Sea corals are vibrant animals, often mistaken for plants or rocks, that build spectacular underwater ecosystems. These colonies of tiny creatures construct immense structures teeming with life, which have fascinated scientists for centuries. The worlds they build support a significant portion of all marine biodiversity.
The Astonishing Life of a Coral Animal
Each coral structure is a colony of individual animals called polyps. These simple, sac-like creatures have a mouth surrounded by tentacles armed with stinging cells. Corals use these tentacles to capture small prey like plankton, which provides nutrients for their growth.
Most shallow-water corals have a symbiotic relationship with microscopic algae called zooxanthellae. Living within the coral’s tissues, the algae use photosynthesis to produce sugars that feed the coral host. In return, the coral provides the algae with a protected, sunlit environment. This partnership is also responsible for the brilliant colors of corals.
Corals reproduce in two primary ways. Through sexual reproduction, many species release eggs and sperm into the water in synchronized spawning events. A fertilized egg develops into a larva that settles to start a new colony. Corals also reproduce asexually by cloning polyps to expand the existing colony.
Hard corals, or stony corals, are the primary reef builders. They secrete a rigid skeleton of calcium carbonate that forms a protective cup around each polyp. In contrast, soft corals have a flexible skeleton made of a protein called gorgonin, allowing them to sway with water currents.
Building Underwater Worlds: Coral Reefs
The immense structures known as coral reefs are built over millennia by hard corals depositing layers of their calcium carbonate skeletons. This slow accumulation, strengthened by organisms like algae and mollusks, creates the massive, three-dimensional frameworks that define a reef.
Coral reefs develop into several distinct formations. Fringing reefs grow directly from the shore, forming a border along coastlines. Barrier reefs are found further offshore, separated from the coast by a wide lagoon. Atolls are ring-shaped reefs that enclose a central lagoon, often formed on submerged volcanic islands.
Reef-building corals require specific environmental conditions to thrive. They need warm water, between 70 and 85 degrees Fahrenheit (22-29°C), and clear, shallow water for their symbiotic algae to perform photosynthesis. These requirements concentrate coral reefs in a band roughly between 30 degrees north and south of the equator.
Why Coral Reefs Are Ocean Treasures
Coral reefs are often called the “rainforests of the sea” for their biodiversity. Though covering less than one percent of the ocean floor, they support an estimated 25 percent of all marine species. Countless organisms, including fish, sponges, and crabs, rely on the reef for food, shelter, and breeding grounds, using its complex structure to hide from predators.
The massive reef structures act as natural breakwaters, absorbing the energy of waves and storm surges. This buffer helps prevent coastal erosion and property damage during powerful storms. The protection they offer is particularly valuable for low-lying coastal areas and island nations.
Coral reefs support major fisheries that provide food and livelihoods for millions of people. Tourism and recreation centered around reefs, such as snorkeling and diving, generate billions of dollars annually. Scientists are also exploring reef organisms as a source for new medicines to treat human diseases.
Coral Reefs in Peril: Understanding Threats and Bleaching
Coral reefs face an array of threats driven by human activities, with the most significant being climate change. Rising ocean temperatures cause corals to become stressed and expel the symbiotic zooxanthellae from their tissues. This is known as coral bleaching, as the coral’s white calcium carbonate skeleton becomes visible through its transparent tissue.
Bleached corals are not dead, but they are weakened and more susceptible to disease, and will starve if high temperatures persist. Another consequence of increased atmospheric carbon dioxide is ocean acidification. As the ocean absorbs more CO2, its pH drops, making it harder for corals to build their skeletons.
Local pressures compound these global threats. These include:
- Pollution from land-based sources, such as agricultural runoff, which degrades water quality.
- Sediment from coastal development, which can smother corals and block sunlight.
- Overfishing, which disrupts the reef’s ecological balance.
- Destructive fishing practices, such as dynamite fishing, that cause direct damage to the reef structure.
The scale of these threats is immense, with scientists reporting significant losses of coral cover in regions like the Caribbean. The frequency and intensity of mass bleaching events are increasing globally, pushing many reef ecosystems toward a tipping point.
Hope for Reefs: Conservation and Our Role
A primary conservation strategy is the creation of Marine Protected Areas (MPAs), which restrict harmful activities like fishing in designated zones. These protected areas allow reefs to recover. They can also serve as reservoirs of marine life that help repopulate surrounding areas.
Scientists are researching coral resilience by identifying and propagating strains that are more tolerant to heat stress. This research informs active restoration projects using techniques like coral gardening and microfragmentation. These methods, which can accelerate coral growth rates, are showing promise in localized recovery efforts.
Individuals can contribute to coral conservation. Actions include:
- Reducing your carbon footprint to address ocean warming and acidification.
- Making sustainable seafood choices.
- Supporting reef-friendly tourism operators.
- Avoiding the purchase of souvenirs made from coral.