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You are here: /main/research/NOWRAMP 2002/features/bleaching


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Coral Bleaching
Posted by Mark Heckman, Waikiki Aquarium - University of Hawai`i, Manoa, Education Team Member
Photography by Jim Watt

Building a reef is a team effort. Reef building corals (1) have evolved a symbiotic partnership with tiny, single-celled marine algae that live inside their bodies. These algae are called zooxanthellae (zo zan thel ay) (2).

Porites evermanni showing partial bleaching. The image to the left shows a bleached coral, Porites evermanni. Not all of the coral is bleached however, and even the sections that are bleached are still alive as is shown by the extended tentacles from the polyps. The lack of color in the bleached areas represents a loss of zooxanthellae that give this coral a brown color.

Imagine that you had plant cells living in your skin. As you breathe and burn up your food products, your body releases carbon dioxide and waste materials - just what a plant needs. The plant cells in your skin would take those up, create more sugars and carbohydrates and give you a big boost of energy that could be used to make more of your body (3). You would have a major growth advantage over others.

Reef building corals have come to rely on their brown, algal plant partners. When a coral "bleaches", it simply means that it does not have the algae present. Typically, the zooxanthellae are lost due to some sort of stress and here is where it gets interesting. Virtually any physiological stress to a coral can cause bleaching. Some (but not all) causes of coral bleaching are:

1. Disease - there are actually a host of coral diseases. Because corals are animals just like us, they and can get sick and then recover or perish naturally.

2. Thermal stress - a temperature rise or fall of several degrees for some critical period of time (hours, days or weeks depending on the amount of change) may cause bleaching.

3. Other environmental stress - pollutants or irritants in the water, changes in the basic pH of the ocean, high sediment loads, from man made or natural sources, etc.

4. Visible and UV (ultraviolet) light stress - calm water for extended periods during summer months leads to too much sun exposure and stress. If a coral is bleached on the top, but not the shaded sides, it may have been UV or visible light stress.

5. Dilution of seawater - freshwater runoff form storm events diluting seawater past optimal conditions for corals can cause stress.

Bleached coral, but it is still alive. Montipora capitata. The image to the left shows a bleached coral (Montipora capitata). From the picture you can see that the tentacles on the polyps are extended and that the coral is still alive. The coral may live for a few weeks without its zooxanthellae but will eventually die if they do not return.

So when it is said that a coral has "bleached", it means that it has lost its zooxanthellae. A coral is only a few cell layers thick and basically transparent like a green house. When the brown zooxanthellae are removed, you can see right through the transparent coral to the white skeleton underneath. The coral looks white (4).

Now the next part of the puzzle - bleached corals are not dead. Often, if the stress is removed and things go well, the coral will regain the zooxanthellae and be fine. But, if not, the coral may eventually die. Define eventually? It depends, but here are some things to consider:

1. Many corals store lipids (fats) that seem to help them survive prolonged periods of stress (think in terms of weeks here) since they cannot rely on the zooxanthellae for their input of extra energy.

2. Some corals grow very quickly (5), so that even if parts of the colony die, re-growth will reclaim the lost area within a year or so.

3. Some bleaching is "normal"; many corals bleach a bit each year from various stress events and recover. We just don't know much about recovery rates at this point.

4. Sometimes one individual will bleach, while its neighbor will not! Obviously there is variation within the coral and zooxanthellae populations. Some deal with stress better than others (just as is true for human populations).

One species of coral is bleached while the others are not. The picture on the left shows a bleached coral head (Pocillopora sp.) among Porites compressa that are not bleached. In this case one species is bleached while the other is not. This shows how species of coral react differently to environmental stress.

5. Corals have survived for millions of years. They have dealt with periodic hurricanes, fresh water floods, hot water events, cold water events and so on. In the long run (hundreds - thousands of years), they are survivors - in the short run (human time scales), a reef may be disrupted for five, ten, fifteen or more years before full recovery is achieved. If the bleaching becomes "chronic" full recovery may not happen until the stress is removed.

There is no doubt that major bleaching events can and have occurred on Pacific reefs in the last twenty years. This bleaching can seriously affect local economies, fisheries and tourism in the time it takes to recover. Long term effects, however, are still being studied.

Coral biology and ecology is a new field. While modern medical research has a background of hundreds of years by thousands of researchers, coral reef research started with Darwin (1842) and built slowly. Many pioneers of modern coral research are still alive, active and working to encourage the coming generation of researchers.

Unfortunately coral reef research is still very poorly funded. This means that many basic questions that need to be answered have not been. These beautiful and vital coral reefs need our help. We need to understand these processes now, not later.

References: Expedition Research Scientists: Dr. Jim Maragos, US Fish and Wildlife Service; Coral Reef Biologist, Dr. Don Potts, University of California, Santa Cruz; Dr. Greta Aeby, Coral Biologist, Hawai`i State Department of Land and Natural Resources.

More information: Coral biology and coral reefs - an excellent series of pages about coral biology, and coral reefs, and coral diseases.

Hot Links

(1) There are many types of corals, some build reefs, some do not. Corals that do not build reefs typically do not have a significant symbiotic relationship developed with the particular type of algae that the reef builders do. The familiar small orange cup corals (various species) that adorn many marine calendars and aquarium exhibits are an example of a non-reef building coral.


(2) Zooxanthellae are dinoflagellates, protozoans that include a number of important single-celled marine species. Many are planktonic, photosynthetic microscopic drifters in the ocean waters. Many luminesce, the bioluminescence of northern waters can be due to Noctiluca, a type of dinoflagellate.


(3) A by product of photosynthesis is oxygen, which we need to process the food we eat into energy and body parts - a nice complementary cycle between plants and animals. If you had symbiotic plants in your skin, they would not only provide extra sugars, but would also release oxygen. For corals, this excess oxygen may not be important, since they are usually not found in oxygen limited environments.


(4) Some corals have accessory pigments. Common colors include blues and reds. When a blue coral bleaches, it will still appear blue; the pigments are part of the coral rather than the zooxanthellae. But the blue will be lighter or more intense than usual since the brown zooxanthellae will be gone.


Ancient coral colony. Porites evermanni at Lisianski Island.(5) Growth rates for corals vary widely. Some thinly branching corals can grow 5 - 7 inches per year per branch. Massive corals on the other hand, have to deposit skeleton over their entire surface and may only add ½ inch per year. Different growth strategies have differing "payoffs". Branching corals may grow fast, but break up easily in storm or wave events. Massive corals can survive storms or waves, but grow over new areas less aggressively.


Talk About It!

Are zooxanthellae plants or animals?

Asked by Diane from Kealakehe Elementary on Feb 23, 2003.
Are zooxanthellae plants or animals? I thought Protozoans were animals and algae were plants. So is this organism both?

Answered by Mark Heckman from Waikiki Aquarium on Jun 30, 2003.
Traditionally, living things were classified into 3 to 5 kingdoms. In the most recent version, Dinoflagellates would have been in the Protista kingdom. They were not classisifed as multicellular Plantae or Animalia, but motile (mobile) single cell organisms, some of which can do photosynthesis -- sort of independent "plantamals" in this case.

With the rise of DNA coding and other advances in biology, evolutionary histories are being continually revised, and one of the more popular systems now is "3 Domains." In this scheme, most of life is in the Eukaryotic domain. These living things (including both plants and animals) all divide their cells the same way (via mitosis).

Within the three domains, the Dinoflagellates are in the Alveolates! The Alveolates have sacs under their cell membranes, and also include the familiar Ciliates (Paramecium), the Forams (of beach sand fame), and others.

So...are they plant or animal? Neither! As in the 5 Kingdom scheme, they are still an independent single-celled type of life that can do photosynthesis like a plant, but move (and sometimes eat) like an animal.

For educational purposes, we may describe them differently depending on the audience. In terms of corals, their most important fuctions are "plant like", so we may refer to them as plants if we do not have time to explain further.

Stepping up one level, I might refer to them as single-celled animals that can photosynthesize. At that point, I usually refer to the diversity within the Dinoflagellate group (which includes the bioluminescent Noctiluca, and red tide linked Gonyaulux, etc.). They may have very ancient links to the plants,but that is far back in history.

For an excellent site on current classification systems, see:

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Halimeda algae.  Click for more details.
Halimeda algae

Spotted Knifejaw.  Click for more details.
Spotted Knifejaw

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