When coral polyps are stressed they become bleached by ejecting tiny algae cells that normally live inside them and provide food and color. This somehow compromises their relationship with surrounding fish communities and reduces the overall dynamism and health of the reefscape.
What is Coral Bleaching?
Published: February 26, 2019
Onscreen: What is coral bleaching? Reefs are built by microscopic animals called coral polyps.
Ruth Gates: A polyp is a little type of animal. A ring of tentacles around a mouth. That is over a stomach. You’ll see thousands of repeated polyps that create a coral colony. Inside the polyp’s tissue live tiny plant-like algae cells. Using the sun’s energy they make food for the coral polyp. In return the coral provides carbon dioxide and shelter.
So the animal and algae combination is absolutely mutualistic and co-dependant. That means both benefit from the relationship and they die if they’re separated.
Onscreen: If the water around the coral gets too warm the animal ejects algae cells.
Gates: The animal now sees the algae as being a foreign body and tries to get rid of it as quickly as possible. Which is a bit of a problem when you think that it’s the algae that’s feeding the animal.
Onscreen: The result is coral bleaching.
Gates: What we’re really talking about here is the visual paling of the normally dark-brown corals turning white. A bleached coral is a coral that is likely to die. A bleached coral somehow doesn’t communicate well with the fish communities. There’s some change in the overall dynamic nature of the reefscape. And you can detect it when you’re on a bleached reef prior to it dying. It is different, it is more resilient, it is quieter. It doesn’t feel right. We have lost 50% of the world’s reefs in the last 30-40 years. That’s...I mean...even when I say it, I have to be honest I still find it shocking and I want to find a reason for that figure to be wrong but it’s not wrong.
Decoding the Weather Machine
Produced and Directed by: Doug Hamilton
Co-Produced by: Caitlin Saks
Director of Photography: Paul Atkins
Digital Producer: Brian Kantor
Additional Footage: Brett Lewis — QUT, David Hannan, Ocean Ark Alliance, The Ocean Agency / Exposure Labs
© WGBH Educational Foundation 2019