A difficult, tedious and error prone task

With more than two hundred known species and the great similarity between many of them, acropora ID can be a really difficult task. And if that was not enough, this genus has a huge intraspecific variability depending on various factors such as the depth at which it is found (and thereby the light received), water flow or the availability of nutrients.

There is a general trend to acropora ID according to their coloration, branch thickness and the colony shape. Nothing more wrong than the usage of these three criteria, which are strongly dependent on factors such as the aforementioned. Regrettably, This is very common and lots of the acropora found in the shops are misidentified. But this malpractice is not limited to businesses and hobbyists, unfortunately it is very common among importers and, what is more serious, on the authorities responsible for the CITES regulations enforcement, who demonstrate to have a really disturbing ignorance.

Genetic classification or morphological classification?

The first thing to know before attempting to ID acropora is the way they are classified. Although there are several, we owe the two most outstanding to two Australian scientists: Carden Wallace and Charlie Veron, both currently active.

Carden Wallace proposes a genetic classification, which is beyond the scope of this handbook. In his classification found 21 groups within the resemblance among species is little more than coincidence, therefore no practical usefulness for the hobbyist who wants to ID acropora. However, Wallace classification could be helpful if it is to intend an evolutionary study of acropora.

Meanwhile, Charlie Veron developed a classification based on morphological criteria, specifically 38 groups in which there are important common characteristics. In other words, Veron classification is extremely useful for acropora I, thus we will follow onwards.

Charlie groups

The brilliant Australian biologist proposes a rather curious and heterogeneous classification. In order to facilitate understanding, I have taken leave of sort it “my way”, judge for yourself:

  1. Group 1. Axial colonies lacking corallites (no visible)
  2. Colonies with axial corallites
    1. Colonies tree shaped
      1. Group 2. Acroporas with immersed radial corallites
      2. Acroporas with exserted radial corallites
        1. Group 3. Irregular branches
        2. Group 4. Buffalohorn-like branches
        3. Group 5. Elkhorn-like branches
        4. Staghorn-like branches
          1. Group 6. Branches growing upright
          2. Branches growing down and backwards (prostrate)
            1. Group 7. Radial corallites rasp-like
            2. Group 8. Radial corallites not rasp-like
            3. Horizontal branches interlocking
              1. Group 9. Sharp-edged radial corallites
              2. Group 10. Rounded-edged radial corallites
        5. Medium sized branches
          1. Highly visible secondary branches
            1. Group 11. Branches and sub-branches differentiated
          2. Group 12. Staghorn-like branches
          3. Branches interlock vertically
            1. Group 13. Sharp-edged radial corallites
            2. Group 14. Irregular radial corallites
          4. Group 15. Branches interlock horizontally
        6. Thin branches
          1. Group 16. Tubular branches
          2. Group 17. Flat branches
    2. Colonies plate-like
      1. Group 18. Thick branches
      2. Group 19. Thin branches
    3. Colonies digitate
      1. Colonies clump-like
        1. Group 20. Cylindrical branches
        2. Group 21. Finger-like branches
      2. Colonies plate-like
        1. Group 22. Other axial corallites
        2. Large axial corallites
          1. Group 23. Radial corallites distinct and intergrade
          2. Group 24. Radial corallites spiny
    4. Colonies corymbose to branching, radial corallites scale-like
      1. Group 25. Colonies corymbose
      2. Group 26. Colonies branching
    5. Colonies forms clumps, branchlets well developed
      1. Group 27. Radial corallites appressed
      2. Group 28. Radial corallites small
      3. Group 29. Radial corallites with flaring lips
    6. Group 30. Colonies plate-like
    7. Colonies form plate-like bushes
      1. Group 31. Axial corallites dominating colony shape
      2. Axial corallites not dominating colony shape
        1. Group 32. Other radial corallites (conspicuous and inconspicuous)
        2. Group 33. Radial corallites with smooth edges
        3. Group 34. Radial corallites with sharp edges
        4. Group 35. Radial corallites appressed
    8. Group 36. Colonies thicket-like
    9. Group 37. Colonies form tangles
    10. Group 38. Colonies bottlebrush-like

 

And now we relax…no way, it is not so simple. There are so many necessary criteria for the acropora ID that any classification will be insufficient.

Let's see it with an example:

I just got a plate-shaped acropora, which group would it belong?? The answer is that it can be in any of the groups 8, 9, 10, 17, 18, 19, 25, 28, 30, 31 and 35 and if it was not enough, it could also be a separate species belonging to other groups such as irregularis (group 7), seriata (group 11), retusa and samoensis (group 21), digitifera and sarmentosa (group 22), latistella, miriabilis and subulata (group 27), selago, tenuis and vermiculata (group 29), maryae, rosaria and verweyi (group 32), secale (group 33) or valida (group 35).

So little…we must keep on working. No matter how well thought is the classification proposed by Veron, we must observe many morphological characteristics to correctly ID acropora.

The guide to acropora ID

This is a work that have held me busy long hours at the computer, for many weeks, and I am very satisfied. So far there was no tool allowing acropora ID using simultaneous and visible filters throughout the process.

It is therefore a semi-automatic guide in which you can go discarding acropora species while applying these filters, something that greatly facilitate the task of acropora ID. It is programmed in a MySQL database which has some advantages such as the operating speed and ease of editing features from my “backoffice”, but also disadvantages such as the inability to properly apply the translation filter (which does not reach such a deep layer). This makes that some texts can only be seen in my native language, Spanish, so I have taken the decision to write the filter in a more universal language such as English because most of my blog readers of are from various countries and it will be easier to understand for them.

The guide to acropora ID is a table with three columns:

    • Species. It shows the name of the species. Clicking on it we will access a page with information and pictures .

 

    • Description. It shows the main identifying features of the species.

 

  • Geographic zone. This information can be useful to confirm when we try to identify wild species with a known origin. The enormous variability makes it impossible to use as a filter, so not do without it, I decided to place it on the table itself.

And a selection area on the right:

  • A button to clear filters. We must press this button every time we want to identify acropora or if we start again.
  • The first fields we find are those relating to genetic and morphological classifications (Wallace and Veron) aforementioned. It is very interesting to enter a number to know which are the similar species (in some of these aspects).
  • 14 filters to ID acropora (see its description later).
  • Three text fields that should not be touched (I will probably make them disappear in the next version)

When no filter is applied we will see a complete list with 170 acropora species. My intention is to add more species as they are discovered, as it has happened with rongelapensis and spathulata to name two of them. I have also the intention to provide further information with more photos and data, this is something that will happen transparently to the user.

Guide for acropora ID

Acropora identifying features

In the classification proposed by Veron, we have seen that he alludes to various characteristics or identifying features, some with a strange name. Knowing them is essential to apply the correct criteria that allow us to ID acropora.

Prepare a good cup of coffee and go for it. We'll start with the most outward and visible features and keep on approaching the less obvious:

 

Colony shape

It is one of the most important features, this is why it is very inaccurate to try to ID acropora from frags.

I consider the following ones:

  • Bottlebrush. With small twigs that tend to be inserted in the main branch perpendicularly which makes it look like a bottlebrush.
  • Bush. With the shape of a thicket or a clump.
  • Cluster. Small groups formed on each main branch.
  • Elkhorn. Tree-like with wide open and flattened branches.
  • Encrusting. The acropora is upholstering a rock, sometimes it grows branches but is not usual.
  • Finger. Shaped like a hand from which sprout many fingers, usually orderly and equally spaced.
  • Staghorn. Tree-like with open branches and sub-branches growing in all directions.
  • Plate or table. Because many times the colony must reach dimensions that are impossible to reproduce in an aquarium I do not like to differentiate between the two types and I just call them table.

 

Acropora can adopt different shapes and although there is usually a dominant one, sometimes we can find two shapes, which I call primary (the most common) and secondary. For instance, the acropora aculeus can be found in the primary shape as a bush and secondary shape as a table , moreover, acropora acuminata may be in the primary shape as a staghorn and secondary shape as a table. I noted it this duality in 51 acropora species

Colony layout

This is one of the easiest features to employ when identifying acropora. At a glance you can discard many species, with only to evaluating the degree of determinism of the colony.

We see two possibilities:

  • Orderly. The branches grow following a regular and deterministic pattern , usually radially, parallel or repetitive.
  • Disorderly. The branches grow chaotically or in a non-deterministic way, giving an overall appearance of disorderly tangle.

 

In the following example you can see two table-like acropora, acropora desalwii (orderly) and acropora jacquelineae (disorderly):

Branch thickness

One of the most inaccurate and difficult to evaluate features in acropora ID. As I mentioned before in this article, corals in general and acropora in particular have a strong morphological dependence of the medium in which they are. This dependence is very pronounced as regards to the thickness of the branches, so that a colony located in an area with strong currents or wave strikes such as the outer reef crest will tend to produce much thicker and robust branches than other acropora of the same species located in shallow water and more protected from the force of the sea.

For this reason I do not recommend to use this feature in the first instance, but rather as a final check or to resolve a tie between two species that share other identifying features.

In order not to complicate it excessively we contemplate the following cases:

  • < 5 mm
  • 5 – 10 mm
  • 10 – 20 mm
  • > 20 mm

Axial corallites

As we know, a corallite is the skeleton which houses a polyp. Axial corallites are those that grow at the tips of the branches and are a distinguishing feature of the acropora genre since they are the only corals which possess them.

Axial corallites can be classified in many ways, the more helpful ones in order to ID acropora are the following::

Shape

  • Tubular. It is the most common in acropora, and there are some variants like the conical tubular corallite which should not be confused with the hemispheric.
  • Hemispheric. The 14% of the acropora have axial corallites with this distinguishable shape, so if we watch this feature the work of identification is much reduced.

 

Size

  • Big. It is considered large when it occupies all or substantially the tip of the branch.
  • Small. It is considered small when it only occupies a part of the tip of the branch.

 

Visibility

  • Prominent. When the corallite protrudes from the tip of the branch or is very visible (regarding its shape, not the color)
  • Not prominent. When the corallite dos not protrude nor is not clearly visible.

 

Here you can see an example of each of these types of axial corallite (except the large, prominent, hemispherical corallites since there are no species with this features):

Radial corallites

They are by far the most abundant and varied, not only in the acropora species but any hard coral. Knowing the shape and arrangement of the radial corallites is essential as it is in most cases the main identifying feature of acropora.

The shapes that can adopt are numerous, These include tubular, nariform, rounded, labial, scale-like, conical and immersed, and besides this, different openings where the polyp live: circular or straight, beveled, oval and incomplete.

Appressed corallites

But in practice, none of the above features are very reliable because of the difficulty to be differentiated by the eyes of a common hobbyist. But there is a third feature on the shape of these corallites that it is easy to identify and help focus the search accurately : the appressed corallite.

Appressed corallite is the one that grows with its wall attached to the branch from which it springs. Sometimes it is not easy to determine whether or not a corallite is appressed (especially in non-tubular ones), the trick is to observe the tip and if its edge is touching the branch. Nearly 60% of the acropora species have appressed radial corallites. We have two options:

  • Yes.
  • No.

Corallite spacing

Not an easy feature to evaluate, except when we try to identify acropora with very spaced and scattered ones or so close that they are tocuching. About one third of the acropora species have clearly separated corallites and another third clearly touching so there is a field for indeterminacy too high (the remaining third).

I advise not to make decisions based on this feature, it is wiser to leave it for a final check once the ID is completed.

We distinguish three options:

  • Separated.
  • Near.
  • Touching.

Corallites of the same size

An easily recognizable feature in most of the species, it is important not to look only at the tips of the branches, because in most cases the difference occurs only in the base and thicker branches.

Nearly 60% of the acropora has one-size radial corallites, so if it is clear this identifying feature we´ll have narrowed down the possibilities a lot. We have two possibilities:

  • Yes.
  • No.

 

In the pictures below you can see examples:

Coenosteum

With this strange name we call the skeleton on which the coral tissue adheres. Each coral species builds a skeleton with its own structure, it's like their own footprint and this can be seen with a microscope. The appearance of the coenosteum only depends on the surface finish of this structure.

In some species it is not easy to determine the degree of roughness of the coenosteum, so to identify acropora species I advise you to only use this feature as a final check, unless you have it very clear.

We see two possibilities :

  • Smooth. No stretch marks on the tissue lining the coralito are appreciated, usually coinciding with those with rounded edges.
  • Rough. In some occasions small streaks are distinguishable in the lining of the corallite, sometimes it is necessary to observe a free coralito tissue. I believe that almost 70% of acropora species are in this group.

 

Polyp extension

This is a subjective characteristic that should be left to the end as a checkup. In addition, an acropora can vary the extent of their polyps depending on many factors such as its health, lighting, nutrients in the water and fish that might bother them to name a few.

I estimate that only the 20% of acropora species tend to have a large polyp extension, among them it is easy to recognize prostrata, millepora, convexa, horrida and latistella to give some examples.

We consider three possibilities :

  • Little.
  • Big.
  • Big PE and long polyps.

 

In the above example we can see a loripes acropora extension of small polyps and extension acropora horrida large polyps.

Depth

This is a feature applicable to wild acropora, normally collected or observed by ourselves since no shop will give you that information. It will not be very useful when trying to ID acropora, however I decided to include it in this guide since I consider it of great interest in order to know the light requirements of each species and therefore its optimal location in the aquarium.

Two possibilities:

  • Shallow water.
  • Deep water.

Abundance

Like the previous one, it only applies to wild acroporas, so it will not be very useful to identify acropora bought in a shop or from other hobbyist. The abundance in the commercial sector is highly variable from year to year and depends on aspects such as supply and demand.

However, I decided to include this feature to allow divers can get an idea of ​​what can be found before making a dive.

Two possibilities:

  • Common.
  • Uncommon.

Final note

I hope this tool, I've created with so much effort and dedication, is useful to all hobbyists and professional trying to identify acropora or know more about this beautiful genus of hard corals.

I'll be happy to hear any suggestions or comments that may enhance its performance, and post any picture of the species you keep it in your tanks.

If you had the patience to get to the end of this article, I can only congratulate you because you must be a passionate of the acroporas as I. Thanks for your attention and good ID.