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Coral Identification by Eric Borneman

This is an attempt to condense and make available a Coral idenitification class offered by Eric Borneman in 2004 that was originally done on a Reef Central thread. The posts instead of being written verbatim will be condensed to what is necessary and use each individual seperate Coral ID process as a seperate example. I have listed the author of the original posts to give credit to those who did the work. I don't want them neglected, but for purposes here, I think the information will flow better if it is not setup as thread but as a navigable set of pages.
I myself have been interested in learning more about coral identification, and wished to make the reef central thread more compact, and useful as a reference. Eric suggested trying it in HTML format which I think has worked much better than as a word type document.

Note that not all corals have been taken fully through the ID process but a lot of information can be gleaned by looking at the photos and the work done on them.
References used by Eric:
1. Wallace, Carden C. 1999. Staghorn Corals: A revision of the genus Acropora, CSIRO publishing, Queensville, Australia. 421 pp.
2. Veron, J.E.N.. 2000. Corals of the World. Volume 1. AIMS, Townsville. pp. 176-443.
3. Veron, J.E.N., and C.C. Wallace. 1984. Scleractinia of Eastern Australia Vol 6, Part 5: Acroporidae. AIMS, Townsville. 485 pp.
4. Wallace, CC., and J Wolstenholme. 1998. Revision of the coral genus Acropora (Scleractinia: Astrocoeniina: Acroporidae) in Indonesia. Zool J Linnean Soc 123: 199-384.
5. Wallace, C, and M. Aw. 2000. Acropora staghorn corals: A getting to know you and identification Guide, Indian Ocean: Southeast Asia: Pacific Ocean. OceanNEnvironment Ltd, Australia. 128 pp.

This page will serve as the text of the learning. Each coral ID will have it's own page as will the process to follow to ID your Acro/Isopora coral.

Background:


Species boundaries in corals were highlighted in JEN Veron's book, Corals in Space and Time. Hybridization, plasticity of growth form, and other factors makes the taxonomy of corals quite difficult in many cases. Acropora is the largest extant genus of hermatypic coral, comprised of 2 species in the Caribbean (and a third that is a hybrid - A. prolifera), and about 160-175 species in and around the greater Tropical Pacific and Indian Oceans. Acropora is also one of the most widespread.
There is considerable question as to the utility of using a Linnaean or Lamarckian categorization scheme in Acropora. That is, using morphology, growth form, and physical characteristics, along with variability, to assign species designations. Use of molecular techniques may show in the relatively near future that efforts to identify species using such classical systematics are too often wrong.
But, that is what we have at present, and that is the focus of this class.
The genus Acropora can be broken up into two subgroups - Acropora and Isopora. The latter lack perisistent axial corallites. Examples of these would be the elkhorn coral, A. palmata in the Caribbean, and the cat's paw coral, A. cuneata and A. palifera in the Indo-Pacific. One might make the argument that in A. palmata, there is a lack of persistent radial corallites, too To me, they all look like axials. Acropora, incidentally, is characterized by the appearance of this special terminal polyp that is housed in a generally elongated corallite called the axial corallite.
Other thigns to know: any species of Acropora can vary in its skeletal morphology - by geographic location, environmental conditions, age, size, season, mechanical impacts, hybridization, etc. Therefore, the single colony you have chosen might or might not be indicative or typical of the species. This will become clear as we begin to look at your specimens.
So, before going much further, let's look at some basic terminology. I am going to list a lot of terms we have to be familiar with before going further.

Growth form of the colony:


Acropora develops as a consequence of the extension of axial corallites, budding of radial corallites, development of new axial corallites, and subsequent branching. Colony growth form varies from a little to a lot, and various growth forms may occur within a colony, within a species, and between species.
  • Digitate - colonies have short, non-dividing, non-anastomosing branches (like fingers of a hand)
  • Hispidose - colonies have numerous short side branchlets projecting outwards from the mainbranch
  • Corymbose - colonies consist of horizontal anastomosing branches and short upright branchlets
  • Tabulate or plate-like - colonies are flat, either with one cental stalk (leg) or attached to the substrate at one side
  • Arborescent - tree-like branches
  • Caespitose - colonies are bushy and branches interlock (anastomose) similarly in three dimensions
  • Massive - colonies are solid and similar in all dimensions
    Other colony growth forms that could be used:
  • Cushion-like - colonies consist of fine branches that grow into the shape of a cusion
  • Encrusting - colonies encrust the substrate, sometimes with short branchlets projeting upwards
  • Prostrate - colonies sprawl across the substrate
  • Staghorn - colonies composed mainly of large antler-like upright branches
  • Thickets - colonies consist of closely compacted upright branches
  • Columnar - colonies form columns
Any given specimen may have more than one colony growth form, and may not fit specifically into any one of these categories. In the case of aquarium corals, generally the colonies are very small and the growth form that might develop in a larger and more mature colony is not yet apparent. With fragments, such as by propagation efforts, this is certainly the case.
Growth Forms , from 1999. Staghorn Corals of the World. CSIRO, Queensland, p. 52.

Skeletal features:

  • Axial corallite - the central corallite that determines the axis or growth on an Acropora. It often lies at the end of a branch, though axials and incipient axials may also form along the sides of branches, or on the surface of plates and encrustations. We will cover the axial corallite in depth in the next few days.
  • Radial corallite - peripheral corallites that like arranged around the axials, often on the sides of branches or between axials on plates and encrustations. Sometimes, they look very much like axials. We will cover the radial corallites in depth in the next few days.
  • Coenosteum - the skeletal matrix that lies between the corallites but not part of the axial or radial corallites. Often, the boundary is indistinct.

So, other corals besides Acropora may have dimorphic polyps/corallites, but the characteristic is pronounced in Acropora. The axial corallites make up the axis of a branch, and the branch can sort of be considered as one long formation of the axial, with the solid part of the branch being the outer wall of the axial, with radial corallites budded off from below the growing axial. Sometimes, as in my photo, it can be difficult to tell which are really axials. Same is true for some hispidose colonies. Also, if growth slows or ceases, axial length by be reduced or even halted. Sometimes, the axial is very long and tubular...sometimes it is short and berry-like.
Contributions of branch thickness may be formed mainly from axials, radials, or both. But the wall of the axial corallites is formed from synapticular rings of varying numbers. If you look down directly into the axial corallite, you will see the septa. In some cases, they are very immersed deep in the corallite, while in others they are more apparent. There will be either six or twelve septa, arranged in one or two cycles. By cycles, I mean that there may be primary septa that are typically larger, thicker, and reach farther towards the center of the corallite. Secondary series are usually smaller, thinner, and do not extend as far towards the center. If you look at enough axials, you might find variation, with sometimes only 4 or 5 of the 6 secondary septa showing any development at all, so that it appears at first look as though there is not a full set. In other cases, primary and secondary are almost indistinguishable. In still others, there are only primary septa. You may also find that one set of primary septa are larger than the rest of them. These are called the directive septa.

Radial corallites typically form as buds from the axials and lie between and around them. By comparison with axials, radials are quite variable. Wallace notes that the terminology for them stems from Dana in 1846 and makes significant use of anatomical terms. The wall of the radials can be complete and tubular, or they can be incomplete, as though an incipient tube-like corallite was chopped off. In contrast to axials with their normally round openings, radials can have round, oval oblique or have an extra bit of skeleton lost at the outer and upper edge (dimidiate) or an extra bit added to the outer and upper edge. The latter case produces nose-like or nariform radial corallites. Additionally, the angle of the radials against the corallum or axial wall can vary from perpendicular (sticking straight out) to totally appressed. The shape of the corallites is also variable as will be seen in the idealized drawing below. As if this isn't enough, numerous variations may occur along a branch and within a colony. Gradations are common along a branch in many species. Radial corallites can also be dimorphic, with more that one type/size occurring on a colony. Finally, one must again consider the septa in each type of radial corallites, the same as with the axials.
It is probably helpful to look at many colonies to become more familiar with all the different sizes, shapes, types, and variations.
Image of radial corallites from Wallace, Carden C. (1999) Staghorn Corals of the World, CSIRO, Queensville. p. 55.

Here is a list of terms:

  • Tubular: tube-like
  • Cochleariform: ear-like
  • Nariform: nostril- or nose-like
  • Labellate: lip-like (can be flaring or straight)
  • Conical: the distal end tapers from a wider base
  • Rounded tubular: berry- or keg-like
  • Appressed: angled so that the inner edge lies against the corallum
  • Sub-immersed: barely sticking out from the corallum with little ridges of wall showing
  • Immersed: totally flat against the corallum
  • Oblique: the opening appears as though a tube was cut at an angle
  • Round: the opening is rounded when viewed from the end
  • Oval: the opening is oval when viewed from the end
  • Dimidiate: the wall forming the opening is incompletely developed

Coenosteum:
Coenosteum is a term used to describe the skeletal material between polyps on corals. In Acropora, the coenosteum consists of the skeletal material formed by layering of synapticular material around the axial and radial corallites. Becuase it has different polyps types creating different corallite types, the coenosteum may be very different around and between corallites. However, this is not always the case, and it can be quite uniform.
There are relatively few terms used here, although a great degree of variation. The costae on coral skeletons are defined as the radial elements outside the corallite wall. A good example can be seen on an average Favia, where these radial skeletal elements are nearly continuous as the move over the wall of a corallite, forming the septa as they converge towards the center of the corallite.
In Acropora, similar radial elements can appear as parallel or nearly parallel raised bands of skeleton. The top edges of the bands can be smooth, or they can have jagged edges that appear like regular "teeth." These "teeth", or dentititons, are also called spinnules. Spinnules means "small spines." Spinnules have various degrees of development, and can be simply little points or shark-like teeth, or can be globular, highly ornately branched, forked, , labellate, lobed, etc. The spinnules tend to be quite regular, although again variation can exist. Finally, the coenosteum can be rather homogenous, forming a net-like appearance whereby it is termed reticulate. There can be a simple or elaborate reticulate structure. There are some excellent photos of examples in Wallace (1999) on p. 57 for those of you who have this book. Many of the photos posted here also have some nice views of coenosteum. The coenosteum very well might be among the most distinct characters on a species, and when coupled with the other characters already assessed, provides a pretty strong morphological description of the coral. Because of the variation in coenosteum, it is hard to describe in words, and often this is a place where comparison to photos becomes useful. It is also the least easily observed in living corals, being mostly to completely obscured by living tissue. It also becomes dificult sometimes where the base of the radial coenosteum joins the axial coenosteum, and this continuous structure sometimes becoomes a gradient of forms if the two have very distinct coenosteal morphologies. In other cases, it can be nearly continuous with very little distinction between the two. This seems to be more common with a simple costate or reticulate coenosteum.

Both Carden Wallace and Charlie Veron have grouped Acroporid species into larger subcategories. Veron lists 38 categories, Wallace has 20. I rather prefer the way Wallace lists them since they are arranged phylogenetically, while Veron separates them morphologically. It may seem easier to deal with the latter, but the descriptives of the categories require almost as much work to translate as would the species itself, and many of the terms seem a bit vague. For example, Wallace would have A. horrida in the A. horrida group, consisting of 8 species related to A. horrida. In general, the members have a lot in common, but in some cases may be quite different in appearance. Not that this really matters, given the variability we have learned about, right? In contrast, Charlie puts them in the "species with middle sized branches and irregular radial corallites" group that has four unrelated or related species in it. I will list them all for you guys in case you do not have these references. It will be helpful for you to be able to move your coral into one of these categories.
Wallace's groups:

  • A. rudis group - 4 species
  • A. humilis group - 8 species
  • A. nasuta group - 7 species
  • A. divaricata group - 5 species
  • A. lovelli group - 3 species
  • A. verweyi group - 1 species
  • A. cervicornis group - 3 species
  • A. muricata group - 5 species ( Wallace refers to A. formosa as A. muricata - Veron disagrees, and says no basis or need exists for changing a distinct and well-recognized taxon. I agree.)
  • A. robusta group - 8 species
  • A. togianensis group - 1 species
  • A. selago group - 8 species
  • A. aspera group - 7 species
  • A. florida group - 2 species
  • A. hyacinthus group - 7 species
  • A. latistella group - 4 species
  • A. horrida group - 8 species
  • A. plumosa group - 1 species
  • A. elegans group - 7 species
  • A. loripes group - 12 species
  • A. echinata group - 8 species
  • then there is the Isopora subgenus, consisting of 4 species (according to Wallace).
Let's do some easy intial work using the process of elimination.
Now, you might note the discrepancy in numbers of species. It is helpful here to note especially the ones with a lot or few species. Without going any further, I would look at the list and think about reproduction and dispersal. If a coral has many relatives, it has probably had ample time to diverge, and is probably a widespread group. By contrast, those with few related members are probably more recent or have significant barriers to dispersal - either being relatively new species or really old ones on their way to the genetic glue factory. In fact, if we look at the A. togianensis group, which consists of A. togianensis, I would thinkthe name means that it is found in and around the Togian Island group of Northeast Sulawesi, Indonesia. In fact, this is the case, and it is restricted to that area. The Togian Islands are a protected area, and are also very hard to reach. It is unlikely any corals collected for the aquarium trade came from here, and so one can immediately almost cross this species and group off the list.
It is notable that using a name is not a good thing to use in making assumptions. Often, a name may indicate a place of first discovery, but the species with all its variations was later found to be very widespread. This brings up another very important aspect in identifying your Acropora corals. The range. Unless you have good reason to believe, or know, that a coral was collected elsewhere, there are relatively few areas of coral collection for the aquarium trade. Veron and Wallace both give the known ranges for the species. Thus, one can immediately eliminate the A. cervicornis group which consists of Caribbean/Tropical western Atlantic species. One can, as one looks at the invidual groups, rule out species that are only known from Australian waters, the Hawaiian islands, the eastern Pacific, the east African coast, and, likely, the Red Sea. Now, if the range goes into that wide Indian Ocean, Indo-Pacific, Central Pacific region, then it must stay in the list of possibiltiies. Mind you, this does not rule out a lot, since the vast majority of Acroporids will be found in areas where it is possible they were collected for the aquarium trade. However, geographical range is a quick a dirty way to eliminate species that yours cannot possiby be with very little work.
Another quick and dirty technique is abundance data. When we begin using our characters, it may be helpful to see whether a species is common or rare. In general, odds are that your coral is probably not some rare coral, simply by virtue of the odds of collection. The same is true with habitat information. A rare Acroporid known only from deep current swept reef slopes or in mangrove areas is not likely to be your coral species, because collectors are unlikely to collect there, or to find it if they were. This is not a definite method of elimination, but it should be helpful.
Words of advice as we proceed, just general bits of trivia regarding preconceived notions if they haven't already been dispelled. Do not trust what the name of the coral was when you purchased it. Do not trust names you have previously associated with your specimen. As with my favorite example, everyone calls whorling or plating Montipora M. capricornis, even though this growth form is very common in Montipora and many many species exist other than M. capricornis....trust me when I say idenitfication of Montipora is hard. Similiary, the hobby tends to call any Acropora with scale-like radial corallites A. millepora, and although the radials are distinct and a good beginning, A, millepora belongs in the A. aspera group which has 7 similar species. Also, there are several others in other groups altogether that are very distinct as adult colonies, but very similar as small colonies or fragments, that have similar looking corallites. Also, do not believe if someone has told you your coral is rare. So, if you have a "rare deepwater Bali Acropora," do not begin by looking for deepwater uncommon corals where Bali is within their range. Just, throw out any preconceived notions, in general, and go from where we are now.
Tomorrow, I will list the characters for each group. Then, we will get a list of groups from each of you and begin moving in on the species.

Please refer to the page named Coral Keys for genus and species groupings
This page put together from the reef central thread using my fingers and HTML from scratch (no fancy editors :) )

This page updated last on December 12, 2006.

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