Corals and Coral Reefs

I have recently launched a new interpretative walk at my workplace at the reef flat accessible by foot via the beach. So in preparation of the walk, I have done much reviewing on corals and coral reefs, and even learned new things along the way. And to help gather my thoughts for this reef flat walk, I thought I should share what corals and coral reefs are on this blog. Enjoy.

The anatomy of a coral polyp. This form of polyp is
known as a stationary polyp.

A reef is simply something located beneath the water's surface. A reef can be abiotic (not a living thing) ie. a ridge of rocks, submerged sandbanks, and shipwrecks, or biotic (living organism) ie. coral reefs. As the name suggests, coral reefs are made up of... CORALS! And yes, corals are living organisms.

Corals are marine invertebrates of the phylum Cnidaria, which sea jellies and sea anemones are part of too. Being cnidarians, corals also have stinging cells for feeding and defense purposes.

The coral head, which is what we see and enjoy on our snorkeling and SCUBA diving trips, and what creates a coral reef, is the colony/colonies of Scleractinian corals - corals that form calcium carbonate exoskeleton (aragonite) for protection. (And since the calcium carbonate exoskeleton is hard and sturdy, Scleractinians are commonly known as hard or stony corals.) Inside the coral exoskeleton are genetically identical individual coral polyps, the coral animal itself.

In Bahasa Malaysia, corals are known as Batu Karang, and "batu" means "rock/stone". I think some exclusive Bahasa Malaysia speakers may just think that corals are really just rocks and will definitely treat coral reefs differently compared to those who know the difference. Just something that conservationists should have in mind when working with local communities.

There are two types of hard corals: hermatypic corals and ahermatypic corals. Hermatypic corals are reef builders, the one that we are all most familiar with (and the corals focused in this entry), and ahermatypic corals are hard corals that do not form reefs.

Top: Some hermatypic brain corals (Mussidae?) and hump corals (Porites sp.?). Bottom: Some ahermatypic solitary mushroom corals (Fungiidae). Both photos taken in Datai bay, Langkawi.

 Another significant difference between hermatypic and ahermatypic corals are the presence of photosynthetic single-celled dinoflagellates, known as zooxanthellae, in hermatypic corals. The zooxanthellae (Symbiodinium) and corals have a mutualistic relationship (benefiting both parties) with each other, whereby corals play host and protect the zooxanthellae within the coral polyps and exoskeleton while excess food created from photosynthesis get transferred into the coral polyps' cells for metabolism, growth, and reproduction. And since the corals respire to produce carbon dioxide and water, the zooxanthellae also benefit from easy access of inorganic carbon and a reducing agent for photosynthesis (visit this page for an animation to easier understand what you just read and this and this pages to learn how corals acquire zooxanthellae). (Acquiring food from their endosymbionts are not the only way corals get food. Being cnidarians like sea jellies, corals have nematocysts as seen in the anatomy figure above, which can be used to stun their prey, like tiny fish, and digest them. But since the synergy between the corals and zooxanthellae is so efficient in most reefs, it is probably just easier for the corals to get food from their microalgae buddies.)

Since the photosynthetic zooxanthellae require sunlight, coral reefs are found mostly in clear shallow waters and in tropical warm waters where sunlight is available all (or almost) year round. That is why recreational divers do not actually have to go super deep on their coral reef dives. (And zooxanthellae are responsible for the colourful submarine landscape!)

Global distribution of coral reefs. You can also check out this really cool Ocean Data Viewer on the global distribution of coral reefs. Both maps show that coral reefs are situated within the tropics.

Importance of Coral Reefs
Other than being great tourist attractions, coral reefs are home to a myriad of organisms (which is also a tourist attraction, heh) that is of importance to our livelihood, especially to coastal areas. Coral reefs are home to high biodiversity and high abundance of fish, crustaceans, mollusks, sponges, sharks and rays, echinoderms, planktons, macroalgae and etc. (apparently 25% of the ocean's biodiversity can be found in coral reefs)!

We can harvest/use these coral reef organisms for seafood (15 tonnes of seafood per km each year from properly managed coral reef), production or in production of many things (uses of seaweed), medical research (antibacterial activity in marine sponges), and they are tremendous carbon sinks, just like rainforests!

Coral reefs are important carbon sinks because they trap a lot of atmospheric carbon dioxide by creating their aragonite calcium carbonate exoskeleton. Atmospheric carbon dioxide enters the ocean everyday to form carbonic acid, which is made of a bicarbonate anion and a "free" hydrogen cation. The corals will use the bicarbonate anions and abundant calcium cations in seawater to form their calcium carbonate exoskeletons, which will then house all the other living things that we benefit from.

CO_{2 +} H₂O ↔ H₂CO₃
Ca²⁺ + 2HCO₃^- --> CaCO₃ + CO₂ + H₂O
Top: Chemical equation of atmospheric carbon dioxide reacting with seawater to form carbonic acid. Bottom: Chemical equation of the combination of calcium cations with the bicarbonate anions from the carbonic acid resulted from the reaction between atmospheric carbon dioxide and seawater.

Ocean Acidification on the Corals' Exoskeleton

You may think that the increase of atmospheric carbon dioxide (ie. from the combustion of fossil fuels) will benefit the corals and the other biota in the coral reef ecosystem because there will be more carbon dioxide available for the creation of calcium carbonate exoskeleton. The increase of carbon dioxide levels in seawater will decrease its pH (becomes more acidic, but not <7 acidic, just higher concentration of hydrogen cations), which can actually hinder the coral calcification process to build reefs; corals must then use more energy to form their exoskeleton. (Read this article for more in depth explanation with plenty of chemical oceanography jargon.)

Nevertheless, coral reefs remain an important carbon sink for now, and hopefully for many years to come.

There is still much to write about coral reefs, especially the threats they face in a period of increase human development and climate change. (The example on how the increase in acidity of seawater affects the calcification of corals is just one of the many, many threats faced by corals.) I guess I will just have to save that topic for another time.

I hope you enjoyed this entry. And like all the other entries in this blog, please let me know how I can improve in any way possible.

Enjoy the ocean responsibly!