The oceans are home to a variety of coral species that form expansive reefs in warm, shallow waters. A reef can resemble outstretched fingers, mushroom caps or tree branches, with environmental variables such as water temperature and the amount of sunlight reaching the reef determining its ultimate size. The National Oceanic and Atmospheric Administration reports that reefs have been a fixture of Earth’s oceans for at least 400 million years, while noting that human activity currently threatens approximately 75 percent of the planet’s reef systems.
- Coral Reefs in Fiji
- Coral Reefs in Tonga
- Coral Reefs in Texas
- Coral Reefs in Kenya
- Coral Reefs in Brazil
- Coral Reefs in Honduras
Biology & Taxonomy
Biologists classify corals as anthozoans, placing these animals in the same class as anemones and sea fans. Scleractinians are the stony corals comprising the largest anthozoan group and are the primary components of coral reefs. These simple creatures use a ring of tentacles around their mouths to snare plankton, which provide their primary sustenance, and also use their mouths to expel bodily wastes.
Corals have a symbiotic relationship with zooxanthellae, algae which convert waste byproducts from coral respiration and digestion into organic material the coral uses to produce nutrients and calcium carbonate, the building block of reef structures. Zooxanthellae are also responsible for giving coral reefs their vibrant range of colors. If reef corals suffer profound physical trauma, they expel their zooxanthellae, which results in the corals’ deaths. After the corals in a reef system perish, the reef turns chalk white from the loss of the zooxanthellae.
Almost all of Earth’s coral reefs lie within the waters between the Tropic of Capricorn and the Tropic of Cancer. Reef ecosystems flourish in waters where average temperatures range between 73 and 77 degrees Fahrenheit, though they can tolerate temperatures between 61 and 95 degrees. Corals require water both shallow and clear enough to allow sunlight to reach the reef, so that their symbiotic zooxanthellae can conduct photosynthesis.
The Ecological Society of America notes that 25 percent of the world’s marine species live within reef ecosystems. Coral reefs support a diverse array of aquatic creatures, from thousands of tropical fish species to other marine vertebrates such as sea turtles. Reefs also provide habitats for invertebrate animals such as sponges, sea urchins and crustaceans, along with other coral species, which include sea fans, that don’t produce reef structures.
Threats to Coral Reefs
Agricultural and industrial activities that result in sediment discharge along tropical coastlines cut off the sunlight zooxanthellae need for photosynthesis, endangering local reef systems. Runoff containing plant nutrients can increase algae growth, depriving corals of oxygen. Snorkelers and commercial harvesters who break off coral fragments for souvenirs or ornaments for home aquariums can inflict extensive damage upon reef systems. Designating coral reefs as protected preserves can help reefs recover from damage inflicted by pollution and harvesting, but global warming presents a more difficult challenge. Reuters reports that carbon emissions from human activity increases ocean acidity which degrades reefs, but countering this threat would require collective agreement and action from the world’s nations to curb their greenhouse gas emissions.
- National Oceanic and Atmospheric Administration: CoRIS, What Are Coral Reefs
- Ecological Society of America: Coral Reefs
- Reuters; Climate Change Fuels “Dire” Threat to Coral Reefs; Deborah Zaborenko; February 2011
- “Ocean”; Fabien Cousteau, et. al.; 2009
- Office of Naval Research Science & Technology Focus: Habitats, Coral Reefs – Characteristics
- Florida Museum of Natural History: Ichthyology, Coral Reefs, Habitat Requirements