Sea fans are among the most beautiful sights seen by divers. Gorgonian sea fans are Cnidarians that build colonies in branching formations that usually are fan-shaped, thus the common name.
Like the Nephtheid soft corals I wrote about recently here on Photo Synthesis, Gorgonians are octocorals: each polyp has eight pinnate tentacles which it uses to capture nutrients suspended in the water column. They are seen most often on reef crests, or jutting out from drop-offs or steep banks in locations where natural currents will sweep plankton and other organic nutrients across the polyps’ tentacles.
Some sea fan species form colonies in a single plane, while others grow their branches in somewhat of a tangle. They come in quite an array of colors, most of which actually are the result of zooxanthellae that live in the structural tissue of the Gorgonian. Beyond giving them attractive coloration, zooxanthellae also produce nutrients through photosynthesis, which benefit the Gorgonians.
By the way, not all Gorgonians form fan-shaped colonies. Some families in this order form long, slender colonies, commonly known as Sea Whips — but we’ll save those for another time. Today I’d like to show you some close-up and macro photos of some Gorgonian sea fan species from several tropical locations around the world to illustrate some of the variability in the morphology of the colonies. (A brief description follows each photo.)
Above is a sea fan from the genus Melithaea, with its polyps open for feeding. The branches of sea fans in this genus tend to intertwine. This one was photographed at Bunaken Island, Indonesia.
The next sea fan, Acabaria sp., forms a fragile looking net-like colony. There are other species in this genus that form denser structures. This one was photographed in the Red Sea, off the coast of the Sinai Peninsula.
The above macro image shows a section of Subergorgia hicksoni, a species widely distributed throughout the Red Sea. This sea fan species is not so beautifully colored, but the colonies grow to an impressive size, very often measuring more than a meter in width. The fans are relatively flat, or gently curving. This species is dominant on many deep reefs in the Red Sea, whereas Acabaria spp. seem to prefer shallower depths.
This purplish sea fan is Gorgonia ventalina, photographed in the Cayman Islands. It forms a tight mesh, and grows in a single plane. G. ventalina is very common throughout the Caribbean Sea. In fact, the sight of this purple fan is almost a hallmark of Caribbean reefs. It tends to live in on patch reefs in relatively shallow water, so it is seen very frequently by divers and snorkelers.
The final photo, below, shows an entire colony of G. ventalina, photographed at West Caicos, in the Turks and Caicos Islands.