Corals are small animals that make up the phlyum Cnidaria with jellyfish, anemones, and hydroids, all of which have stinging organs or cnidocysts. Corals consist of two types: hard (scleratinain) and soft. Primarily the hard corals build the coral reef framework. Coral reefs are often referred to as the rainforests of the ocean, as they have expansive biodiversity.
Location: Coral reefs are primarily located in warm waters, as corals cannot tolerate temperatures below an average of 18°C. Thus, their habitats are primarily located between 23°N and 23°S latitude in clear, tropical oceans. The current also affects where they can locate. Additionally, corals form anywhere from the surface of the water to approximately 150 feet (45 meters) deep, as they depend on sunlight for their survival.
Three of the most common types of coral reefs are barrier reefs, fringing reefs, and atolls. Barrier reefs occur most frequently in the Indo-Pacific and Caribbean in locations that are farther offshore than fringing reefs are located. Fringing reefs are often found in Hawaii and the Caribbean, right along the shorelines of islands and continents. Atolls are most often found in the Indo-Pacific. They consist of a series of low coral islands, which surround a central lagoon. The world's largest coral reef is the Great Barrier Reef in Australia. It is over 1,200 miles (1,900 km), which is greater than the distance from Los Angeles, CA to Seattle, WA.
Taxonomy: As mentioned above, corals belong to phylum Cnidaria along with jellyfish, sea anemones, and hydroids. The soft and hard corals make up the class Anthoza with 6,000 species altogether. The reef building or hermatypic corals are part of the order Scleractinia. Currently there are no recorded sightings of freshwater corals.
How corals live: Typically the waters around coral reefs have poor nutrients but still manage to produce a wide diversity of life. This is possible through an energy equation. Most corals have a symbiotic relationship with zooxanthellae, which is a small microalgae. The microalgae is able to do photosynthesis through the abundant available sunlight, fulfilling almost all of the corals' energy needs. In exchange, the coral gives its waste products, which are nutrient-rich, to the algae. This relationship allows coral to grow close to the surface so the zooxanthellae can perform its functions in relatively clear waters.
Plants: In addition to the zooxanthellae, the primary plants in coral reefs are seagrasses and algae. These plants provide food and oxygen for the animals that inhabit the reef. Additionally, seagrasses provide shelter for juvenile reef animals, such as lobster and conch.
Animals: Even though coral reefs make up only 1% of the ocean floor, nearly 25% of life in the ocean resides in them. Coral reefs can be used as resting points for animals traveling in the ocean as well as permanent homes for other animals. The corals themselves are the most prevalent animals in the reefs. Corals come in a variety of colors including red, white, green, pink, purple, blue, and orange. These colors are due to both the zooxanthellae, which resides in their tissues, and natural pigments.
Additional animals that permanently reside in coral reefs include sponges, sea urchins, worms, sea stars, fish, sharks, lobster, octopus, rays, shrimp, and snails. Just as the coral and zooxanthellae work together as team, many other coral reef inhabitants also work as teammates. This process is called symbiosis.
Reproduction: Coral reefs reproduce both asexually and sexually. Asexual reproduction occurs as coral fragments break off and start a new colony. There are several corals, such as staghorn, which rely on this method almost exclusively. Sexual reproduction is rare and often synchronous. Corals have mass spawning events only a few nights every year. During the events, the spawn are released and will cover the entire water surface.
Benefits for humans: Coral reefs are important to humans for a variety of reasons. Reefs absorb the impact of strong storms and waves, which protects the land from harsh weather. Reefs provide food for humans, including conch, lobster, and shrimp. Coral reefs are a large tourist attraction. Coral reefs are providing more and more medicines and medical tools each year. For example, certain types of coral skeletons can be used in reconstructive bone surgery as a human bone substitute.