What Is Coral Bleaching?

As ocean enthusiasts, many of you probably already know that the ocean’s reefs have undergone extensive coral bleaching over the last decade, but do you know what that actually means?

Corals are animals in the Cnidarian family that are related to jellyfish, sea anemones and hydroids. Specifically, stony corals, which make up most of the world’s shallow-water reefs, are a colony of animals.

Each coral is made up of thousands of tiny individual polyps living inside a hard outer calcium carbonate skeleton. Corals slowly add more calcium carbonate to their skeleton, and over long periods of time these ancient structures can become massive.

Each polypʻs tissue is nearly translucent; the coral's beautiful colors come from algae that lives inside the animal's tissues called zooxanthellae.

These algae, often referred to as the coral’s endosymbiont (endo=internal; symbiont=symbiotic organism), have a mutualistic relationship with the coral, the coral provides protection for the algae, and the algae provides food and oxygen for the coral through photosynthesis.

When coral bleaches (appears stark white) the animal has lost or expelled its algae, and you are seeing through the polyp to the skeleton of the animal. There are several causes that can lead to coral bleaching, however the largest threat corals are currently facing is rising ocean temperatures.

When ocean temperatures rise above a certain level, sometimes as little as 1°C/2°F, the coral's stress response is to expel their symbiotic algae, leaving them without their food source. The corals are still alive when they are bleached and are supported for a short period of time by a small lipid reserve, but they cannot survive without their endosymbionts for long.

If temperatures remain elevated, the polyps will starve and die. However, if the temperatures drop back down into the tolerable range quickly, the coral can recolonize the algae and has a chance at survival and recovery. Ultimately, the real threat is sustained elevated temperatures.

Corals are much more likely to bleach if they are already vulnerable and stressed before sea temperatures rise. Local stressors like overfishing, pollution, and coastal development, which leads to sedimentation, can exacerbate the global stressor of rising ocean temperatures.

It can take up to a decade for corals to recover from a bleaching event, but these bleachings are becoming more and more frequent, leaving little time in between for reefs to recover.

Although coral reefs cover less than 1% of the sea floor, they support 25% of the oceans biodiversity. They are also incredibly important to coastal communities that rely on fishing, tourism, and the storm protection reefs provide.

Coral reefs are vital to the health of our oceans and coastal areas, but it is estimated that the oceans will be too warm to support coral reefs by 2050. These amazing and resilient animals first evolved around 240 million years ago and have survived through many different climate regimes.

Let’s hope that they find a way of persisting through the unprecedented anthropogenic forces acting upon them now.


What Can You Do to Help Coral Reefs?

As the number 1 cause for coral decline is increased sea surface temperatures caused by the emissions of greenhouse gasses. We need strong environmental policy that will curb those emissions immediately.

That means we need to:

  • Vote for politicians whose priority is climate action.

  • Write to current policymakers, urging them to take more drastic measures to reduce emissions and slow climate change.


In your personal life, you can:

  • Eat an ocean-friendly diet - sustainably caught seafood, not supporting commercial farming that emits methane and covers the earth in pesticides.

  • Reduce plastic consumption, particularly single-use plastics.

  • Use chemical-free personal care products.

  • Dispose of your waste accordingly.

  • Be an ocean advocate and educate your friends and family on this issue!

It is important to remember that conservation needs support from people in all fields, not just scientists.


Written by Jessica Glazner (@jess_mermaid)


Jess is a marine biologist from Hawai`i. She is currently a master’s student at the University of Hawai`i, Hilo studying coral reef resilience and completing an internship with The Nature Conservancy’s Hawai`i Chapter.

Jess is also a scuba instructor and owns her own dive operation, Liquid Cosmos Divers, on the Kohala coast of Hawai`i island. With over 3000 dives in Hawai`i she has an intimate understanding and connection to the local reefs, and this relationship is what fueled her to return to school, so she can have a significant impact on coral reef conservation.



24 views0 comments