Coral Reefs 101
Coral Reefs 101
Everything you need to know about what coral reefs are, how they form, what threatens them, and why they matter.
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Sources: NOAA, International Coral Reef Initiative (ICRI)
What are coral reefs?
A coral reef is an underwater structure built entirely by living organisms. Despite looking like rock or plant formations, reefs are built by tiny animals called coral polyps, each one smaller than a fingernail.
Coral polyps
Each polyp secretes calcium carbonate to form a hard skeleton. Over hundreds and thousands of years, these skeletons accumulate to form the complex structures we call reefs. The polyps themselves contain symbiotic algae called zooxanthellae, which give coral its color and provide up to 90% of its energy through photosynthesis.
Hard corals
The primary reef builders. They produce hard calcium carbonate skeletons that form the reef's structure over centuries.
Soft corals
Flexible and feather-like, these do not build reefs but add to biodiversity and provide habitat for fish and invertebrates.
Coral Triangle
The Philippines lies within the Coral Triangle, where 76% of all coral species are found. It is the world's center of marine biodiversity.
How reefs grow.
Coral growth is slow. Most hard corals grow between 0.3 and 10 centimeters per year. A reef structure can take thousands of years to form, which is why losing one to bleaching is so serious.
Sexual reproduction
Once a year, many coral species release millions of eggs and sperm simultaneously in a mass spawning event. Fertilized larvae drift until they find a suitable hard surface to settle on.
Fragmentation
Coral fragments broken off by storms or fish can reattach to new surfaces and grow into new colonies. This is the principle behind coral nursery restoration techniques.
What reefs need
Clear, warm (but not too warm) shallow water with good light penetration. Salinity between 32 and 42 parts per thousand. Low nutrient levels. Stable water temperature between 23 and 29 degrees Celsius.
What threatens coral reefs.
Climate change and bleaching
When water temperatures rise just 1 to 2 degrees above normal for extended periods, corals expel their symbiotic algae and turn white. The 2023 to 2025 global bleaching event damaged 84% of the world's monitored reefs.
Ocean acidification
As oceans absorb more CO2, seawater becomes more acidic, making it harder for corals to build and maintain their calcium carbonate skeletons.
Overfishing
Removing key fish species disrupts the ecological balance that keeps reefs healthy. Herbivorous fish control algae growth that would otherwise smother coral.
Destructive fishing
Dynamite fishing and cyanide fishing physically destroy reef structures and kill entire reef communities instantly. Both remain problems in parts of the Philippines.
Pollution and runoff
Agricultural runoff, sewage, and plastics increase nutrient levels and smother coral. Sediment from coastal development reduces the light corals need to survive.
Careless tourism
Divers and snorkelers who touch or stand on coral, sunscreen chemicals, and anchor damage from dive boats all cause direct physical harm to reef ecosystems.
Why reefs matter.
Biodiversity
Coral reefs support 25% of all marine species despite covering less than 1% of the ocean floor. They are the oceans' nurseries.
Coastal protection
Reefs absorb 97% of wave energy, protecting coastlines from erosion, flooding, and storm damage. Their structural value to coasts is estimated at $9 billion per year globally.
Food and livelihoods
More than one billion people depend on reef fisheries as their primary protein source. In the Philippines, 1.5 million fisherfolk depend on reef-associated species for their income.
Medicine
Reef organisms have contributed compounds used in treatments for cancer, HIV, cardiovascular disease, and pain management. Many more remain undiscovered.
Now you know what we're working to protect.
Reef Without Borders funds real restoration dives in Cebu, Philippines. Every coral we plant will be documented and published. Every dollar is traceable to a specific reef, a specific dive, and a specific outcome.
U.S. 501(c)(3) nonprofit • EIN 42-2691547 • Tax-deductible
