a nice sea fan (photo: Shelagh McNally)

By: Shelagh McNally

Sea Fans or gorgolina are the delicate, lace-like plants you see when snorkeling or diving the coral reefs. Though they look like small trees, sea fans are actually animals. There are thousands of different species of sea fans and most remain a mystery. Research into these animals has only just begun.

Because sea fans are not light dependent they can find their niches in sandy gullies, potholes, and the deeper regions of a reef. Alcoves, overhangs, and vertical surfaces provide ideal habitats for some species. And, while most gorgolina live in tropical reefs, sea fans have been found throughout the world’s oceans, including polar seas, from the low intertidal zone to near the bottom of the deepest trenches.

eretoka (photo: 22sept_carol)

They are part of the family known as coelenterate. Coelenterates, a group that also includes jellyfish, sea anemones, and hydroids, are unique among animals in their relatively simple body structure. While humans have a body with two halves, Sea Fans have one built with a circular symmetry where the body parts radiate outwards from a central point much like the spokes of a bicycle wheel. All have hard skeletons, made of calcium carbonate or a tough fibrous horn-like protein, known as spicules. Some species even have a combination of both. The spicules in the tissues of a sea fan give it physical support and make it unpalatable to most predators. They also help scientist identify the various species acting like a kind of fingerprint.

This skeletal structure gives them great flexibility to sway with the water currents. This is important because they are dependent on currents for their food. During development, sea fans grow perpendicular to strong, prevailing ocean currents and wait for the seawater to deliver plankton banquets. To catch the plankton, their branches are covered with thousands of tiny individuals, called polyps. When a sea fan feeds, these polyps open and bloom like a flowering tree in the spring. From each polyp eight feathery tentacles burst forth to catch the food. Together, the sea fan’s branches, polyps, and tentacles create a trap that captures the small particles brought by the current. Unlike hard corals, which have individual stomachs, Gorgonians share a common digestive system and any food caught by an individual feeds the entire colony. This is why it is so important not to pull up sea fans while snorkeling. Killing one can starve a whole colony.

Little is known about reproduction in most sea fans. It is thought that the polyps release egg and sperm into the current where the fertilized eggs develop into a larval stage, (known as a planula). The planula settles into the coral reef to become a founder polyp, which gradually grows into a new sea fan. To protect themselves, these soft corals have chemical compounds that are anti-fungal, anti-bacterial acting to fish and other invertebrate predators.

Sea Fan forest (photo: Shelagh McNally)

Their importance in the coral reef is not yet completely understood. Unlike hard coral, sea fans do not build up the coral reef. The scientific team of Robert Van Syoc and Gary C. Williams has been investigating the ecological relationships between sea fans and barnacles. They have discovered an interesting relationship. Coastal currents contain numerous barnacle larvae searching for a steady place to attach. When barnacle meets sea fan, a slow-motion battle between the two sedentary creatures begins. While healthy parts of the sea fan are protected by nematocysts, (small polyps which sting much like a jellyfish), injured tissues are vulnerable and open to barnacle larvae attaching itself. Sea fans respond to this invasion by trying to repair the damage, overgrowing the developing barnacle with new spicule-rich tissue that forms a conspicuous lump. Frequently, the barnacle hangs on and is able to keep a space in the sea fan’s tissue open for feeding. Most times the sea fan is the victor, and the barnacle becomes encased in the sea fan’s tissue and dies. So sea fans may be responsible for keeping the coral reef healthy and free of destructive barnacles.

reef (photo: Shelagh McNally)

The most common kind of sea fans in the coral reef off Quintana Roo are the Common Sea Fan (Gorgonia ventalina), the Venus Sea Fan (Gorgonia Flabellum) and the Deepwater Gorgonia (Iciligorgia Schrammi). The Common Sea Fan is identifiable because of its bright purple color. It can grow to an impressive height of two meters or five feet. Colonies form large fans that grow in a single plane as a tightly meshed, interconnected network of branches, each with a flat compressed surface. Although some are found at depths of 100 feet, most grow on the seaward side of shallow reefs and slopes where there is water movement. This sea fan has been traditionally used by Caribbean housewives as a sieve for sifting cornmeal, cassava and other grain. The Venus Sea Fan moves to face the water currents and so it is able to trap a greater number of food organisms. Small side branches form at right angles giving it a more dimensional look than the Common Sea Fan. It grows to a maximum of 3 feet and the color varies depending on its locality. Most found here are a bright lime-green. The Deep Water Gorgonia is found at levels deeper than 50 feet. Unlike its other two relatives, it has a more defined skeletal structure. The color may vary from red to brown.

These exotic animals are currently in danger from a fungal disease. This disease causes variable-sized lesions and can even kill a whole colony at some sites. As an integral part of the coral reef, they need to be protected for everyone to enjoy their delicate beauty.