Gorgonian Corals: Sea Fans and Sea Whips

Rising from the seafloor, whether as an intricate network of intertwining branches or a single meandering twig-like structure towering over the seabed, sea fans and sea whips bring a dazzling flair of colour to their marine environments. They belong to the genus of soft coral called Gorgonia, and are found in all the world’s oceans, from shallow coastal waters to thousands of metres deep. There are over 500 species of these colourful sea creatures, with just over 170 species in Indian waters alone. While their structure may closely resemble that of plants or algae, just like their hard coral relatives, they are actually colonial animals (collective life forms of individual organisms). Despite being classified as soft corals, gorgonians have a skeleton comprising primarily of a tough protein known as gorgonin. This provides the colony with a strong yet flexible structure, and it is also where they derive their name from. This tough but flexible skeleton, along with their rapid vertical growth, allows gorgonians to tower above their competitors (which include other corals and algae), and then take on several forms, ranging from the simple elongated, unbranched structure of sea whips to large, complex reticulate and branching paddle-like structures of sea fans. Like other corals, many species of sea fans and sea whips have microscopic algae, called zooxanthellae, living within their tissues. These photosynthetic associates produce food in the presence of sunlight, providing sea fans and whips with nutrients. Alongside this, the individual polyps of the gorgonians also help trap floating particles such as plankton, which provide them with an additional food source. To optimise their ability to feed on these floating particles, sea fans often grow in current-exposed areas, like steep slopes, with their flat surface facing the direction of the current. This allows more water to flow over their surface and increases their ability to collect food particles coming through. 

Gorgonians come in a variety of brilliant colours, from bright oranges and yellows (top) to pinks and purples (bottom right). This diverse colour palette stems from the different colours of zooxanthellae that form a symbiotic relationship with the gorgonians and live within their tissues. This image captures the different forms gorgonians come in, from broad reticulate sea fans (orange) to the branching bush type (pink/purple, bottom right), and long unbranching sea whips (blurred dark lines in the background, centre and right). 

A close look at this sea fan reveals the individual polyps that cover its surface, giving it a fuzzy appearance. Each polyp has eight tiny arms which surround a central mouth. They use these arms to trap small plankton and move them into their mouth. The intricate fractal pattern (geometric shape) that is created by the rapidly dividing branches of a sea fan helps increase surface area and the number of polyps, which helps them trap more food particles suspended in the water.

Feather stars (relatives of starfish) often perch at the very top of sea fans. This is because, like sea fans, they are filter feeders. They use their feathery arms to trap small plankton and food particles from the water. By positioning themselves at the very top of a sea fan, they are in the best position to filter out the food particles that come with water currents.

Many gorgonians produce toxic organic compounds that protect them from predation, and, as a result, they are fairly long-lived. Flamingo tongue snails and spindle cowries, for instance, are vibrantly coloured marine snails that are among the few species that prey on gorgonians. While gorgonians may not serve as a food source for many marine organisms, their almost fractal-like skeletal network provides an intricate maze for other small marine critters to call home. Some of these are brittle starfish, feather stars, shrimps and crabs, and even small species of goby fish.

Sea fans and sea whips are often home to a plethora of small marine critters like (1) whip coral shrimps (Miropandalus sp.), (2) brittle stars, (3) spider crabs (Xenocarcinus depressus), and (4) spindle cowries (Phenacovolva sp.), to name a few. Many of these species are uniquely adapted to resemble their host gorgonian and will spend their entire life on a single gorgonian. However, the relationship between many of these species and their host gorgonians is still unknown.

Gorgonians function as ecosystem engineers, providing homes for a plethora of organisms. Many of these creatures are uniquely adapted to live amongst the arms of a particular gorgonian species and nowhere else. They mimic the gorgonian down to the smallest detail. Bargibant’s pygmy seahorse and Denise’s pygmy seahorse, for example, are found exclusively on gorgonians. The former is only found on gorgonians of the genus Muricella. Once the larvae of these species settle on a gorgonian, they undergo a transformation that allows them to perfectly resemble a small branch of that particular gorgonian in terms of colour and texture. After this, they will spend their entire adult life on a single gorgonian. 

Whip coral gobies (Bryaninops spp.) are small fish that form associations with whip coral. They are attracted to whip coral by chemical cues that the coral emits. The gobies then feed on toxic algae that try to grow on the coral.

Some of the toxins that gorgonians produce to deter predators also show strong antimicrobial properties. Bottlenose dolphins have been observed rubbing their bodies against gorgonians, and we believe this may be their attempt to use the gorgonians as a natural disinfectant. Dolphins are not the only ones utilising these organic compounds; several compounds extracted from gorgonians have been identified as potential candidate drugs by the pharmaceutical industry. 

Both images show the same sea fan, a few months apart, (1) first as a healthy sea fan and (2) then smothered by algae. Why this sea fan was unable to cope with the algal bloom is unknown, but it could be due to changing water conditions, such as an influx of sewage (organic matter) that promotes algae growth, or the overfishing of herbivorous fish that would otherwise feed on the algae.

Gorgonians, such as those of the genus Corallium, are harvested, and their skeletal structure is processed to make red coral stones used in jewellery. However, more pressing threats are anthropogenic (human-induced) disturbances such as bottom trawl fishing and climate change. Bottom trawl fishing involves dragging large nets across the seabed, often targeting creatures like shrimp and flounder, but in the process, uproots anything and everything in the area. Rising water temperatures due to climate change stress the zooxanthellae associated with many gorgonians, leading to the expulsion of the zooxanthellae by the gorgonians and subsequent bleaching. If these adverse conditions persist, gorgonians will eventually die. Losing gorgonians doesn’t just mean the loss of a few colourful flourishes that rise from the seafloor, but also the diverse biomes that they support.