What Are Coral Reefs?

A coral reef is an underwater structure built entirely by living organisms. Despite looking like rock or colorful plants, the physical structure of a reef is made of calcium carbonate — the same compound as limestone — secreted over centuries by tiny marine animals called coral polyps.

Coral reefs are sometimes called the rainforests of the sea, and the comparison earns its weight. They cover less than 0.1 percent of the ocean floor yet provide habitat, shelter, and food for an estimated 25 percent of all known marine species. The biodiversity packed into a single healthy reef system is extraordinary by any ecological measure.

Coral Polyps

Individual coral animals are called polyps. Each polyp is a tiny, soft-bodied organism, typically just a few millimeters in diameter, with a cylindrical body, a central mouth surrounded by tentacles, and a hard calcium carbonate cup called a corallite that it secretes beneath itself as a skeleton.

Polyps are cnidarians — the same animal phylum as jellyfish and sea anemones. Their tentacles carry stinging cells called nematocysts that they use to capture tiny drifting prey called zooplankton at night, though most of the energy that fuels reef-building corals actually comes from photosynthesis.

This relationship between coral and zooxanthellae is also why bleaching happens. When water temperatures rise above a coral's thermal tolerance, even by one degree Celsius for an extended period, the coral expels its zooxanthellae. Without the algae, the coral loses its primary food source, turns white, and if conditions do not improve, it starves and dies.

Hard Corals and Soft Corals

Not all corals are reef builders. The two major categories — hard corals and soft corals — play very different structural roles in reef ecosystems.

How Reefs Form

A coral reef begins when a single coral larva, called a planula, settles onto a firm substrate on the seafloor — a rock, a shell fragment, or even a piece of debris. The planula attaches and begins secreting a calcium carbonate base, then divides and clones itself repeatedly to form a colony of genetically identical polyps.

As each polyp builds its calcium carbonate cup beneath itself and the colony grows, the accumulated skeletons of both living and dead polyps layer up over time. At the surface of this structure, living coral tissue covers the topmost layer. This is what gives reefs their color and their biological activity.

Types of Reef Formations

Charles Darwin was one of the first scientists to classify coral reef formations systematically, and his three-category framework — fringing reefs, barrier reefs, and atolls — remains the foundation of how reef scientists classify reef structures today.

How Coral Reefs Grow

Reef growth depends on a delicate balance between calcium carbonate production by living corals and calcium carbonate erosion by biological and physical forces. When a reef is healthy, production outpaces erosion and the reef grows. When a reef is stressed, the balance tips the other way.

Parrotfish play a surprising role in this equation. They bite off chunks of coral skeleton to feed on the algae inside, grinding it into fine white sand that accumulates on the seafloor and beaches. A single large parrotfish can produce several hundred kilograms of sand per year. The white sandy beaches of tropical islands are, in large part, the product of parrotfish digestion.

For a reef to keep pace with rising sea levels and warming temperatures, it needs optimal conditions: clean water, controlled fishing pressure, stable temperature, and an adequate supply of coral larvae to replace dead colonies. Reef Without Borders works to restore those conditions at our sites in Cebu.

How Corals Reproduce

Corals reproduce both sexually and asexually, and both strategies matter for reef recovery and restoration work.

What Reefs Need to Survive

Coral reefs are highly sensitive ecosystems with narrow environmental tolerances. Get any one of the following conditions wrong and the reef begins to struggle. Get several wrong at once and collapse can happen within years.

1. Warm but not too warm water: Most reef-building corals thrive between 23°C and 29°C (73°F to 84°F). Sustained temperatures above 30°C trigger bleaching. Below 18°C and coral growth stops entirely.
2. Clear, shallow water: Zooxanthellae need sunlight to photosynthesise. Most coral reef growth occurs in water shallower than 30 meters. Sedimentation and pollution that cloud the water starve corals of the light they need.
3. Saltwater salinity: Reefs require stable ocean salinity. Freshwater runoff from floods or heavy rain, if intense enough, can cause localized coral death by diluting salinity beyond the coral's tolerance range.
4. Hard substrate: Coral larvae need a firm surface to settle on. Bare rock, shell fragments, or existing dead coral skeleton work. Sandy bottoms or silty seafloor do not.
5. Moderate water movement: Gentle to moderate currents bring food, remove waste, and distribute coral larvae. Completely stagnant water encourages algae overgrowth and reduces reef health.
6. Low nutrient levels: Paradoxically, reefs thrive in relatively nutrient-poor water. High nutrient levels — from sewage, agricultural runoff, or fish waste — promote algae blooms that can smother coral.

Where Are Coral Reefs Located?

Coral reefs are found in tropical and subtropical ocean regions, primarily between the Tropic of Cancer and the Tropic of Capricorn — a band that encircles the globe roughly 30 degrees north and south of the equator where ocean temperatures remain warm enough year-round to sustain reef growth.

Global Threats to Coral Reefs

The threats facing coral reefs fall into two categories: global threats driven by climate change, and local or direct threats driven by human activity at the site level. Both matter. Addressing only one is not enough.

Global threats include:

Ocean warming is the most urgent global threat. As ocean temperatures rise due to increased atmospheric carbon dioxide, coral bleaching events are becoming more frequent and more severe. Between 2014 and 2017, the longest global bleaching event on record affected more than 70 percent of the world's reef areas. Scientists project that at 2°C of global warming, up to 99 percent of coral reefs could experience annual severe bleaching conditions.

Ocean acidification, a separate consequence of rising CO₂, weakens the calcium carbonate structures that reef-building corals depend on. As seawater absorbs more atmospheric CO₂, its pH decreases, making it harder for corals to build and maintain their skeletons.

Local and Direct Threats

Local threats do not cause climate change, but they significantly reduce a reef's ability to resist and recover from it. A reef that is already stressed by overfishing, pollution, or physical damage is far less resilient when a bleaching event arrives. This is why local conservation work matters even in the context of a global climate problem.

Local and direct threats include:

Why Coral Reefs Matter

The case for protecting coral reefs is not sentimental, though they are genuinely beautiful. It is ecological, economic, medical, and cultural — and it is urgent.

How You Can Help

The scale of the reef crisis can feel overwhelming. But the science is also clear that reef conservation works when it is funded, targeted, and sustained. Here is how you can be part of what changes the outcome.