Far from the sunlit shallows, ancient organisms thrive in the deep ocean. Among these are black corals, long-lived animals that grow in complex, branching forms. For centuries, their dark skeletons have been objects of fascination. These corals are found in oceans around the world, forming communities that are foundational to their deep-sea environments.
Biological Characteristics of Black Corals
Though they share a name with their shallow-water relatives, black corals possess a distinct biology. They are animals, not plants, belonging to the class Anthozoa and the order Antipatharia. The name “black coral” is misleading, as the living tissue, or polyps, that cover the skeleton can be an array of colors. The name refers to the colony’s inner skeleton, which is characterized by its dark, jet-black or brown coloration.
This internal skeleton is not made of the calcium carbonate found in many reef-building corals. Instead, it is composed of successive layers of protein and chitin, a flexible and durable organic material. This composition gives the structure a toughness that allows it to bend in strong underwater currents. The surface of the skeleton is also covered in microscopic spines.
Black corals are among the most long-lived animals on the planet. Scientists determine their age by analyzing growth rings in the skeleton, similar to dating a tree. Radiocarbon dating of specimens has revealed ages exceeding 4,000 years, making them some of the oldest known marine organisms. Their growth is exceptionally slow, with some species growing only a few micrometers in diameter each year.
Deep-Sea Habitats and Ecological Importance
Black corals are predominantly found in deep-sea environments, on continental slopes, seamounts, and canyon walls below 164 feet (50 meters). They thrive in these dark waters where strong currents deliver a steady supply of food. Unlike shallow-water corals that rely on a symbiotic relationship with photosynthetic algae, black corals are filter feeders. They extend their polyps to capture tiny particles of organic matter, called “marine snow,” that drifts down from surface waters.
In the vast and often featureless deep sea, these corals function as ecosystem engineers. Their complex, tree-like structures create a three-dimensional framework that provides shelter, feeding grounds, and nursery areas for a multitude of other species. This structural complexity establishes biodiversity hotspots, creating habitats often referred to as “animal forests.” These forests become home to numerous invertebrates and fish.
The presence of these coral forests enhances the biodiversity of their environments. They play a part in the flow of energy from the open water to the seafloor community. Fish and other mobile organisms find refuge within their branches, while stationary animals attach themselves to the coral’s skeleton. The health of these communities are indicators of the broader health of the deep-sea ecosystem.
Human Interaction and Conservation Status
The dense, dark skeleton of black coral has been prized for centuries, primarily for making jewelry and decorative items. This demand has led to significant harvesting pressure on black coral populations. Because these organisms grow so slowly and take more than a decade to reach reproductive maturity, they are highly vulnerable to overharvesting. Once a mature colony is removed, it can take hundreds of years for the population to recover.
Beyond direct harvesting, black corals face other human-caused threats. Deep-sea fishing practices, particularly bottom trawling, cause immense damage. Heavy nets dragged across the seafloor can destroy these fragile, ancient colonies. The destruction of these habitats has lasting consequences for the species that depend on them.
Due to their slow growth and vulnerability, all black coral species are listed under Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). This agreement regulates the trade of black coral to prevent over-harvesting and ensure sustainability. Despite these protections, illegal harvesting continues to threaten populations, making conservation a continuing challenge.