Many people don’t think about the concept of coral as a growing organism. Coral organisms are some of the oldest and longest living animals on the planet. In fact, the geological record shows that ancestors of current coral ecosystems were formed a minimum of 240 million years ago; therefore, the most established coral reefs on the planet today are estimated between 5,000 and 10,000 years old.

If you plan to add a coral to your aquarium, under the optimal conditions and proper care, it will grow and change structure as it ages. Thus it is important to understand coral structure and growth habits before exploring your coral options.


 Coral 101

Corals are indeed animals. While sessile, they rely heavily on their relationship with plant-like algae called zooxanthellae. Combined with the symbiotic, mutualistic relationship with zooxanthellae, coral organisms have built some of the largest biological structures on Earth. To make it short, coral provides a shelter for zooxanthellae, while zooxanthellae essentially provides the coral with nutrients. To learn more about zooxanthellae click here to check out my previous article all about corals and their relationship with zooxanthellae.

Since coral permanently attach themselves to a hard substrate like the ocean floor (sessile) it is no wonder people assume they “take roots” like plants. Corals definitely fall into the animal category of life because unlike plants, animals can’t make their own food. Animals must acquire food or energy from plants and animals by consuming them. Corals do this by extending tiny arms (very similar to a tentacle) out into the ocean to capture their food and bring it to their mouth to eat.


Coral structure and formation

When examining coral structure more often than not you are looking at many individual coral organisms together as they tend to live grouped together in a colony like setting. When looking at what you might call “a single coral” organism you are more than likely looking at hundreds or even thousands of coral polyps. These individual coral polyps are soft bodied and thus vulnerable so they must secrete a hard outer shell or skeleton. This skeleton is made up of limestone or calcium carbonate and this is the park of the coral organism that attaches to the substrate whether it be rock, the ocean floor, or even the skeletons of other polyps (dead or alive).


Coral structure is heavily dependent on the type of coral you are examining. Now, most people imagine hard or stony corals when picturing coral in their head (like the above photo).  Stony or hard coral polyps grow, die, and endlessly repeat this cycle until over time they create a limestone foundation for coral reefs to build upon.  This endless cycle of growth, death, and regeneration for individual polyps gives rise to many substantial coral colonies that have the potential to live a very, very long time.

Soft corals are less common in the minds of many and include organisms such as sea fingers and sea whips. Soft corals are ahermatypes or non-reef building corals and because of this do not always need or have zooxanthellae. As their name suggests, they are soft and bendy resembling trees or plants. They tend to live shorter lives than their shard or stony counterparts. (see below)



Scientists tend to divide coral reef formations into four distinct categories:

  1. Fringing reefs: These are the most common coral reef structure. These reefs grow mostly close to coastlines near islands or continents and are separated from the coast by narrow,shallow lagoons.
  2. Barrier reefs: Like fringing reefs, barrier reefs generally form parallel to the coast line. However, barrier reefs are separated from the coastline by deeper lagoons. They earned their name from the barriers in navigation they can create when at their shallowest point they can often reach the surface. The most famous coral reef in the world, The Great Barrier Reef in Australia, is an example of a barrier coral reef structure
  3. Atolls: These structures are rings of coral organisms that create well protected lagoons normally located off the coastline, out in the ocean. Atolls commonly form when fringe reefs around islands sink into the ocean over time or when the sea level rises around them (typically where underwater volcanoes are found.
  4. Patch reefs: These small reef structures are isolated that grow from the open bottom of a island platform or continental shelf. They most commonly are found between fringing reefs and barrier reefs. They can vary greatly in size and have rarely been witnessed to reach the surface of the water.



Coral reef growth

In order to understand coral reef growth, let’s start from the beginning of the life cycle. Many coral organisms begin as free-swimming larvae. When out in the ocean, the larvae attach themselves to a solid substrate like a submerged rock or another mature coral organism. These larvae develop into the polyp that then, like discussed previously, secretes a calcium carbonate skeleton to protect its soft body from predators. As you can probably imagine, growing a calcium carbonate skeleton can be very energy costly and this is where zooxanthellae is critical for the development of a new coral organism.

Of course different coral types and species grow at different paces. Coral growth is very dependent on the environmental conditions they are exposed to. Corals are extremely in tune with their environment and even the most minor fluctuations can be deadly for them. Temperature, salinity, pH, turbulence, light, and food abundance are just a few factors of coral growth both out in the ocean and in an aquarium setting. The majority of coral species grow best in warm water that ranges from 70-85 ° F or 21-29 ° C.  Clear, shallow waters where sunlight is plentiful are where the most productive coral reefs flourish. Plenty of sunlight allows the photosynthetic zooxanthellae to do their part at maximum potential.

As one can infer, the massive coral organisms you may see out in the ocean or in a commercial aquarium are the slowest growing. These extra-large species grow at a rate between 5 to 25 mm or .2-1 inch a year in length. As opposed to branching corals that can grow much faster. In optimal conditions dependent on species, they can grow up to 20 cm or 8 inches in their branches a year.


Coral in aquariums

As I touched on briefly already, corals require a narrow set of parameters to not only grow and thrive but to survive, so they can be tricky to care for in a personal salt water aquarium. It is important to keep conditions as consistent as possible, as even a 2 degree change could kill certain coral species.

Established aquariums are the best place to introduce your new coral organism. It is very important to do your research before purchasing a coral. Check out this great youtube video on how to successfully have and grow coral in your saltwater aquarium:



 Coral conservation in the ocean

Coral reefs in the ocean are struggling and disappearing at an alarmingly, increasing rate. Pollution and rising ocean temperatures are a major problem for coral reefs. Now, looking at how delicate coral environments are it is no wonder that the massive changes we are seeing in ocean parameters, like temperature , it is no wonder we are seeing coral bleaching in even the most established coral reef ecosystems.

coral infographic

Coral bleaching events are occurring in massive occasions all around the globe due to rising ocean temperatures and pH. It is without a doubt that global warming and pollution go hand in hand with this big problem for coral.  In the most recent news, the National oceanic and atmospheric administration (NOAA) has released a new prediction on the fate of coral reef systems around the globe. They suspect that within the next two years that 6% of coral reefs will be destroyed due to bleaching events. While 6% may seem insignificant at first glances that could lead to more than a third of coral disappearing forever. It is important before buying coral for an aquarium to understand the critical condition coral health is in the ocean, and to understand the responsibility we have in protecting, conserving, and saving coral.  Check out these awesome graphics on the basics of coral bleaching and what to you can do to help coral out in the ocean!

coral infographic


References and photos courtesy of: