Coralmania by Eric Borneman

Taxonomy in Aquarium Corals:
Part 3 - Everything Else - Soft Corals, Zoanthids, and Corallimorpharians

In the first two parts of this series, I have described difficulties in establishing the identity of corals, especially to a species level. I have focused on the Scleractinia, or stony corals, because of all the groups (and as difficult as they are!), they are the easiest to identify in most cases. For groups without any real physical material to examine, identification becomes even more difficult.

In stony corals, I explained, one is faced with using limited traits to establish a proper identity. Even if skeletal characters are visible in a living coral at all, reaching a species identification ranges from relatively easy for a few, to difficult or impossible for the majority. Soft corals and gorgonians do have some skeletal elements to use in identification, but they are not visible or useful at all in the living coral. Zoanthids and corallimorpharians have no skeletal elements at all. Thus, in these groups, taxonomy is based on other characteristics. As with stony corals, identification may be more precise using molecular techniques. Only a handful of stony corals have been examined in this manner, and they are FAR more well-studied, as a group, than any discussed here. Consequently, the identification of some of these animals, even if it were possible for the aquarist using various skills and techniques, is completely negated by the fact that the actual identification may not be known at all. In the first part of this article, I described the use of type specimens, where a specimen in question is compared to a known species example. For many of the organisms here, no type specimens exist, and many found in museums or collections are likely to be incorrectly identified, so that no valid comparisons are possible at all. To compound the problems in species identification, even generic or familial level identifications may be extremely difficult or not properly classified.

Soft Corals

The octocorals (soft corals, stoloniferans and gorgonians) are hard to identify, even to a genus level. Part of the problem is that there are no visible skeletal structures in living corals to help aid in the identification process. Most soft corals contain spicules of calcium carbonate, called sclerites, within their tissues. The shapes and sizes of the sclerites are often different in the polyps, the base, and the interior. They may be loosely present throughout the tissue (as in members of Xeniidae), tightly packed (as in some Sinularia spp.) or even fused (as in most gorgonians axial material). Furthermore, sclerites can be variable. The color and arrangement of the sclerites must also be considered for accurate identification. The sclerites and their features are used in classical taxonomy, and these must be prepared from living tissue and then examined, usually by scanning electron microscopy. Even if one were to extract the sclerites and look at them under a microscope, it is the distinct patterns of bumps, shapes, and protuberances (tubercles) that must be considered. Obviously, this is beyond the reach of most aquarists. Also useful in octocoral taxonomy are chemical isolates from the secondary metabolites that are produced, although variability may make these studies somewhat less accurate. Finally, genetic typing is probably the most accurate method of identification.

Frederick Bayer (1981) made what I consider to be the best key to the species of Octocorallia. However, much of the needed information to key-out species is not available to aquarists in living corals. I have drawn heavily from this key in Aquarium Corals (Borneman, 2001), and made use of visually distinct elements in the identification sections of the octocoral section. Since this writing, a very important work has been published entitled Soft Corals and Sea Fans: A Comprehensive Guide to the Tropical Shallow-water Genera of the Central-West Pacific, the Indian Ocean and the Red Sea (Fabricius and Alderslade 2000). In my mind, this book is a "must have" for anyone interested in the octocorals from this region. In fact, outside of a few other limited or less-accurate sources, this may be the only book of its kind.

The following is a practical method of establishing the identity of living octocorals:

Identification of soft corals begins with noting the presence of polyps having eight tentacles that normally have feathery pinnules along their edges.

Step 1. Determine the growth form of the octocoral:
Branched (and branching patterns)

Step 2. Determine if the coral contains both autozooids and siphonozooids (dimorphic) or if there are only autozooids present (monomorphic). Autozooids are the primary feeding polyps and are usually the most visible. Siphonozooids are usually smaller, appearing as small bumps without tentacles on the coral surface, and are used mainly for water flow within the colony.

Step 3. Note the features of the colony
Is there a capitulum or polypary (a localized polyp-bearing surface)?
Are primary or secondary branches sterile (lacking polyps)?
Are sclerites visible through the tissue?
Are any sclerite arrangements visible (such as supporting "baskets" around polyps)?
Does the colony have a rough or smooth feel?
Is the colony soft or firm?
Is the colony slippery?
Does the colony contract or collapse?
Are the polyp tentacles highly pinnate? Do they look like feathers or are the pinnules barely visible on the tentacle?
Do the polyp tentacles sit atop of stalks (anthosteles)? The anthostele is the little rod-like element that the tentacles are perched upon in Sarcophyton and to a lesser degree in a few other genera.
Do the polyps have visible, raised, or contrasting calyces into which they withdraw?
Do calyces, if present, have a notable shape (rounded, angular, bumpy)?
What are the locations of the polyps on branchlets (alignment)?
Do the polyps arise from a mat?
Is there a significant coenenchyme (matrix in which the polyps are embedded)?
Are the colors of the colony indicative of a taxon (see guidebooks for specifics)?
Are the polyps a different color from the colony?
Are there behavioral characteristics:
Do polyps pulse? Does the colony shed a mucus tunic? Does the colony contract at night?
Can the polyps withdraw completely (retractile), contract (contractile), fold on themselves or are they completely non-retractile?
Are there specific branching patterns (single rods, fingers, nets (fans), anastomosing (interconnected) branches, branching in a plane, branching in multiple planes (bushy), lobes, hinges, etc.)?
If branching, are the branches stiff or flexible?
Do secondary branches arise directly opposite each other, in alternating series, as Y-shaped divisions, or randomly?
Does the colony drop branchlets?
Are there stolons (mat-like bases from which the polyps arise or are embedded) present? Solid layers or rootlets?
Does the colony have a strong odor out of water?
How large is the colony?
Is it zooxanthellate or azooxanthellate?

Step 4. Use the characteristics to assign a family or genus level designation, if possible, using various identification guides, such as Fabricius and Alderslade (2001).

Lobophytum sp., note dimorphic polyps: siphonozooids and autozooids in photo above.
The colony has a short to non-existent stalk, and finger-like lobed projections. It is quite firm, indicating extensive sclerite support.

Also note gorgonian in background. It is purple, displays pinnate, planar branching, and the polyps arise along the edges of the branches (you cannot see this from the picture, though). It is also zooxanthellate. It is a Psuedopterogorgia sp.

Klyxum sp.; note sterile main trunk, primary, and secondary branches. The colony is quite flaccid, indicating a comparative lack of sclerite support. The polyps are not retractile, and only one type are present (monomorphic; autozooids).

Part 3: Zoanthidea

The zoanthids are a horrible lot to classify, and most current taxonomy is incomplete or inaccurate. Identification is based on numerous and often variable characteristics. These may include the tentacle number and arrangement, nematocyst composition, chemical taxonomy, and other features of the soft anatomy. The major delineation is whether or not they are macrocnemic or brachycnemic (i.e. the completion of the fifth pair of mesenteries). This information is not practically available in living animals, nor are most of the other required characteristics. Unfortunately, the classification of most zoanthids is probably questionable, with very few true species being accurately characterized and assured (Burnet et al. 1997). Characteristics used to identify zoanthids, including some characteristics that can be used by aquarists in living corals, are as follows:

1. polyps: colonial or solitary
2. polyps: imbedded in coenenchyme or not imbedded
3. polyps: connected by stolons or not connected
4. polyps: contain sediment inclusions or do not contain sediment inclusions
5. polyps: color may be indicative in some cases
6. zooxanthellate or azooxanthellate
7. associations: many zoanthids associate commensally with other organisms, such as sponges.

Part 4. Corallimorpharia

The last, and perhaps worst, group in which to try and establish taxonomy are the corallimorpharians, or mushrooms. Daphne Fautin and some of her associates are just beginning a long project to identify the phylogeny and taxonomy of these animals. There are few references on the taxonomy of corallimorphs, and the best are concerning the Caribbean species (den Hartog 1980). Earlier descriptions by Oskar Carlgren, who studied the Pacific animals, are unfortunately probably not accurate because of inadequate use of defining characteristics (den Hartog 1980). New molecular probes and genetic studies have been done for a few species (Chen 1995a,b,c 1996a,b). Like the zoanthids, characteristics used to identify species are generally unavailable to the aquarist, especially in living polyps. Corallimorphs do not even have basic delineations like incomplete mesenteries or incorporation of sediments with which to separate certain groups. Often, nematocysts (number and type) and the distribution of surface and marginal tentacles are used in identification. In terms of physical characteristics, Aquarium Corals (Borneman 2001) discusses the features that may be used to distinguish corallimorphs to a genus level, if in fact the genera names are correctly assigned at all.

The visual characteristics potentially used as a guide are:
1. lack or presence of surface tentacles and their shape (verrucae (short tentacles) papillae (bumps and warts), etc.)
2. lack or presence of bare zones between tentacles of upper oral surface and margins
3. lack or presence of marginal tentacles
4. relative stiffness of base
5. potentially the ability of animal to trap prey by infolding, or if a prey capture response is present at all.
6. maximum size
7. mode of reproduction (laceration, fission, budding, or other)
8. color may be somewhat indicative in some cases.
9. degree of coloniality - large colonies, small colonies, isolated individuals.


Fraught with uncertainty, identification of the groups discussed here is sometimes possible to a family or genus level. Practically, or even absolutely, unavailable distinguishing characteristics are a hallmark of many of the organisms. To compound the frustration lies the fact that real systematics and proper identification of many of these groups is in its infancy and remains largely unknown. Perhaps surprisingly, aquarists may even be more familiar with some of these groups than those who study them.

In concluding this series on coral identification, I hope that the articles have proved enlightening and perhaps even helpful. I am quite sure I have caused some degree of confusion, frustration, and...just perhaps... interest in further explorations of identifying the coral animals we keep in our aquariums.

If you have any questions about this article, please visit my author forum on Reef Central.


Bayer, Frederick M. 1981. Key to the genera of octocorallia exclusive of Pennatulacea (Coelenterata: Anthozoa) with diagnoses of new taxa. Proc Biol Soc Wash 94: 902-47.

Borneman, E. H. 2001. Aquarium Corals: Selection, Husbandry and Natural History. Microcosm/TFH, Neptune City. 464 pp.

Burnett, W.J., J.A.H. Benzie, J.A. Beardmore, J.S. Ryland. 1997. Zoanthids (Anthozoa, Hexacorallia) from the Great Barrier Reef and Torres Strait, Australia: systemics, evolution and a key to the species. Coral Reefs 16: 55-68.

Chen, Chaolun A., Bette L. Willis and David J Miller. 1996. Systematic relationships between tropical corallimorpharians (Cnidaria: Anthozoa: Corallimorpharia): utility of the 5.8S and internal transcribed spacer (ITS) regions of the rRNA trancription unit. Bull Mar Sci 59: 196-208.

Chen, C.A., and D.J. Miller. 1996. Analysis of ribosomal ITS1 sequences indicate a deep divergence between Rhodactis (Cnidaria: Anthozoa: Corallimorpharia) species from the Caribbean and the Indo-Pacific/Red Sea. Mar Biol 126: 423-32.

Chen, C.A., et. al. 1995. Systemic relationship within the Anthozoa(Cnidaria: Anthozoa) using 5'-end of the 28SrRNA. Mol Phyl Evol 4: 175-83.

Chen, C.A., et. al. 1995. Spatial variability of size and sex in the tropical corallimorpharian Rhodactis (=Discosoma) indosinensis (Cnidaria: Corallimorpharia) in Taiwan. Zool Stud 34: 82-7.

Chen, C.A., et. al. 1995. Sexual and asexual reproduction of the tropical Corallimorpharian Rhodactis (=Discosoma) indosinensis (Cnidaria: Corallimorpharia) in Taiwan. Zool Stud 34: 29-40.

den Hartog, J.C. 1980. Caribbean shallow water corallimorpharia. Zool Verh 176: 83pp.

Fabricius, K. and Alderslade, P. 2001. Soft Corals and Sea Fans. AIMS, Townsville. 264 pp.

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Taxonomy in Aquarium Corals: Part 3 by Eric Borneman -