Difference between hard and soft coral, situation of coral in Kenya

What is coral? Is it a plant, an animal or a rock? A question I have asked many times when providing marine education lessons.

The answer is simple and complex at the same time, so we will break it down.

Coral is a sessile animal that relies on its symbiotic relationship with a tiny algae plant called zooxanthellae to build the largest living structures on Earth. So even though they may exhibit some characteristics of plants and are often mistaken for rocks, coral is in fact an animal!

They fall into the phylum Cnidaria and the class Anthozoa. They are close relatives of jellyfish and anemones, all of which have stinging tentacles.

Perhaps you’ve come across this meme where mum, dad and baby were playing the game ‘under the sea’. Baby wanted to be a shark, dad a scuba diver, and guess what a tired mama chose to be, of course a coral! So she could just sit there and not have to move around! Genius!

Mama was wise because nearly all coral species are permanently attached to the seafloor! This means they are stuck in one place. They don’t swim or move around to evade predators or pursue their prey. They rely on their stinging cells for protection and to immobilize their prey. 

Close up of a coral poly; Image from National Geographic Society 
Photographed by Jennifer Bright, MYSHOT 

Think of a coral structure as an architectural design!

Coral reefs have been termed as exquisite examples of marine architecture and this is why!

It all begins with a soft bodied animal called a polyp. One coral is made up of hundreds to thousands, or even millions, of tiny genetically identical polyps living together as a colony. When one polyp multiplies, it secretes a hard outer skeleton of limestone (calcium carbonate) that attaches either to rock or the dead skeleton of other polyps.  As the polyps live, reproduce, and die, they leave their skeletons behind. A polyp will go through a cycle, grow, reproduce and die, leaving new coral limestone skeleton behind.  The cycle repeats over time and slowly a coral reef is formed as new living polyps continue to cover layers of these skeletons.

Have you ever heard of the terms “hard coral” and “soft coral”? They are the general classification of corals. There are around 6000 known species of hard coral, also known as the ‘reef building’ corals. 

Hard corals are reef builders, while soft corals are   no reef builders!

Soft and hard coral in Kenya
Image by Ewout Knoester, REEFolution Foundation

Breaking this down more scientifically, hard corals are also known as scleractinian or stony corals and are technically what builds reefs. The hard corals that form reefs are called hermatypic corals. Colonial hard corals, consisting of hundreds to hundreds of thousands of individual polyps, are cemented together by the calcium carbonate ‘skeletons’ they secrete. The calcium carbonate (CaCO3) secreted is   of crystal form called aragonite.

Soft corals, though present in a reef ecosystem, do not form reefs. The term soft coral already speaks volumes about the process of their formation. Soft corals known as Alcyonacea and ahermatypic coraldo not produce a rigid calcium carbonate skeleton, instead their bodies are supported by tiny limestone spike-like structures called spicules. They are however mostly colonial, like hard corals; what appears to be a single large organism is actually a colony of individual polyps combined to form a larger structure.

 Visually, soft coral colonies tend to resemble trees, bushes, fans, whips, and grasses. Most soft corals are also brightly coloured, unlike hard corals, yet this is not always due to the zooxanthellae living within their tissues as many soft corals do no exhibit this symbiotic relationship, and a lot of the range of colours comes from simple pigmentation much like any other animal. Based on these differences it is easy to tell apart soft corals from hard corals.

Does this coral look like a brain to you?

Brain coral.
Image by Kidadl, Brain Coral Facts You’ll Never Forget.

Reef building corals exhibit a wide range of shapes with common names derived from their appearance. For instance, a brain coral will resemble a human brain and so on.  I will share here a simple classification that I found easy to understand on the major species of hard coral by International Coral Reef Initiative (ICRI); “If you come across a hard coral resembling finger like or clumps of cigars, and have no secondary branches, you are in the presence of a sub-massive coral. If you come across table-like structures that often have fused branches, you are in the presence of table corals. Elkhorn coral has large, flattened branches while foliose corals have broad plate-like portions rising in whorl-like patterns. Thin layers growing against a substrate are encrusting corals and hard to miss corals are the massive corals forming a ball-shaped or boulder-like structure and may be as large as a house! Lastly the mushroom corals resemble the unattached tops of mushrooms.” 

Even though given the term hard coral because of their hard outer skeleton of limestone, hard corals are very fragile, and they are particularly sensitive to changes in temperature. A 1-2°C increase of temperature above 29°C persisting for several weeks is enough for many hard corals to show (or display) a phenomenon known as coral bleaching.

Bleaching is when corals are stressed by changes in conditions such as temperature, light, or nutrients, and they expel the symbiotic zooxanthellae living in their tissues causing them to lose their colour and in some cases turn completely white, hence the term bleaching. Bleached corals are not dead corals but zooxanthellae which have now been ejected from a bleached coral tissue were its primary food source, providing the polyps with necessary nutrients. Therefore, if conditions remain unfavorable for too long, bleached corals starve and eventually die, often leading to mass coral death.

The status of coral in Kenya has changed over the years, global climate change and human activities being the biggest contributors. As global temperatures continue to rise due to climate change, the frequency of mass coral bleaching events continues to increase. Kenyan coral reefs average hard coral cover currently sits at 18%, with fleshy algae at 34%! When coral cover declines, fleshy algae increases.

According to the coral reef status report for the Western Indian Ocean, the hard coral cover in Kenya decreased by almost 8% during the 1998 bleaching and mortality event. Coral cover remained low between 1999 and 2003. In 2013 a slow recovery was documented.

Between 2015 and 2016, the worst global mass coral bleaching event to have ever occurred vastly reduced global populations of coral, yet no mortality event was recorded in Kenya, and since, cover has continued to increase slowly until today.

In Kenya, hard coral cover is highest in fully protected areas and Community Conservation Areas. The coral genera Acropora and Pocillopora dominate in these regions. The number of coral species in each reef varies: Some species tend to be more resilient compared to other. In Kenya the diversity of corals is high on the South coast compared to the North coast, dominated by Porites and Acropora in the south and Porites in the North coast.

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