Gorgonian corals, known as sea fans and also called horny corals, belong to the Phylum Cnidaria (the “C” is silent), which means “nettle” in Greek and includes hydroids, anemones, jellyfish and corals. All cnidarians are carnivores and their tentacles contain tiny harpoons (nematocysts) that can be fired to repel predators and catch food.
There are two cnidarian forms, polyps and medusae. Polyps are attached to a surface on one end by the pedal disk, while the oral disk at the other end is unattached and ringed with tentacles. Medusae are unattached, upside down versions of polyps. Some cnidarian lifecycles include polyps and medusae. Sea fans however, are always polyps (actually, colonies of polyps).
Gorgonians are also members of the Class Anthozoa, which includes sea anemones, corals and sea pens. All have a flower like circle of tentacles and a central mouth opening. Additionally, gorgonians are in the Subclass Octocorallia, so named because their polyps have eight tentacles. Gorgonians belong to the Order Alcyonacea, which includes fixed colonial forms without an axis cylinder.
The four gorgonians described here represent two different families: Gorgoniidae (Eugorgia rubens and Leptogorgia chilensis) and Plexauridae (Muricea fruticosa and Muricea californica).
Sea fans are invertebrates with internal skeletons secreted by the polyps. The stiff but flexible skeleton consists of a horny material strengthened by calcium carbonate spicules and covered by a soft, fleshy layer of skin and mucus. Embedded in the skin are retractable polyps. Their gastrovascular cavities are connected by numerous tiny channels so water and nutrients can flow freely to all members of the colony.
Gorgonians reproduce two ways, by cloning (also called fission) and by spawning. Each sea fan is a colony of genetically identical, same sex polyps. The sea fan grows as the polyps clone themselves. Sea fans also produce either eggs or sperm that are released into the water simultaneously when triggered by some unknown factor. When fertilized eggs hatch, free-swimming larvae emerge, which eventually settle down on the substrate and morph into polyps. Every sea fan begins with a single polyp. Since these creatures are sessile (stationary) and eat plankton, they depend upon currents and surge to bring them food, which is why they grow on the tops and sides of reefs, usually perpendicular to the current flow.
Sea fan predators can include a tiny snail (Neosimnia barbarensis) that eats their flesh and lays its eggs on their branches. Besides a white shell, I have red and brown Neosimnia shells and wonder if they absorb the color of the gorgonian they eat.
Yet another gorgonian predator is a bright yellow colonial zoanthid anemone, which gradually grows over the sea fan’s branches and polyps, killing them. These anemones usually begin to grow on a small part of the gorgonian’s exposed central axis.
New research on gorgonians is revealing previously unknown facts and the scientific classification of many are very likely to change.
In Colors of Brown, Red, Purple and Gold
Each of the articles I write for this feature starts with an image search. I quickly discovered I had photographs of brown, red and purple gorgonians. I also had images of either California goldens or orange gorgonians. They look very much alike and it was impossible for me to figure out what species they were. So I sought the advice of a gorgonian expert, invertebrate zoologist Beth Horvath of Santa Barbara’s Westmont College. Beth quickly determined that some of my photos definitely depicted California goldens. Others might be orange gorgonians. Still others might be species more commonly found in Baja or the Sea of Cortez. The only way to confirm their identities would be lab work on the actual gorgonian photographed. This article features brown, California golden, purple and red gorgonians because I know the sea fans depicted are definitely those species and they are common in SoCal waters.
Brown gorgonian (Muricea fruticosa): First of all, none of my photographs show a brown gorgonian that is actually brown. The thick branches are red and the polyps are white. This sea fan grows as tall as three feet and is found from Point Conception to Baja, in depths from 50 to 100 feet. In Guide to Marine Invertebrates, Alaska to Baja California, Dan Gotshall says, “The colonies are able to survive in some of the most polluted nearshore waters, as well as uncontaminated offshore waters.”
California golden gorgonian (Muricea californica): The exact hue of the California golden’s polyps varies from almost white to bright yellow to almost orange. The thick branches are reddish-brown. Also called the robust gorgonian, the California golden can be three feet high and ranges from Point Conception to Panama. It is found in depths ranging from 40 to 180 feet. Gotshall says, “As the colony grows, annual growth rings are formed in the skeleton.”
Purple gorgonian (Eugorgia rubens): This is one of the easiest sea fans to identify. It is smaller than its close cousins, flat (as opposed to bushy), bright purple, and has white polyps that are found only on the edges. Guide to Marine Invertebrates provides the following description: “The branches form an interwoven pattern on one plane.” Purple gorgonians can reach about 12 inches in height and range from SoCal to Baja in depths from 80 to 100 feet. Off Anacapa they are common in much shallower water.
Red gorgonian (Leptogorgia chilensis): This is another easy-to-identify gorgonian. It is the only one found in our waters that resembles a bush rather than a fan. It is bright red, with white polyps. It grows to heights of three feet. While guidebooks indicate it is found from Monterey to Baja, in depths from 30 to 200 feet, Beth Horvath has determined that the red gorgonians found north of Point Conception may be a different species. I would consider it SoCal’s most common gorgonian because I have seen and photographed it off most of the eight Channel Islands.
Sea Fan Stats
Eugorgia rubens, Leptogorgia chilensis
Muricea fruticosa, Muricea californica
Reproduction: Sexual and asexual