Gorgonians are a diverse group of marine animals often recognized by their striking, plant-like appearances, commonly known as sea fans or sea whips. These organisms are not plants, but rather a type of soft coral, belonging to the phylum Cnidaria, which also includes jellyfish and sea anemones. They are colonial, sessile animals, with individual polyps living together in a colony anchored to the seafloor.
Physical Characteristics
Gorgonians exhibit a wide array of forms, ranging from broad, branching fans to slender, whip-like structures and even bushy or encrusting shapes. The internal support for these colonies comes from a flexible central axis composed primarily of gorgonin, a protein similar to keratin. This semi-rigid protein allows the colony to sway with water currents, aiding feeding.
Embedded within the gorgonian’s soft tissue are microscopic, calcareous structures called sclerites, which provide additional support. These sclerites can vary in shape and arrangement, and their specific configurations are often used to identify different genera or species. The surface of the gorgonian colony is covered by numerous tiny polyps. Each polyp possesses eight tentacles, often pinnate, with feather-like side branches.
Habitat and Distribution
Gorgonians are widely distributed across the world’s oceans, inhabiting diverse marine environments from shallow, sunlit tropical reefs to the cold, dark depths of the deep sea. They attach to stable hard surfaces on the seafloor, such as rocks, other corals, or various stable structures. Their presence is documented at depths from a few meters to several thousand feet.
These corals are particularly abundant in regions like the Caribbean and the Indo-Pacific, where they contribute to the visual complexity of reef ecosystems. Deep-sea gorgonians can form extensive “animal forests” on continental shelves and slopes, including areas like the Northeast Channel in the Atlantic Canada, where species such as Paragorgia arborea and Primnoa resedaeformis can be found at depths exceeding 400 meters. Their distribution is influenced by factors such as temperature, salinity, and the availability of suitable hard substrates for attachment.
Ecological Role in Marine Environments
Gorgonians function as ecosystem engineers, constructing complex, three-dimensional structures. These intricate forms create shelter and nursery grounds for a variety of marine life, including fish, crustaceans, and other invertebrates. For example, pygmy seahorses are known to live exclusively on certain sea fan species, utilizing the gorgonian’s structure for camouflage and protection.
Gorgonians are filter feeders. Their polyps extend eight tentacles into the water column to capture plankton and other particulate organic matter. In some shallow-water gorgonian species, a symbiotic relationship exists with photosynthetic algae called zooxanthellae, which provide nutrients through photosynthesis. However, many deep-water and some shallow-water gorgonians do not host these algae and rely solely on filter feeding for their energy.
Relationship to Other Corals
Gorgonians belong to the subclass Octocorallia, distinguishing them from hard, reef-building corals classified under Hexacorallia. A primary distinguishing feature is their skeletal structure; gorgonians possess a flexible, internal skeleton made of gorgonin protein. This internal framework allows them to sway with currents, unlike the rigid, external calcium carbonate skeletons of hard corals.
Another difference lies in their polyps; octocorals, including gorgonians, consistently have polyps with eight tentacles. In contrast, hard corals, or scleractinians, typically have polyps with tentacles in multiples of six. While hard corals build large, stony reefs, gorgonians contribute to the biodiversity and three-dimensional complexity of marine ecosystems without building massive calcium carbonate structures.