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Life’s A Bleach – In Honour of Earth Day

Because it was Earth Day last Friday, I thought it an appropriate time to slot another environmentally focused article in before we resume our global diving trek.

There are so many topics we could look at (melting ice caps, over-fishing, toxic pollution, dynamite fishing, species close to extinction, great garbage patches or vortexes in mid-ocean gyres, etc.), but we are going to limit ourselves to one massively worrisome issue in this posting: Coral Bleaching.


Corals are principally found in the world’s reef systems.  These systems are, in fact, expansions of really old coral colonies.  Coral reefs are some of the most complex ecosystems found on our planet.  Although they cover less than 1 percent of the ocean floor, our reefs are home to over 25 percent of all marine life in the oceans.  In the last 25 to 30 years, the reefs have suffered some horrendous environmental tragedies.  Some of those disasters have simply destroyed large tracts of corals, but most recently the biggest threat is the epidemic of coral bleaching.  It doesn’t take any particular mental agility to realize that the loss of our coral reefs will also negatively impact the 25 percent of ocean life that calls the reefs home.

(Credit: YellowMagpie)

(Credit: Barry B. Brown)

Before we get into the nitty-gritty of why we think coral bleaching is occurring and what steps are being done to help, let’s dig a little deeper into what a coral reef actually is.

You may be surprised that corals are living creatures, part of the Animal Kingdom.  Well, more precisely, corals themselves are not a living animal.  When we see a single solid coral “head” structure, it is, in fact, a colony composed of thousands upon thousands of genetically identical tiny living soft-bodied organisms, called “polyps”.

(Credit: NOAA)

Although each polyp is an individual creature, once they become part of a colony, they effectively become a single organism.   The individual coral polyps are generally tiny, typically being only a few millimeters in diameter.  These fascinating little critters (most closely related to anemones and jellyfish) work so closely in tandem with each other, that they even reproduce collectively.  Some polyps can even reproduce both sexually and asexually, some of them are even hermaphrodites and do it all themselves and some simply split themselves to form new polyps!

One of the most spectacular effects though is to see corals spawning at night:

Blue Planet on YouTube

There are also two “families” of coral: “hard” and “soft” corals.  The polyps of the different types act slightly differently too.

(Credits: and

In “hard” coral, which are the reef-builders in the oceans, the polyp secretes a hard, protective skeleton of calcium carbonate (limestone), called a “calicle”.  When multiple identical polyps amass (individual polyps can survive alone, but this is relatively rare), the limestone secreted by the polyps, as well certain other functions (such as a system of gastrovascular canals between polyps) connect all of the polyps together, thus becoming one coral “head”.  Over time, as these colonies grow, they join other colonies and thus eventually become a reef.

(Credit: Natural History Museum – UK)

“Soft” coral instead develops a woody core and a fleshy cover and acts more like plant or tree-like structures under water.   These coral types don’t build the reef structures, but they usually can be found co-existing in the reefs.

(Credit: Sierra Club)

A fascinating fact about hard corals is that they are, for the most part, in fact, translucent (clear).  They get their colours as a result of the manner in which they get the nutrients and energy they need to survives.  This is because most corals tend to feed in only two ways: using the by-products created through the process of photosynthesis; and, by predation.

Photosynthesis is the primary source of nutrients for coral polyps.  The energy they gain from the nutrients comes from a symbiotic relationship that they have developed over millennia with photosynthetic, unicellular, zooxanthellae algae.  The majority of hard coral polyps act as hosts to the algae.  It is the algae that carry the colour we see in the multi-hued vista underwater on a sunny day.  As photosynthesis occurs, the algae produce nutrient rich by-products that the polyps feed on.  Additionally, the algae absorbs any carbon dioxide naturally released by the polyps and through the process of photosynthesis produces oxygen, which is then used by the polyps. The whole process is crucial to the polyps, which tend to live in fairly nutrient-poor waters.  It is also highly beneficial to the algae, which gains a relatively immobile and sheltered environment.  The photosynthetic needs of the algae also play a huge part in the locations of reefs, as shallow, tropical waters receive a lot of sunlight, a critical factor in the process.

The other method by which a lot of coral feed themselves is a simple predatory function.  Usually during the night, many coral also extend nematocyst-capped tentacles that capture plankton and even small fish.

(Credit: George Cathart)

So what is coral bleaching and how is it caused.  Coral bleaching provides a fascinating insight into the symbiotic relationship between the coral polyps and the algae.  As we have seen, the majority of corals live in warm and shallow tropical waters.  There are some deep-water corals (such as the Darwin Mounds of Cape Wrath, Scotland – where some corals have been found as deep as 3000 m/9,800 ft), but deep water corals are few and far between.

(Credit: DEEPSEAS Group)

Outside of those few hardy cold and deep corals, almost every coral reef has some standard, but pretty specific requirements.  Given that most of the hard corals rely on the zooxanthellae algae for food and oxygen, the needs of the algae become one with the coral.  The algae need sunlight to survive and photosynthesize, thus the reason why most coral reefs are found in shallow waters.  In order to maximize the sunlight absorption, the water also needs to be fairly clean and clear.

The kinds of coral that are reef-builders also work best in warm waters, so again, this is why most reefs are found in tropical climates.  Finally, ocean corals obviously live in salt water, so they don’t do well with fresh water nearby.

So briefly recapping, in order for corals to remain healthy and continue to grow, they need shallow, warm, clean, clear and salinated water.

In the last half a century a lot has happened to change virtually all of those characteristics of a healthy coral environment.  First of all, water temperatures have been steadily rising, largely due to global warming, but also influenced by such phenomena as El Niño.  Especially in the summer, the temperatures simply become too hot for the corals to remain healthy.


The increase in water temperatures has been identified as probably the largest single contributing factor to coral bleaching.

On top of the warmer waters, however, there has also been a massive increase in water-borne pollution in modern times.  Pollution comes in many forms.  In some cases, the pollutants themselves become sedimentary and coat the corals, blocking out the sunlight needed by the symbiotic algae.  In other instances, the pollution simply clouds the waters generally, again preventing sunlight from reaching the coral.  Finally, the pollutants may include sewage or fertilizers, raising nutrient levels and drastically altering the ecological balance of the coral community.  Where the corals are around landmasses, increased human activities on those islands have also tended to create more runoff.  The runoff itself may again cloud the waters, add to the sediment levels, carry harmful or toxic pollutants (like fertilizer) or introduce fresh water onto the coral reefs.

The bleaching itself occurs whenever the coral is threatened by changes to its environment.  Basically, the coral goes into self-preservation mode.  The usually harmonious algae places a fair amount of stress on the individual polyps.  When conditions are good, the polyps are able to handle this stress.  When conditions worsen, the polyps are unable to handle this stress, as well as the extraneous environmental stressors.   In reaction to deteriorating environmental conditions, therefore, the polyps release the algae as a short-term stress relief, on the basis that they can always reacquire the algae again at a later time.  As the algae provide much of the pigmentation of the coral structure (particularly the brown hues), when they are expelled on mass in bad environmental conditions, the coral polyps resume their translucency and we see the corals as “bleached”.  If those conditions persist, the polyps eventually die.

The reality is that when we see large reef structures, there are millennia of dead polyps already in the reefs.  After polyps die they eventually become sediment that sinks through the reef and their calcified exoskeletons remain.

The difference between the regular death of a poly and coral bleaching, however, is that individual polyps don’t die at the end of their natural life cycles., but due to the environmental changes.  These deaths occur on mass and cause damage to the reef systems from which they simply cannot recover.

Without our reefs, there would be a catastrophic shift in the ecosystems throughout our oceans.

What can we do to prevent it?  Well, with major temperature increases, it is incredibly difficult to resolve coral bleaching at the grass roots level, but every little helps.  Some dive spots globally have initiated plans to at least encourage further reef growth, in efforts to stave off such a decline in coral reefs. Most, effectively create coral-building friendly environments, often using basic structures for the corals to grow on, even sometimes encouraging the growth with electrical pulses.  The success of many of these projects is simply too early to tell.

(Credit: GaiaDiscovery and TreeHugger respectively)

While I promoted signing petitions and getting involved in stopping shark finning in the last post, that isn’t as simple when it comes to the reefs. But you can still help do your bit.  Get online and find out about reef preservation and conservation practices out there that you can support.  Heck, if you can, even maybe plan a holiday where you go and help out on one of the world’s many projects underway.  Forget Earth Day, make it Earth Vacation…..better yet, make it Earth EveryDay.