The following is a practical method of establishing the identity of living Scleractinian corals. In a later installment, I will explain how to use a systematics key for more accuracy in identification efforts. Throughout this article, I will try and use the same corals in photos as examples so that the reader may become familiar with a few, rather than overwhelming with variation.

While growth form cannot be used to positively identify a coral, it can be a useful tool. Many corals may have a determinate growth form; in other words, all members of a species adopt a certain form. However, many others may be extremely plastic and able to adopt many growth forms, some of which may be very different from what is thought of as the "norm." Nevertheless, noting a coral's growth form may allow for a process of elimination with similar looking types. A specific series of terms is used to describe these growth forms. Some of these are listed below. For a definition of these terms, use a reference source such as Veron (2000) or others.

Massive or submassive:

Branching or arborescent:

Foliaceous:

Laminar:

Encrusting:

Columnar:

Tabulate:

Solitary:

The type of structure polyps produce with their corallite homes to make a coral colony is an important characteristic used in identification. It is the basis for many genus level designations. Unfortunately, as with the growth form, it may not always be apparent, because the living tissue may obscure the corallites (Figure 1). This is especially true in massive and submassive corals displaying cerioid and phaceloid forms, as in the Family Faviidae. Furthermore, variations of meandroid and cerioid species exist where a given genus or species can display variations intermediate between, or possessing both types of corallites (Figure 2).

Figure 2. This Platygyra sp. shows both meandroid and cerioid corallites on its surface.

In these situations, it may be helpful to examine areas of the coral that may have suffered partial mortality, exposing some areas of the skeleton. There, one may see at least a portion of the skeleton to determine what types of corallites are present. Fortunately, many of the other forms are quite obvious and require little more than a quick glance. While it is probably not possible to use these traits conclusively to establish the identity of a coral, they provide a way to either confirm identity in conjunction with other characters, or to allow for a process of elimination with similar looking types. For a definition of these terms, use a reference source such as Veron (2000) or others.

Plocoid

Phaceloid

Meandroid

Cerioid

Flabellate or Flabello-meandroid

One of the more important characteristics of corals, from family to species level, is the size of the corallites. For a completely retracted specimen, or one where bare areas of skeleton are exposed, all that is required, in most cases, is to use a metric scale ruler and measure the diameter of the corallites. Because corallites may vary in size, choosing a number of them and taking the mean (average) may be required. If there are considerable variations in size, this may be a characteristic all unto itself. Also, some groups may have two or three consistent size classes present. In this case, the mean would be taken across a number of each size class.

Furthermore, the appearance of the corallites within the corallum may provide useful identifying characters. Observe the corallite features and note if they project outward, project inward, or lie flat against the rest of the coral surface. Depending on the coral, these may be important characters.

Figure 3. Note how the corallites of this Montastraea cavernosa and Turbinaria patula project above the surface of the coral.

Although, these features may or may not be apparent in the living coral, the specimen should be examined carefully for any such characters. Note as well the general corallite shapes, their regularity, and any patterns they present on the living tissue. For example, note the petaloid shapes of Pavona spp. (Figure 4) or the cone-like hydnophores of Hydnophora exesa (Figure 5).

Figure 4. This Pavona decussata displays the flower-like petaloid corallites that characterize this genus and a few others.

Figure 5. This Hydnophora exesa illustrates several things: First, the cone-like hydnophores are a distinguishing feature of this genus. Second, an area of recently exposed skeleton is visible and allows consideration of skeletal characteristics important in identification. Third, it is clear how much of the skeleton is obscured by living tissue, even if withdrawn.

Figure 6. The skeletal component and living coral, Cynarina lacrymalis. Notice how the diagnostic taxonomical features are either visible or concealed by living tissue.

As mentioned, many characteristics may become more noticeable when the animal is taken out of the water and the polyps are withdrawn (Figure 7). Finally, a very few species (e.g. Oulastrea spp.) have colored instead of white skeletons. Here, it must be ascertained if any coloration is truly skeletal, or if it was caused by encrusting, boring, endolithic, or overgrowing organisms.

Figure 7. A withdrawn and partly bleached Trachyphyllia geoffroyi displays fine dentitions along the upper margin of its septa. While clearer in areas where the skeleton has broken through the tissue, it can still be seen quite clearly in places even through the tissue.

Another corallite feature to be examined is their mode of asexual division. Examine the specimen and determine if the polyps bud by intratentacular budding, as in Favia spp, or by extratentacular budding, as in Montastraea spp. Examine the margin of the coral and elsewhere to see if other buds are developing, as such budding may be indicative of a characteristic type of asexual growth or reproduction.

Note in these Faviids how a corallite divides in equal or nearly equal halves, also referred to as intratentacular budding. In contrast, the coral below displays numerous extratentacular budding events (two shown). Also, I have shown what the living coral shown in the top photo looked like when alive (inset). While the living coral appeared to be a Favia sp., it was later found to be Favites sp. after seeing the skeleton.

Other features of a coral may be important in establishing an identity. Bumps, hillocks, whorls, lobes, the presence of axial corallites (Figure 8), and other unusual demarcations of the skeleton can be used in some cases, making sure they are actually features of the coral and not caused by other commensal or associated organisms.

Figure 8. Note the large axial corallite at the end of the branch tip of this Acropora sp. This special corallite is a hallmark of the genus, although the subgroup Isopora lacks these corallites and includes species such as A. palmata (Elkhorn coral) and A. cuneata/palifera (Cat's Paw Acropora).

Occasionally, and I stress occasionally, living polyp tissue can be used in identification and taxonomy. The polyp or tentacle shapes are used to distinguish some genera and species, or simply aid in basic identification; for example, Plerogyra spp., Euphyllia spp., Alveopora v. Goniopora spp.). Even more rarely, coloration may be possible to use in distinguishing genera (e.g. Halomitra sp.v. Sandalolitha spp., various Tubastraea spp., and others). I emphasize that there is no case of which I am aware where coloration will be a criteria in taxonomical determination, but may provide clues to separate similar groups or species. Generally, the variation in coloration is too great to be used at all. In another few cases, the degree of tentacle development may be useful information (e.g. Pachyseris spp.). Behavioral characteristics might also be helpful; for example, knowing if the polyps are extended by day or by night, and if they are clear (transparent) or colored (opaque), could help in assessing a general level of identification (e.g. Pectinia spp. v. Montipora spp.). The number and distribution of mouths present might also be useful in some cases, and could perhaps explain some of the skeletal features obscured in the living coral (e.g. Fungia spp. v. Polyphyllia spp.). For example, numerous mouths along a continuous stretch of similar tissue may indicate that a coral is meandroid rather than some other form (e.g Favites sp. and some Goniastrea sp.).

Figure 9. Notice the multiple mouths along the obscured flabello-meandroid skeleton of this Catalaphyllia jardinei.

In many attempts, an aquarist may find there is simply not enough key information available in the living coral to establish an identification with any confidence. For others, there may be more than enough information through careful observation and consideration to go all the way to a species level identification. The best attempts, I have found, are those in which there is some amount of exposed skeleton to examine. Unfortunately, encrusting and eroding organisms quickly foul exposed skeleton, and physical injury may have made such areas either deceptive or useless. Ideally, a freshly exposed area is available for examination. In these cases, one may even be able to take notes and measurements of the skeletal elements such as septa, costae, columellae, and even the smaller features of these elements. If an identification is truly important to an aquarist, a sample of the coral could be taken, and the tissue removed to examine these features thoroughly. This can take place over a longer time than is possible with a living coral that is probably being examined outside of the tank water. However, as I discussed in the last article, even having such material available or present in no way ensures that an accurate identification can be made, or that one is even possible with any given specimen. For many, (if not most) corals, the skeletal features used in taxonomy are obscure, variable, require practice and expertise to interpret, and could involve the use of specialized equipment and resources. In the majority of cases with living aquarium corals, a genus level identification is all that will be possible, and even this may be difficult in some cases.

An Echinophyllia sp; note large corallites, raised from the surface at all angles. If the corallites were all angled towards the margin, it would characterize Mycedium sp., a family member.

Example:

Figure 10. This is a solitary coral, free-living, with one mouth. It is also divided into clear pie shapes. From this, we know it is a member of Family Funigiidae, genus Diaseris sp. Species level identification in this coral is relatively simple since there are only two species. One is smaller (up to 40mm), found on soft substrates often in currents, and has thick, uneven, beaded septa. The coral pictured is 58mm across, has even, thin, finely dentate septa and was found in a deep, flat submerged reef. This coral is Diaseris fragilis.

Information in coral resources, such as Corals in Space and Time: The Biogeography and Evolution of the Scleractinia (Veron 1995) describe the inherent difficulties and uniqueness of corals to resist classical definitions of species. As an example, consider the case of Montastraea annularis in the Caribbean. This coral adopts three fairly consistent growth forms, largely depending on its depth. Plating, lumpy, and columnar/massive forms all exist. At first, these were thought to be distinct species, but in view of the morphological variation of corals, and the nature of corals to adopt different growth forms in different conditions, they were later assigned as a single species. However, some years ago, genetic studies showed them to actually be separate species - M. annularis, M. franksii, and M. faveolata. However, some degree of overlap is now becoming apparent, again calling into question the real answer to the question of what are distinct species. Compounding this is the apparent capability of corals' specificity to host different species or strains of zooxanthellae. This host-specificity further complicates the speciation problem in corals, and is only now beginning to be investigated.

Species are often described as the smallest unit of reproductively capable organisms; a group of organisms sharing certain characteristics that are capable of interbreeding. Corals, however, defy this definition in their capacity to hybridize, form chimeras, and to self-fertilize. Classical taxonomy in these organisms involves almost solely physical skeletal characteristics, apparent in fossil records, and unfortunately often subject to great variation with continuums being common across geographical and geological space and time. It is now recognized that many other factors besides skeletal features may come into play, and that molecular methods may be a better way to accurately separate species in corals. Even using such techniques, a question exists as to what level of variation or difference is enough to separate two similar taxa from one, or vice-versa.

In the next installment of the series, I will discuss the identification (and problems inherent to identifying) the non-Scleractinian corals.

Veron, J.E.N. 2000. Corals of the World. Australian Institute of Marine Science,
Townsville. 3 Volumes.

Veron, J.E.N. 1995. Corals in Space and Time: The Biogeography and Evolution of the Scleractinia. Comstock/Cornell, Cornell University Press, New York. 321 pp.

Encrusting purple montipora (photo 5) courtesy of Carlos Chacon
All other photos courtesy of Eric Borneman