Antipatharians
Antipatharians, also known as black corals or thorn corals, are a cosmopolitan order of soft deep-water corals. They are characterized by their jet-black or dark brown chitin skeletons and colored polyps. These corals are found in almost every oceanic location and depth, with the exception of brackish waters, although they are most commonly observed on continental slopes under 50 m deep. Black corals have both sexual and asexual reproduction methods and provide essential shelter, food, and protection for other marine organisms.
Originally classified in the subclass Ceriantipatharia along with ceriantharians, black corals were later reclassified under Hexacorallia. While they have been historically used by Pacific Islanders for medicinal purposes and rituals, their modern use is primarily for jewelry manufacturing. Unfortunately, black corals are facing a decline in population numbers, which is expected to continue due to poaching, ocean acidification, and climate change.
Contrary to their name, black corals are rarely black and can display a variety of colors such as white, red, green, yellow, and brown, depending on the species. The name "Antipatharia" is derived from the Ancient Greek word "antipathes," meaning "against disease." In Hawaiian, a black coral is referred to as "ʻēkaha kū moana," which translates to "hard bush growing in the sea," and it is the official state gem of Hawaii. In Malay, black corals are known as "akah bahar," likely named for their affinity for low-light depths.
From a taxonomic perspective, black corals have historically posed challenges due to poor-quality specimens and limited distinguishing morphological characteristics. However, research efforts by marine biologists Dennis Opresko and Tina Molodtsova between 2001 and 2006 helped establish the current taxonomic system. According to this system, black corals fall under the order Antipatharia, which comprises seven families, 44 genera, and 280 distinct species. These corals can be identified by their black, flexible skeletons and lack of protection against sediment. They have small or medium-sized polyps and chitin skeletons lined with tiny spines.
Black coral skeletons grow in unique patterns, including whips, trees, fans, or coils, ranging in size from 10 to 300 cm. The polyps residing within these skeletons are gelatinous and have six tentacles, unlike soft corals that have eight. The colors of these polyps can vary widely. Sweeper tentacles, which can grow up to 15 mm long, are also present in some corals. While individual polyps are either male or female, entire colonies are typically hermaphroditic. Black corals lack muscular development and protection against abrasive materials, making them susceptible to damage from sediment. Thus, they tend to inhabit areas near crevices and rely on undersea currents for feeding and dispersal.
Black corals are found throughout the world's oceans, with approximately 75% of species located below 50 m. They are observed in reef environments, where they contribute to reef building, but can also be solitary colonies on isolated outcroppings. Attachment to a hard surface is crucial for their growth, often occurring in areas of strong undersea currents that facilitate food consumption. Although the distributions of black coral species are not well understood, recent research suggests that shallow species, such as Antipathes grandis, can be found spanning from the 🇮🇳 Indian to the 🌊 Pacific Ocean.
Carnivorous in nature, black corals primarily feed on meiofauna, such as zooplankton. Their polyps possess an oral disk that acts as the coral's mouth, surrounded by tentacles for capturing and digesting food. Many corals have a fan-like shape to maximize their ability to catch meiofauna. Polyps are often present downstream of the coral to optimize energy usage.
Overall, black corals are a vital part of marine ecosystems, providing habitat and sustenance for numerous organisms. However, their conservation is essential to combat the threats they currently face.
