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CORAL REEFS

RAINFORESTS OF THE SEA

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. 

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What is a coral?

CORAL BASICS

  • 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?

ENERGY AND NUTRIENTS

  • 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?

FRAGMENTATION & SPAWNING

  • 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?

CRITICAL HABITATS

  • 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.  

 
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REEFS UNDER THREAT

 

CORAL REEFS ARE DISAPPEARING

Although reefs have survived the last five mass extinctions, they can no longer keep up with the speed of global environmental changes caused by human activity.  

We have lost more than half of the world’s coral reefs in the last 30 years, and without immediate action, all shallow water coral reefs are projected to vanish by the end of this century.  

Reefs are under threat as a result of both local and global stressors. The combination of these threats is putting reefs under extreme pressure.

IMPACTS TO LIFE ON EARTH

Losing all of our shallow water coral reefs would be like losing every rainforest on the planet. We don’t yet know how catastrophic and complex the impacts of this would be – humanity has never experienced the loss of an entire biome. 

 

Around 70% of all atmospheric oxygen is produced by the ocean. Coral reefs are a critical part of the balance of life in the sea.  

There is no way to know how severe the consequences of losing all of our planet’s coral reefs.

 

We do know that losing coral reefs causes: 

  • Damage to coasts, shorelines, homes, and businesses 

  • Economic fallout from the loss of fishing and tourism 

  • The collapse of the balance of life in the oceans. 

GIVING REEFS A

FIGHTING CHANCE

A healthy reef can better withstand bleaching events, can recover from hurricane damage, and is better able to cope with disease outbreaks. Healthy reefs therefore have a greater chance of surviving climate change impacts in the short to medium term

 

To help give reefs a fighting chance, it is imperative that local stressors are removed and mitigated in the short term, while we work to tackle climate change.

 

What's killing our coral?

local stressors

  • Direct damage: improperly dropped anchors and careless divers and snorkellers will destroy hundreds of years of coral growth.   

  • Pollution: Agricultural run-off and sedimentation harm coral as they impair photosynthesis, encourage algal overgrowth, and cause disease outbreaks.

  • Fishing: Overfishing of herbivorous species and keystone species including sharks and other predators drastically changes ecosystem dynamics.  

  • Curio trade: The removal of species for the trade in souvenirs alters the balance of the ecosystem as live animals and plants are taken from the reef and dead shells and crustaceans are prevented from rejoining the cycle. 

  • Disease: Degraded ecosystems and polluted water cause severe disease outbreaks. For example, a disease that destroyed populations of the long spine sea urchin in the Florida Keys allowed for a phase shift in the ecosystem from a balanced one to one that is dominated by algae. Stony Coral Tissue Loss Disease (SCTLD) is a new and lethal disease currently sweeping through the Caribbean and Florida Keys.  

  • Coastal development: Removal of mangrove areas, dredging, extraction of sand and coral rock, run-off and sedimantation from construction sites all have dramatic impacts on reef ecosystems. 

  • Plastics: Endanger marine animals if ingested, pose a threat as floating obstacles for entanglement, and can affect the photosynthesis of marine plants. Microplastics, when ingested, have also been linked to a greater occurrence of disease in coral.  

  • Invasive species: Alter the relationship between predator and prey and changing the number of individuals among species at different trophic levels. 

  • Chemicals in sunscreen: Affect the development of young coral before they attach to substrate. There is also evidence that they cause DNA mutations in coral reproduction and development.  

Climate change

GLOBAL STRESSOR

  • Climate change: The single greatest threat to coral reefs stems from the anthropogenic increase of atmospheric carbon dioxide.  

  • Warming water: As the oceans are a sink for many global processes, they absorb much of the heat the planet generates.  

Corals, like most animals, can only survive in a narrow temperature range. The ideal temperature range for coral is between 73- and 84-degrees Fahrenheit. Coral bleaching occurs when temperatures rise too far above this range.

 

Bleaching is the result of ocean warming as the zooxanthellae cannot withstand the rising temperatures and they abandon the coral, leaving the corals' tissues transparent and revleaing their skeletons, causing the stark whiteness that gives this process its name.

 

The consistent rise in ocean temperature is exacerbating the frequency and severity of bleaching events to the point where corals cannot recover. 

  • Ocean acidification: The absorption of atmospheric carbon dioxide also causes the ocean’s pH to drop, becoming more acidic, creating a host of problems for marine life. 

Acidification effectively "dissolves" a corals' calcium carbonate skeleton (think of tooth enamel in soda pop).  The increase in carbon changes the water chemistry, causing an imbalance between the hydrogen and carbonate ions which makes producing a calcium carbonate skeleton significantly harder. 

  • Increasing frequency of major storms: The increase in hurricanes poses a great risk to coral reefs. Hurricanes produce an outstanding amount of wave energy and reefs absorb the vast majority of it. This energy will destroy decades of coral growth and degraded reefs struggle to recover. 

  • Sea level rise: As glaciers melt and sea levels rise, shallow-water corals will struggle to survive in deeper water.

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