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Gorgonians are corals in the subclass Octocorallia and the order Alcyonacea. They are divided into two main groups, the highly varied Scleraxonia and the suborder Holaxonia. The Scleraxonia group has an inner axis composed of dense or fused skeletal elements, the sclerites. These sclerites are also found in soft corals, but rarely form solid axes there. In contrast, the Holaxonia has a flexible axis, made of a protein called gorgonin. Both groups contain branched and fan-like colonies of polyps, but only the Scleraxonia has encrusting forms.
Gorgonians in all tropical reef systems may, like other corals, be either zooxanthellate or azooxanthellate, having or lacking photosynthetic symbiotic dinoflagellates, respectively. In shallow reefs of the tropical western Atlantic, zooxanthellate forms are dominant and are often abundant reef components. Although gorgonians are common in the wider Indo-Pacific and Red Sea, especially on reef slopes, shallow reefs are usually devoid of anywhere near the number of gorgonians found in the Atlantic. As is true of most coral taxa, diversity in the Pacific is much greater than in the Atlantic, although there are proportionately fewer zooxanthellate species. Colors, as well, tend to be bright to shockingly gaudy in the Pacific, with uniform purple-greys and reds being most common in the Atlantic.
Unfortunately, for the aquarist, the gorgonians of the Pacific are generally very difficult to keep alive in tanks. There are a few zooxanthellate species that can be expected to survive relatively well. The more desirable species, though, such as some of the bright seafans and brilliant blue arborescent colonies, are usually doomed to a short lifespan in captivity. It is likely that a lack of adequate foods is to blame. In contrast, Caribbean species, particularly those commonly collected, are mostly photosynthetic and tend to fare well in captivity. There are exceptions; commonly collected are the orange and red tree gorgonians, Diodogorgia nodilifera and some look-alikes, and the orange tree gorgonian, Swiftia exerta, which lack zooxanthellae and share similarly dismal success in most aquaria with their aposymbiotic Pacific brethren. There are many gorgonians in both oceans which are variably common but are rarely or ever intentionally targeted by collectors.
Despite their good survival in tanks, and ease of propagation, the photosythetic gorgonians have a number of issues which may be important to aquarists; many ship poorly and foul their bag water. Having collected gorgonians and seafans, the amount of purplish-gray secretions and mucous of some species is astonishing, and transport seems nearly impossible if more than a few hours elapse from ocean to tank. Furthermore, these species are well-endowed with rich chemical arsenals of secondary metabolites which must be considered in terms of their impact on sensitive tankmates. Gorgonians produce an incredible diversity of complex and biologically active compounds, many of which are being investigated for pharmacological potential.
Another potential drawback to gorgonians is their size. Some species become absolutely huge, and few tanks would be able to maintain an adult colony of some of the large fans and tree-like colonies. Others, fortunately, remain "aquarium-sized." The encrusting gorgonians, such as Erythropodium spp. and Briareum spp., can be very aggressive and fast-growing, quickly overtaking almost anything in their way. It is best to consider carefully the purchase of these species, and make allowances to prevent a tank from quickly becoming a living carpet of encrusting gorgonian.
This is a very large group of corals, most of which will never be seen in the aquarium trade. Properly selected, gorgonians make exceptional candidates for aquaria and for captive propagation. Many of them, however, are currently unable to be maintained in closed systems, with a very few having occasional reports of success in an individual tank. It is best to leave the species with difficult husbandry needs in the ocean. If, however, one finds themself in possession of one of the more beautiful and hard-to-keep species, it should be mentioned that it will likely need a very large amount of fine particulate food sources, possibly including phytoplankton as a component. Detritus, bacteria, and pseudoplankton in very small size classes may be the best choices, if available. Remember, seafans should always be positioned perpendicularly to water flow! Other tips on gorgonian placement and husbandry are listed in the book, Aquarium Corals, by Eric Borneman.
Text by Eric Borneman. Photos by Reef Central members.
