One piece of trivia I've learnt while looking stuff up for this post: the genus name Antipathes, from which the whole group of the Antipatharia derives its name, was coined to refer to the supposed ability of black coral to cure illnesses and protect against evil. It almost goes without saying that I found no indications that this evaluation was warranted.
The black corals of the Antipatharia are a group of colonial, sessile cnidarians that are found in marine waters around the world. They are predominantly deep-water animals, found mostly below the level of light penetrance. Those individuals that are found in shallower waters still keep to secluded habitats out of the light. Some of the shallowest communities are found in New Zealand at depths of only 4 m in the fiords of the South Island, where a rich concentration of tannins in the top layer of the water prevents light from reaching even that far down (Wagner et al. 2012). Black corals have been harvested in many parts of the world for jewellery (and also for their supposed curative properties referred to above), but they are very slow-growing animals. At least one colony subjected to radiocarbon dating was estimated to be over 4000 years old (Roark et al. 2009).
Colonies of antipatharians may be highly branched, or they may form an unbranched whip (the latter forms are sometimes referred to as wire corals or whip corals). They may be only a few centimetres tall, or they may reach a length of several metres in the case of some wire corals (Wagner et al. 2012). The core of the colony is a stalk composed of chitin that varies in colour from jet black in the main stem to golden yellow at branch tips. The stalk is lined with spines that may be simple cones, or may be covered with denticles, or may even be branched and antler-like. In life, the stalk is encased in living tissue, so black corals are not actually black. Unlike other skeletonised cnidarians in which the polyps are recessed within the skeleton, those of antipatharians are entirely external to it. As a result, black corals are rarely found in locations where there is a lot of moving sediment in the water, as they lack the ability to entirely retract the polyps to protect them from abrasion. The individual polyps are usually only a few milimetres wide and up to a few centimetres tall when extended. All antipatharian polyps have six tentacles and six primary mesenteries; depending on the species, there may also be four or six secondary mesenteries, though members of the family Cladopathidae lack secondary mesenteries altogether.
The most recent classification of the Antipatharia divides it between seven families, some of which have been recognised only very recently. Because their deep-water habitat makes the study of live colonies difficult, and many features of the minute polyps become obscured in preserved material, earlier classifications focused heavily on features such as the branching arrangement of the colony, or the morphology of the spines on the skeletal axis. However, these features may be influenced by environmental factors, and their significance may have been overestimated. For instance, a molecular phylogenetic analysis by Brugler et al. (2013) found that the unbranched wire coral genus Cirrhipathes was polyphyletic and not separated from the branched genus Antipathes. Nevertheless, Brugler et al. did find that the higher-level relationships within the Antipatharia were mostly concordant with morphology, including the distinction of the seven families. These relationships included a divergent position for Leiopathes, the only genus with six secondary mesenteries; a clade including the bathyal families Schizopathidae and Cladopathidae, in which the polyps are transversely elongated; a close relationship between the families Myriopathidae and Stylopathidae, with polyps that are not elongated and have relatively short, subequal tentacles; and an association of the families Antipathidae and Aphanipathidae, in which the sagittal tentacles tend to be quite elongate relative to the lateral tentacles. There was still, of course, room for investigation: one notable anomaly is that the type species of Antipathes, A. dichotoma, was identified as a member of 'Aphanipathidae' rather than 'Antipathidae'. If correct, this would mean that aphanipathids should be called antipathids, while antipathids would be... something else.
Brugler, M. R., D. M. Opresko & S. C. France. 2013. The evolutionary history of the order Antipatharia (Cnidaria: Anthozoa: Hexacorallia) as inferred from mitochondrial and nuclear DNA: implications for black coral taxonomy and systematics. Zoological Journal of the Linnean Society 169: 312-361.
Roark, E. B., T. P. Guilderson, R. B. Dunbar, S. J. Fallon & D. A. Mucciarone. 2009. Extreme longevity in proteinaceous deep-sea corals. Proceedings of the National Academy of Sciences of the USA 106 (13): 5204-5208.
Wagner, D., D. J. Luck & R. J. Toonen. 2012. The biology and ecology of black corals (Cnidaria: Anthozoa: Hexacorallia: Antipatharia). Advances in Marine Biology 63: 67-132.