Coral tales – The good, the bad and the algae

We have heard our scientists talk about coral reefs, coral bleaching, and coral reef restoration in Seychelles. We’ve heard that reefs are important to us for what they provide – food, protection from storms, and beautiful beaches, as well as revenue and employment in the fishing and diving sectors. We’ve heard that they support rich communities of marine life, from sharks to flatworms, and are home to 25 per cent of all marine life.

These mesmerizing and colourful creatures that inhabit the ocean's depths have long been a subject of fascination for scientists and dive enthusiasts alike.
But how many of us really know what corals are and what dangers they face?

Here is a brief primer.

Plant or animal?

Despite their plant-like appearance corals are biologically classified as animals (Photo - Athina Antoine)

Despite their plant-like appearance, corals are biologically classified as animals. They belong to the phylum Cnidaria, which are marine invertebrates related to jellyfish and sea anemones. They are made of tiny tentacled animals known as polyps that secrete a calcium carbonate exoskeleton to house themselves in. The polyps’ tentacles are used to capture prey and defend themselves against predators.

Polyps are amazing architects. They build the limestone structures we know as coral reefs. Hundreds of them grow on top of each other over many years and slowly form these bustling underwater cities.

Our coral species include these important reef builders, which inhabit tropical oceans.

Coral reefs are bustling underwater cities

Enter the algae

Most corals have a symbiotic relationship with millions of single-cell algae known as zooxanthellae that live in the polyps. The algae provide them with food and energy through photosynthesis - just like plants - transferring up to 95 per cent of the food they produce to coral polyps. The zooxanthellae in turn receive protection, as well as some nutrients from waste produced by coral polyps. Corals also owe their colour to the algae that live within them.

Corals truly represent a unique intersection between plant and animal - a reminder of just how complex and wondrous they are.

Until they break up

Bleaching occurs when corals expel their algae partners

Unfortunately, when sea temperatures become abnormally high, this relationship can break down. Corals become stressed by high temperatures, causing them to expel their algae partners. The loss of the algae leaves the tissue transparent, revealing the white skeleton and giving the coral the appearance of having been “bleached”.

Can they survive the break-up?

Without the algae, corals lose a major source of energy and become increasingly susceptible to disease and starvation. Corals can only survive for a limited time without the algae. Unless temperatures decrease and polyps are able to reacquire the zooxanthellae, the whole colony can die.

Corals can recover from bleaching, but if the environmental stress continues for a long time, they eventually die. A dead reef cannot provide ecosystem services

Coral healers

Scientists are assisting reefs to recover through coral reef restoration programs

Coral reefs face unprecedented threats today and many are at a tipping point. This is why organisations and scientists are assisting reefs to recover through coral reef restoration programs that grow corals and then plant them in damaged reefs.

For the last decade, Nature Seychelles has been rehabilitating coral reefs affected by bleaching within the Cousin Island Special Reserve MPA through the 'Reef Rescuers' program. This project is currently in its third phase and is funded by the Adaptation Fund through the UNDP and the Seychelles government.