Octocorals are a diverse group of marine animals found across the world’s oceans. Often referred to as “soft corals,” they captivate with their wide array of forms and vibrant colors, resembling everything from delicate plants to intricate fans. These organisms contribute significantly to the underwater landscape, creating living structures that enhance marine environments.
Anatomy and Structure
The name “octocoral” reflects a defining characteristic: each individual polyp possesses eight tentacles. These tentacles are feather-like, with numerous small side-branches called pinnules that increase their surface area. This eight-part symmetry distinguishes octocorals from their stony coral relatives, hexacorals, which typically have tentacles in multiples of six.
Octocorals possess an internal skeleton, contributing to their soft and flexible nature. This internal skeleton is composed of microscopic, calcareous elements called sclerites, embedded within their tissues. Unlike the massive external skeletons of stony corals, these scattered sclerites provide support and rigidity while allowing octocorals to sway with ocean currents. The arrangement and morphology of these sclerites are also used by biologists to classify different octocoral species.
Types and Habitats
Octocorals exhibit diversity in their forms and are found across various marine environments, from shallow tropical waters to depths of 4000 meters. One prominent group is sea fans, also known as gorgonians, which often display a flattened, fan-like branching structure. These corals commonly thrive on reef walls and slopes, orienting themselves to filter food particles from passing currents. Their intricate structures can vary, with some appearing like small trees or “bottle brushes” rather than flat fans.
Sea whips are another common type, characterized by their long, slender, and often unbranched or sparsely branched forms. They are found in areas with moderate to strong currents, anchoring themselves to hard substrates and extending into the water column. Sea pens are a distinct group adapted for life in soft, sandy, or muddy seabeds. They have a fleshy, bulbous peduncle that anchors them in the sediment, with polyps arranged along a central stalk, often resembling a quill pen.
Other octocorals include fleshy soft corals that lack a rigid central axis and rely on hydrostatic pressure or dense sclerite arrangements for shape. Blue corals, while considered octocorals, are unique as they produce a massive, stony skeleton of aragonitic calcium carbonate, similar to hard corals. This wide range of forms allows octocorals to occupy diverse niches, from encrusting surfaces to forming tall, tree-like structures that provide vertical relief in otherwise flat environments.
Ecological Role in Marine Ecosystems
Octocorals play a significant role within marine ecosystems, contributing to the health and structure of underwater habitats. Their complex, three-dimensional growth forms provide shelter for a variety of marine life. These intricate structures serve as nursery grounds and refuges for small fish, crustaceans, and other invertebrates, shielding them from predators and strong currents. Their presence supports a high diversity of other organisms.
Beyond providing physical habitat, many octocorals are filter feeders, playing a role in water clarity and nutrient cycling. They capture small food particles, such as plankton and organic matter, from the water column using their eight pinnate tentacles. This helps process and remove suspended particles, contributing to water clarity. Their upright forms allow their polyps better access to food as water flows past the colony.
Threats to Octocorals
Octocoral populations face several threats to their long-term survival in marine environments. Physical damage from human activities, such as bottom-trawling fishing gear, is a direct danger. These heavy nets can sweep across the seafloor, breaking off or dislodging coral colonies, causing widespread destruction. Such damage can take many years for slow-growing species to recover, if at all.
Climate change also represents a threat through ocean warming and acidification. While some octocorals show more resilience to bleaching than hard corals, prolonged elevated temperatures still lead to stress and mortality. Ocean acidification, from excess carbon dioxide absorption, reduces carbonate ion availability, making it harder for octocorals to form and maintain their calcareous sclerites. Pollution from land-based sources, including runoff containing sediments and chemicals, degrades water quality and negatively impacts octocoral health and growth.