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Coral reefs are complex and ancient habitats. They have been a feature of life on Earth for around 500 million years.  

They are a critical component of life in the ocean. Often referred to as “rainforests of the sea,” coral reef ecosystems are one of the most biodiverse in the world.  

Around 25% of all marine species rely on coral reefs, including sharks and sea turtles, crustaceans, and schooling fish. 

But we currently stand to lose all shallow-water coral reefs by the end of this century. 


What is a coral?


  • Coral is a colonial animal.  

  • Coral colonies are made up of individual coral polyps.  

  • They are a member of the phylum Cnidaria alongside jellies and anemones.  

  • Hard corals grow in a six-fold structure, meaning that they will always grow tentacles in a multiple of 6, whereas soft corals grow in an eight-fold structure.  

  • Hard corals have a hard skeleton. This skeleton is made of calcium carbonate – limestone. 

  • These skeletons are a framework of molecular crystals made in their calcifying space and excreted to form the reef structure. They do this by absorbing carbonate (CO32-) and calcium (Ca2+) ions from the water then bonding them together to create the calcium carbonate structure. 

  • Coral skeletons fuse together to create coral reefs. 


How do corals feed?


  • Corals get their energy in two different ways.  

  • Most of a corals’ energy is provided by their “zooxanthellae” – an alga which has a symbiotic relationship with coral. This alga lives inside coral tissues and photosynthesizes energy for them both. 

  • Zooxanthellae produce around ninety percent of the coral’s energy. They are also responsible for corals’ distinctive colors. 

  • Corals capture the remaining ten percent of energy themselves. 

  • They use their tentacles to gather food as it drifts by using their “nematocysts” – stinging cells used for defense and hunting. Nematocycts function like harpoons with a barb that shoots forward from a spring to pierce their prey. (Other Cnidarians, like jellyfish, are more famous for their nematocycts!)  

  • Corals’ prey includes plankton and copepods. 


How do corals reproduce?


  • Corals reproduce in two different ways - sexually and asexually.

  • On a healthy reef, when corals break, the broken pieces can lodge in cracks and crevices and grow into new colonies that are clones of the parent. This is asexual reproduction and is called “fragmentation”.   

  • Corals also reproduce sexually by “spawning”. Spawning is the synchronized release of "gametes" (sperm and eggs) that then mix together in the water to create new, genetically unique coral babies.

  • Most corals will spawn just once a year during a full moon.  


Why do coral reefs matter?


  • Coral reefs are vital ecosystems.  

  • More than 275 million people worldwide live in the direct vicinity of coral reefs and around 850 million people live within 100 km of coral reefs. 

  • Coral reefs support more than a million different species. 

  • They provide marine life with nursery, spawning, and hunting grounds.   

  • Coral reefs provide crucial coastal protection. They act as a physical barrier between ocean and land, dissipating up to 97% of wave energy, mitigating hurricane damage and coastal erosion, and protecting coastal infrastructure including the homes and businesses of more than 500 million people around the world.  

  • The protection that reefs offer allow for sea grass beds and mangrove ecosystems to flourish. These ecosystems are also important habitats in their own right, but together they create positive feedback loops, ensuring the balance of life in shallow seas.  

  • Reefs underpin commercial and recreational fishing industries, as well as tourism-based economies.  

  • Coral reefs have an annual estimated economic value of around $9.9 trillion globally.  

Coral Biology