Black coral is a fascinating marine organism whose reputation for rarity is complex and often misunderstood. While the term “black coral” might suggest a highly endangered species, the truth is more nuanced, depending on depth and location. Black corals (order Antipatharia) are biologically widespread across the globe’s oceans. However, their presence in accessible shallow waters is limited, which is why they are perceived as rare and vulnerable to human activities.
What is Black Coral (Antipatharia)?
Black corals are not defined by the color of their living tissue but by the material beneath it. The living polyps covering the branches are typically vibrant, appearing white, red, orange, or green, depending on the species and location. The common name comes from the underlying skeleton, which is made of a tough composite material of chitin and protein that is jet-black or dark brown.
These organisms belong to the class Hexacorallia. Unlike stony corals (Scleractinia), which build hard, calcium carbonate skeletons, black corals possess a flexible but rigid internal skeleton. This skeletal structure often features tiny, thorn-like spines, giving them the alternate name of thorn corals. Black corals can grow into diverse forms, including branching bushes, fans, or single whips, and they do not rely on symbiotic algae for food.
The Reality of Rarity: Distribution and Abundance
The order Antipatharia is remarkably widespread, with approximately 222 to 280 species identified across all oceans, from the Antarctic to the Arctic. This global distribution means that black coral is not biologically rare in terms of overall abundance. The vast majority of species reside in the deep sea, primarily inhabiting the dimly lit mesophotic and bathyal zones.
These deep-water populations often form dense aggregations referred to as “coral gardens” or thickets, typically found at depths ranging from 60 meters down to several thousand meters. Only a small fraction of species occurs in shallow waters accessible to divers, usually in cryptic environments like caves or beneath ledges. This scarcity in accessible reef ecosystems is the primary reason for the public perception of black coral as rare. Shallow-area species are frequently targeted by the commercial jewelry trade, increasing their localized vulnerability.
Extreme Lifespans and Slow Growth Rates
The vulnerability of black coral populations results from their extreme longevity and sluggish growth rate. Black corals are among the longest-living marine invertebrates on Earth; colonies of the genus Leiopathes are estimated to live for thousands of years. Scientific age dating has confirmed some individual colonies have existed for over 4,000 years, with one specimen estimated to be 4,265 years old.
This longevity is linked to an exceptionally slow growth process. Radial growth, the thickening of the internal skeleton, is often measured in just a few micrometers per year, with rates ranging from 5 to 30 µm annually. Linear extension of branches is also slow, sometimes only a few millimeters in a year. Because of this slow life history, damage or removal of a colony represents an impact that takes centuries or millennia to recover from, making populations highly susceptible to disturbance.
Conservation Status and Protection Measures
The primary threats to black coral populations are related to human activities. Deep-sea fishing practices, such as bottom trawling, can devastate entire coral gardens by crushing or ripping up colonies that have taken centuries to grow. Targeted harvesting for the jewelry trade, which values the polished black skeleton, has historically depleted shallow-water species, particularly in regions like Hawaii.
To regulate the international trade of this ancient resource, the entire order Antipatharia is listed on Appendix II of CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora). This listing does not ban trade but requires that all international commercial transactions be accompanied by an export permit. This requirement ensures that the trade is monitored and does not threaten the survival of the species in the wild, providing a legal framework for their protection.