|Black coral colony|
Milne-Edwards & Haime, 1857
Antipatharians, also known as Black corals , are an order of deep-water tree-like corals. Black corals were previously classified in the taxon Ceriantipatharia with the ceriantharians, but were later reclassified under Hexacorallia.
Black corals are a group of approximately 280 corals. Those 280 species are divided into seven known families, which are further divided into 45 known genera. Black Corals are cosmopolitan, being found at depths and locations around the world. However, they are primarily deep-sea corals, and are abundant in deep-water tropical and subtropical zones of the ocean.
Despite its name, black coral is rarely black, and depending on the species it can be white, red, green, yellow, or brown. Black corals get their name instead from their black skeletons, which are made of protein and chitin. These skeletons are unique among stony coral. Some black corals, like black wire corals, grow as a single spiral coil. Other grow into fan shapes, or into an elaborate system of tree-like branches.
A layer of cenocarc forms a kind of "bark" around the coral skeleton. Polyps that live inside of this bark are small and gelatinous, and have six tentacles that will sting any small animals that float by. Black corals are carnivorous, with the coral's polyps allowing it to feed mostly on meiofauna..
They are firmly attached to the seafloor, and frequently grow where currents sweep by, which allows them to feed on plankton that is swept by. The suspected evolutionary reason that they are fan-shaped is to catch this plankton. Many corals also have an adaptation where they have polyps only on the downstream side of the coral, allowing them to feed on more animals without wasting energy on unnecessary polyp
Due primarily to the slow life cycle of black coral, little is known about its life cycle and reproduction. Similar to other cnidarians, the life cycle of these corals involves asexual as well as sexual reproduction. Asexual reproduction (also known as budding), is the first method of reproduction used by a black coral during its lifespan. It builds the colony by directly creating more skeleton, growing new branches and making it thicker. This new skeleton, in turn, creates more and more skeleton, allowing the colony to grow. "Growth rings" created by the expansion of the skeleton can be used to estimate the age of a colony. Asexual reproduction may also occur when the ends of the coral are broken off.
Sexual reproduction, on the other hand, happens later in life, after the coral colony is established. A colony will produce both eggs and sperm, which meet to create a larvae that uses currents to disperse and settle in new areas. The larval stage of the coral, called a planula, will drift along with the currents until it finds a surface on which it can grow. Once the larva settles, it metamorphoses into its polyp form and creates skeletal material that attaches it to the seafloor. The coral will then begin to bud, which will create new polyps, which will eventually form a colony. In some black corals that have been closely examined, colonies will grow roughly 6.4 centimeters (2.5 in.) every year. Sexual reproduction will first occur after some 10 to 12 years; the colony will then reproduce annually for the rest of its life. A large 1.8 meter (6 ft) tall coral tree is somewhere between 30 and 40 years old. The estimated natural lifespan of a black coral colony is around 70 years. However, some species may live much longer. In March 2009, scientists released the results of their research on deep-sea (depths of ~300 to 3,000 m) corals throughout the world. They discovered specimens of Leiopathes glaberrima to be among the oldest living organisms on the planet: around 4,265 years old. They show that the "radial growth rates are as low as 4 to 35 micrometers per year and that individual colony longevities are on the order of thousands of years".
Black corals around the world provide a unique environment for crustacans, bivalves, and fish. Whip coral (Cirrhipathes species), for example, host as many as six other species. Whip coral gobies and barnacles permanently inhabit the skeleton. The goby and shrimp quickly hide on the skeleton's opposite side when a threat approaches. The goby and damselfish lay their eggs on the skeleton. The damselfish then bites off the polyps to expose the nesting site.
Black corals have historically been associated with mystical powers and medicinal properties, though more recent harvesting has been for use as jewellery. The best studied and regulated black coral fisheries are in Hawaii, where harvesting has been conducted since the 1960s. In the Caribbean harvesting is typically conducted to produce jewellery for sale to tourists, and has followed a boom-and-bust cycle, where new black coral populations are discovered and overexploited leading to rapid declines. For example, Cozumel, Mexico, was famed for dense black coral beds that have been harvested since the 1960s leading to widespread black coral population declines. Despite better black coral management in Cozumel, including no harvesting permits issued since the mid-1990s, the black coral population had failed to recover when assessed in 2016. Black coral is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), species that are not necessarily threatened with extinction, but may become so unless trade in specimens of such species is subject to strict regulation in order to avoid utilization incompatible with the survival of the species in the wild.
List of families according to the World Register of Marine Species: