| Black coral | |
|---|---|
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| Black coral colony | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Cnidaria |
| Subphylum: | Anthozoa |
| Class: | Hexacorallia |
| Order: | Antipatharia Milne-Edwards &Haime, 1857 |
| Families[1] | |
| |
Antipatharians, also known asblack corals orthorn corals,[2] are anorder of soft deep-watercorals. These corals can be recognized by their jet-black or dark brownchitin skeletons, which are surrounded by their coloredpolyps (part of coral that is alive). Antipatharians are acosmopolitan order, existing in nearly every oceanic location and depth, with the sole exception of brackish waters. However, they are most frequently found oncontinental slopes under 50 m (164 ft) deep. A black coral reproduces bothsexually andasexually throughout its lifetime. Many black corals provide housing, shelter, food, and protection for other animals.
Black corals were originally classified in the orderCeriantipatharia along withceriantharians (tube-dwelling anemones), but were later reclassified underHexacorallia. Though they have historically been used byPacific Islanders for medical treatment and in rituals, its only modern use is making jewelry. Black corals have been declining in numbers and are expected to continue declining due to the effects ofpoaching,ocean acidification andclimate change.
Despite its name, a black coral is rarely black, and depending on the species can be white, red, green, yellow, or brown. The corals derive their name from their black skeletons, which are composed ofprotein and chitin.[3] Black corals are also known as thorn corals due to the microscopic spines lining their skeletons.[4]
The name Antipatharia comes from theAncient Greek wordantipathes ("against disease"). In theHawaiian language, a black coral is calledʻēkaha kū moana ("hard bush growing in the sea"); it is the official state gem ofHawaii.[5] InMalay, thecorals are calledakah bahar ("root of the sea"), likely named for their tendency to grow at low-light depths.[6]
Black corals have historically been difficult to classify due to poor-qualityspecimens. They have few distinguishing morphological characteristics, and the few that there are vary across species, similar to other corals. When black corals were first documented byHenri Milne-Edwards andJules Haime, two French zoologists in 1857, all species of Antipatharia were placed in thefamilyAntipathidae.[4] From 2001 to 2006, marine biologists Dennis Opresko and Tina Molodtsova helped transform the taxonomic system into what it is today.[1] A 2007phylogenetic study confirmed the new taxonomic system.[7]
Blacks corals are classified in theorder Antipatharia with 7 families, 44 genera, and 280 distinct species.[2] The families are Antipathidae, Aphanipathidae,Cladopathidae,Leiopathidae,Myriopathidae,Schizopathidae, andStylopathidae.[8] Black corals can be distinguished from other corals by their black, flexible skeletons and near-total lack of any kind of protection from sediment. All black corals have small or medium-sizedpolyps and achitin skeleton, lined with small spines.[9]
List ofgenera according to theWorld Register of Marine Species:[10]
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The skeletons of these corals grow in many patterns unique to this order, such as whips, trees, fans, or coils. These range in size from 10 to 300 cm (3.94 to 118 in), though polyps can be as small as 1 mm (0.0394 in) in size.[4][11] Skeletons are also lined with tiny spikes.[4] These spikes are roughly 0.5 mm (0.0197 in) in size, and vary widely in terms of size, length, proportion, and sharpness.[2] A layer of "bark" forms around the skeleton as the coral grows. The polyps that live inside this bark are less than 2 mm (0.0787 in)[12] and are gelatinous and have sixtentacles (the same as hard corals and unlike soft corals, which have eight).[13] These polyps can be nearly any color.[3] Some corals also have "sweeper tentacles", which can grow up to 15 mm (0.591 in) long.[12] Though individual polyps are either male or female, entire colonies are typicallyhermaphroditic.[14]
Unlike the vast majority of other corals, black corals have no protection against abrasive materials such as sand and rocks and lack muscular development which can help the corals to hide. These factors can lead to sediment tearing the soft tissue, resulting in death. In response, corals live near crevices, which allows much of their body to be protected.[9]
Black corals occur throughout all the oceans from the surface down to the deep-sea, though nearly 75% of species are only found at depths below 50 m (164 ft). The sole oceanic area in which black corals have not been found arebrackish waters, though they can inhabit areas with decreasedsalinity.[15] Black corals are found on reefs, and may contribute to overallreef building, but are also often found as solitary colonies on isolated outcroppings. Most individuals require a hard surface for attachment. They will frequently grow where underseacurrents flow, which allows them to feed on themeiofauna that is swept by. Since undersea currents benefit the corals, they will often grow on or by geographic structures that cause currents, such ascontinental slopes,cliffs,caves, or underseaplateaus.[9] Species distributions of black corals are poorly understood, though many deep sea black corals have large distributions, and more recent work has indicated that shallow black coral species—such asAntipathes grandis—can be found spanning from the Indian to the Pacific Ocean.[16]
Black corals are carnivorous, with the coral's polyps allowing it to feed mostly on meiofauna such aszooplankton.[17][4]Cnidarians have anoral disk in their center which serves as the polyp's mouth; this disk is surrounded bytentacles, whichstings and digests food.[9] The reason many corals are fan-shaped is to catch meiofauna. Many corals only have polyps on the downstream side of the coral,[17] allowing them to catch nearly the same number of animals without wasting energy keeping unnecessary polyps alive.[17]
Vertebratepredation is not a major threat to black corals.[15] There are rare reports ofparrotfish andbutterflyfish gnawing and eating at the polyps of black corals, but even if a polyp is gnawed off, it will not affect the coral as a whole. The skeleton of a black coral is hard and inert, due to its composition of protein and chitin, making it nearly inedible. Though black coral skeletons have been found in the stomachs ofgreen sea turtles andsharks, these incidents are rare; it has thus been suggested that black corals are not a major part of any vertebrate diets.[9] In contrast,gastropods such asmuricids andovulids feed on corals regularly, including black corals.[9] Thesemollusksmimic the fauna that the coral typically feeds on and is taken inside of the coral. They will then consume the polyps from the inside out.[9][verification needed] Some sea snails, such asCoralliophila kaofitorum andPhenacovolva carneptica, overlap completely in distribution with various black coral species (they are not found in places without these corals). This suggests that they arespecialized predators of black corals, feeding mostly on these corals.[15]
Black corals around the world provide a unique environment forcrustaceans,bivalves, and fish. Some species, such asDascyllus albisella andCentropyge potteri inhabit specific coral trees. Due to this abundance of species, nighttime predation around the coral beds has been observed.[18][19][20]
Due to the slow life cycle and deep-water habitats of black coral, little is known about their life cycle and reproduction.[8] As with other cnidarians, the life cycle of these corals involves both asexual and sexual reproduction. Asexual reproduction (also known asbudding), is the first method of reproduction used by a black coral during their lifespan.[17] Once a polyp is anchored, it builds a colony by creating a skeleton, growing new branches and making it thicker, similar to the growth of a tree. This method of growing creates "growth rings" which can be used to estimate the age of a colony.[21] Asexual reproduction can also occur if a branch breaks off and a replacement is needed.[17] Though light is not required for growth or development, mature colonies will grow towards light. Why they do so is unknown.[22]
Sexual reproduction in these corals remains largely unstudied. It occurs after the coral colony is established; a colony will produceeggs andsperm, which meet in the water to create larvae that use currents to disperse and settle in new areas.[17] The larval stage of the coral, called aplanula, will drift along until it finds a surface on which it can grow. Once it settles, itmetamorphoses into its polyp form and creates skeletal material to attach itself to the seafloor. It will then begin to bud, which will create new polyps and eventually form a colony.[17] In areas with ideal conditions, black coral colonies can grow to be extremely dense, creating beds.[9] In some black corals that have been closely examined, colonies will grow roughly 6.4 cm (2.52 in) every year. Sexual reproduction occurs after 10 to 12 years of growth; the colony will then reproduce annually for the rest of its life. The male to female polyp ratio is 1:1, with females producing anywhere from 1.2 million to 16.9 millionoocytes.[23] A large 1.8 m (5.91 ft) tall coral tree is somewhere between 30 and 40 years old.[17]
The estimated natural lifespan of a black coral colony in theepipelagic zone is 70 years. However, in March 2009 around 4,265 years old specimens ofLeiopathes glaberrima were found at depths of nearly 300 to 3,000 m (984 to 9,840 ft), making them some of the oldest living organisms on earth. The researchers showed that the "individual colony longevities are on the order of thousands of years."[24][25] Rarely, black corals will grow too large to support their own weight, and collapse.[9]
Black corals have historically been associated with mystical andmedicinal properties inIndonesian,Chinese, andHawaiian culture.[9][26] More recent harvesting has been for use as jewelry.[26][27] ManyIndo-Pacific peoples believed that black coral has curative and anti-evil powers and made them intonecklace andbracelets; however, black corals are not ideal for jewelry-making due to it beingsoft as opposed tostony,[4] causing jewelry made with it to dry out and break.[4] If a real black coral is boiled in milk, it will smell ofmyrrh; this test can be used to determine if a sample is genuine.[28]
The best studied and regulated black coral fisheries are in Hawaii, where they have been harvested since the 1960s.[26][29] In the Caribbean harvesting is typically done to produce jewelry for sale to tourists, and has followed aboom-and-bust cycle, where new coral populations are discovered andoverexploited leading to rapid declines.[26] For example,Cozumel, Mexico, was famed for dense black coral beds that have been harvested since the 1960s[30] leading to widespread black coral population declines.[31] Despite improvements in management in Cozumel, including no harvesting permits issued since the mid-1990s, the black coral population had failed to recover when assessed in 2016.[32] Though it is illegal to move black corals across international borders without authorization, as they are listed inAppendix II of the Convention on International Trade in Endangered Species (CITES), it is still possible to buy them.[33]
Though various methods have been proposed for quicker and more efficient rejuvenation of black coral colonies, none have worked to the point where these corals could be commercially farmed.[34]
Though black corals are not listed on theIUCN Red List, a number of factors threaten them today. The largest single threat is poaching— though the majority of black coral fisheries are heavily regulated, there is still a black market for the corals.[35] Particularly on tropical islands andMadagascar, the market for illegally-harvested black corals is large.[35][36] Due to overfishing of mature corals, in some areas nearly 90% of corals are juveniles (less than 50 cm (19.7 in) tall.)[37]
Global warming is the primary threat to black corals worldwide, as well as all other corals.[38] Though black corals rarely builds reefs (the most threatened areas), threats caused by climate change such ascoral bleaching, rising sea temperatures, changing underwater currents, and changingsalinity andpH also affect deep-sea corals.[39] Invasive species such asCarijoa riisei, which were introduced to Hawaiian waters by humans, may pose a significant threat to black corals.[36]
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