Coral
Introduction
Coral refers to a group of marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically live in compact colonies of many identical individual polyps. The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.
Biology
Structure
A coral "head" is a colony of myriad genetically identical polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. Each polyp excretes an exoskeleton near the base. Over many generations, the colony thus creates a large skeleton characteristic of the species. Individual heads grow by asexual reproduction of polyps.
Polyps
Coral polyps are tiny, soft-bodied organisms related to sea anemones and jellyfish. At their base is a hard, protective limestone skeleton called a calicle, which forms the structure of coral reefs. Reefs begin when a polyp attaches itself to a rock on the sea floor, then divides, or buds, into thousands of clones. The polyp calicles connect to one another, creating a colony that acts as a single organism. As colonies grow over hundreds and thousands of years, they join with other colonies and become reefs. Some of the most famous coral reefs, such as the Great Barrier Reef, are actually made up of thousands of smaller coral reefs.
Reproduction
Corals can reproduce both sexually and asexually. An individual polyp uses both reproductive modes within its lifetime. Corals reproduce sexually by either spawning or brooding. About 75% of all hermatypic corals "broadcast spawn" by releasing gametes—eggs and sperm—into the water to spread offspring. The gametes fuse during fertilization to form a microscopic larva called a planula, typically pink and elliptical in shape.
Ecology
Habitat
Coral habitats are a subset of ocean habitats. Coral habitats include the subtropics. They are most commonly found at shallow depths in tropical waters, but deep water and cold water corals also exist on smaller scales. Hermatypic corals often form large reefs in tropical waters. Conversely, solitary corals, notably some Scleractinia, some mushroom corals (such as Fungia), and the polyp-less Heliopora, are found throughout the ocean.
Relationship with other species
Corals are keystone species and play a critical role in the marine ecosystem. They provide shelter and food for a significant proportion of marine life, assist in carbon and nitrogen fixing, aid in nutrient recycling, and contribute to the formation of tropical islands and coastlines.
Threats and preservation
Coral reefs are dying around the world. In particular, coral mining, pollution (organic and non-organic), overfishing, blast fishing, disease, and the digging of canals and access into islands and bays are localized threats to coral ecosystems. Broader threats are sea temperature rise, sea level rise and pH changes from ocean acidification, all associated with greenhouse gas emissions. In 1998, 16% of the world's reefs died as a result of increased water temperature.