an hour. Also try a shallow dish versus a deep cup. Which warms fastest? Shallow, still seawater heats up quickly too.
Kids also ask
Is bleached coral dead?
Not necessarily. Freshly bleached coral is usually still alive but starving. If conditions improve in time, it can recover. If the stress goes on too long, it may die, and seaweed soon covers its skeleton.
How can we help coral?
Saving energy and walking or biking more helps slow global warming. When snorkelling, never stand on, touch or take coral, and keep rubbish out of the sea. Small habits add up.
In the game, “Coral bleaching” is a nature card on Coral Undersea.
Your child finds this card by reaching the right place or meeting the right weather on the island, and lights it up by answering the question on the back. Tap the button below and the game shows your child which island it is on and how to find it. Mochi Isle is a browser learning game for ages 5 to 18; the main island and the daily lesson are free.
Find “Coral bleaching” on the island ↗ All cards from Coral Undersea