| Soft coral | |
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| Cladiella sp. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Cnidaria |
| Class: | Octocorallia |
| Order: | Alcyonacea Lamouroux, 1812[1] |
| Suborders | |
See text | |
| Synonyms | |
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Alcyonacea are an order ofsessilecolonialcnidarians that are found throughout the oceans of the world, especially in the deep sea, polar waters,tropics andsubtropics. Whilst not in a strict taxonomic sense, Alcyonacea are commonly known assoft corals. The term "soft coral" generally applies to organisms in the two ordersPennatulacea and Alcyonacea with their polyps embedded within a fleshy mass ofcoenenchymal tissue.[2] Consequently, the term "gorgonian coral" is commonly handed to multiple species in the order Alcyonacea that produce a mineralized skeletal axis (or axial-like layer) composed of calcite and the proteinaceous material gorgonin only and corresponds to only one of several families within the formally accepted taxon Gorgoniidae (Scleractinia). These can be found in order Malacalcyonacea (taxonomic synonyms of include (unaccepted):Alcyoniina,Holaxonia,Protoalcyonaria,Scleraxonia, andStolonifera.[3]
Common names for subsets of this order aresea fans andsea whips; others are similar to the sea pens of related orderPennatulacea. Individual tinypolyps form colonies that are normally erect, flattened, branching, and reminiscent of afan. Others may be whiplike, bushy, or even encrusting.[4] A colony can be several feet high and across, but only a few inches thick. They may be brightly coloured, often purple, red, or yellow. Photosynthetic gorgonians can be successfully kept in captiveaquaria.
About 500 different species of gorgonians are found in the oceans of the world, but they are particularly abundant in the shallow waters of the Western Atlantic, includingFlorida,Bermuda, and the West Indies.[5]
The structure of a gorgonian colony varies. In the suborder Holaxonia, skeletons are formed from a flexible, horny substance calledgorgonin. The suborder Scleraxonia species are supported by a skeleton of tightly grouped calcareous spicules. Also, some species encrust like coral.[6]
Measurements of the gorgonin and calcite within several long-lived species of gorgonians can be useful inpaleoclimatology andpaleoceanography, as their skeletal growth rate and composition are highly correlated with seasonal and climatic variation.[7][8][9]
Soft corals contain minute, spiny skeletal elements calledsclerites, useful in species identification. Sclerites give these corals some degree of support and give their flesh a spiky, grainy texture that deters predators. In the past, soft corals were thought to be unable to lay new foundations for future corals, but recent findings suggest that colonies of the leather-coral genusSinularia are able to cement sclerites and consolidate them at their base into alcyonarian spiculite,[10] thus making them reef builders.
Unlikestony corals, most soft corals thrive in nutrient-rich waters with less intense light. Almost all use symbiotic photosyntheticzooxanthella as a major energy source. However, most readily eat any free-floating food, such as zooplankton, out of the water column. They are integral members of the reef ecosystem and provide habitat for fish, snails, algae, and a diversity of other marine species.
Despite being dominated by "soft corals", the order Alcyonacea now contains all species known as "gorgonian corals", that produce a hard skeleton made from gorgonin, a protein unique to the group that makes their skeletons quite different from "true" corals (Scleractinia). These "gorgonion corals" can be found in suborders Holaxonia, Scleraxonia, andStolonifera.
Many soft corals are easily collected in the wild for the reef aquarium hobby, as small cuttings are less prone to infection or damage during shipping than stony corals. Nevertheless, home-grown specimens tend to be more adaptable to aquarium life and help conserve wild reefs. Soft corals grow quickly in captivity and are easily divided into new individuals, and so those grown byaquaculture are often hardier and less expensive than imported corals from the wild.
Each gorgonian polyp has eighttentacles, which catchplankton and particulate matter for consumption. This process, calledfilter feeding, is facilitated when the "fan" is oriented across the prevailing current to maximise water flow to the gorgonian, hence food supply.
Some gorgonians contain algae, orzooxanthellae. Thissymbiotic relationship assists in giving the gorgonian nutrition byphotosynthesis. Gorgonians possessing zooxanthellae are usually characterized by brownish polyps.
Gorgonians are found primarily in shallow waters, though some have been found at depths of several thousand feet.[4][6] The size, shape, and appearance of gorgonians can be correlated with their location. The more fan-shaped and flexible gorgonians tend to populate shallower areas with strong currents, while the taller, thinner, and stiffer gorgonians can be found in deeper, calmer waters.[4]
Other fauna, such ashydrozoa,bryozoa, andbrittle stars, are known to dwell within the branches of gorgonian colonies.[11] Thepygmy seahorse not only makes certain species of gorgonians its home, but also closely resembles its host, thus is well camouflaged.[12] Two species of pygmy seahorse,Hippocampus bargibanti andHippocampus denise, are obligate residents on gorgonians.H. bargibanti is limited to two species in the single genusMuricella.
Gorgonians produce unusualorganic compounds in their tissues, particularlyditerpenes, and some of these are important candidates for new drugs.[13] These compounds may be part of thechemical defenses produced by gorgonians to render their tissue distasteful to potential predators.[14]Bottlenose dolphins in theRed Sea have been observed swimming against these tissues, in what is thought to be an attempt to take advantage of the antimicrobial qualities ofditerpenes.[15] Despite these chemical defenses, the tissues of gorgonians are prey for flamingo tongue snails of the genusCyphoma,nudibranchs, thefirewormHermodice spp., and their polyps are food forbutterflyfishes.[16] Amongst the nudibranchs which feed on soft corals and sea fans are theTritoniidae and the genusPhyllodesmium which specialises in eatingXenia species.[17]
The World Register of Marine Species lists thesesuborders andfamilies:[18]
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