Through coral restoration efforts in St. Croix, Curacao, the Florida Keys, and other areas, I’ve learned ways that marine conservation efforts can contribute substantial knowledge to our own hobbyist-level coral rehabilitation efforts. Much of what I’ve observed in my own aquariums since 2007 aligns with research efforts across the Western Atlantic and Indo-Pacific. However, there are some nuances and differences in aquarium-based coral disease assessment, especially since the coral’s location is not a useful determination factor in disease identification. This coral disease assessment information is for general purposes and is not for medical or professional advice.
Since coral diseases are typically only distinguishable at the microscopic level (which is not accessible to everyone with corals in their care), we need a way to work at the macroscopic level. Using common terminology to describe an unhealthy coral by lesion characteristics can assist in creating a “usable diagnosis,” track healing progress (or decline), and support data aggregation for further research. Below is a mixture of corals with various lesions, both from my home aquariums and from scuba diving in the Western Atlantic region.
Coral lesions are changes to the tissue of a coral, such as infections, wounds, growth anomalies, or other injuries or diseases. Click a coral with a lesion below to read more about coral disease assessments, or scroll down to browse various ways lesions can manifest.
Coral Disease Assessment – Gross Lesion Types (Primary):
Although there are generally three accepted types of coral tissue-related gross lesions (i.e., discoloration, loss, or growth anomaly), I’ve also included other soft tissue lesions (not sufficiently described in literature from what I can find) and skeletal anomalies (may lead to gross lesions). In the research literature, gross lesion types seem to only encompass horizontal plane changes (coloration, tissue loss) rather than vertical plane changes (soft tissue bumps, extensions), while growth anomalies tend to focus on the tissue rather than the underlying skeletal deformity. However, in my experience, there are skeletal deformations without initially identifiable tissue lesions (e.g., boring sponges). Maybe those do not belong here, but they seem to be relevant outliers that could help in our coral’s care through detailed coral disease assessments.
Tissue Discoloration (Color Change) (Bleaching and Pigmentation) Lesions:
Intact yet abnormally colored tissues are characteristics of tissue discoloration or color change lesions. Bleaching is the spectrum of a loss of color, ranging from slight fading to complete transparency (such as the following example). However, there are pigmented discolorations as well (although less common).
This coral has an extreme lesion of tissue discoloration (bleaching). Despite the nearly complete loss of pigmentation, the coral made a full recovery.
Here is a bleached Trachyphyllia coral, but it retained a fluorescent color (pigment?) I have multiple examples of this type of Trachyphyllia bleaching (but few of other species), and I have not yet figured out a sure-fire way to save them. If I had to totally guess, it appears like a self-digestive response to a stressor; the coral slowly seems to starve despite attempted interventions.
Growth Anomaly Lesions:
Growth anomalies are still considered lesions for descriptions of patterns, progression, and margin (but typically not of color). Genetics or environmental stressors may trigger these changes, and the specific characteristics are used for coral disease assessment.
Gall (Explained) Growth Anomaly:
Galls are skeletal deformations instigated by an inhabiting creature, such as a crab or fish.
A gall crab‘s home is circled in red (small tube toward the top of the coral). This is an example of a growth anomaly, where the coral grew around the gall crab’s tube. Although gall crabs are commonly associated with dying coral, the gall crabs are likely not a sufficiently substantial environmental stressor to be the root cause.
Here’s a little troublemaker Blenny (ok, but in all fairness, it’s pretty cute). Notice the growth anomaly, where the coral’s skeleton is growing down into the Blenny’s home.
Similarly, this maze brain type coral has a lovely structural pattern surrounding a creature’s home (I did not stick around long enough while scuba diving to determine the inhabitant). The structure is interesting to me – the coral terminates many times down at the hole. I wonder if this provides the coral some type of benefit – maybe it functions as a food distribution source (i.e., the inhabitant’s leftover’s/waste become the coral’s next meal).
Lesion Relief:
Lesion Reliefs are other Growth Anomalies and include umbonate lesions (rounded growth), exophtic lesions (growing out beyond the surface), nodular lesions (small mass), and bosselated lesions (small bits that protrude). An example of exophtic lesions is below. While I do have other photographic examples of lesion reliefs, they may not be clear enough to distinguish between the other three lesion relief types.
This staghorn Acropora coral was covered in these growth anomalies. During my quick swim past while scuba diving, I was unable to discern the cause. The growths (exophtic lesions) appear like the skeleton has broken through the tissue and then become covered with cyanobacteria.
Growth Anomalies of Unknown Cause (GAN):
Growth Anomalies of Unknown Cause include various areas of accelerated growth, chaotic polyp arrangement, “gigantism,” and other as yet undetermined causes of unusual growth.
It appears some algae may have colonized prior areas of damage on this coral and created growth anomalies.
Tissue Loss Lesions:
Various coral predators (e.g., flatworms, fish, sea stars, other corals), infections (e.g., viral, bacterial, fungal), mechanical interference (e.g., falling rockwork, directed water flow, sedimentation), chemical interference (e.g., high alkalinity with low nutrients), and as-yet unexplained issues may all cause tissue loss. The tissue loss lesion appearance may provide clues to the root cause, such as blenny bite marks shown previously.
Another coral stung this Fungia coral, which resulted in a tissue loss lesion. Although the damage was severe, the coral did survive (click here to learn more).
Other Soft Tissue Lesions:
“Elegance Coral Syndrome” (ECS) is what I’d consider an outlier for gross tissue lesion types. The initial signs are external mesenterial filaments with retracted polyps and stubby tentacles. This isn’t a tissue pigmentation issue, tissue loss, or a growth anomaly (at least to my non-medically trained brain). Do we need a new category for soft tissue changes beyond simply its presence/absence and color?
Skeletal Lesions:
Skeletal anomalies do not fall within the tissue response categories (according to some sources, but other sources include them), but it is a gross change to the coral that can result in one or more of the lesion types above, especially tissue loss or growth anomalies. I’ve included them here for completeness.
Skeletal damage from poor fragmentation, damaging rockwork (or storms, if in the ocean), fish bites, and other root causes can result in lesions. This Euphyllia coral was poorly fragged, which resulted in a tissue loss lesion and ultimately death.
On the left is a slice of a Euphyllia coral skeleton that was infested with a boring sponge, while the Euphyllia coral skeleton slice on the right was not affected. Notice the dissolved areas on the left. While this type of damage is not a tissue lesion, it may result in lesions.
Coral Disease Assessment – Lesion Location:
Where the lesion is located on a coral can provide clues toward the coral disease assessment for a root cause. For example, certain bacteria are sediment-associated, and sediment typically aggregates along the base of the coral. As another example, some fish are corallivorous, and they eat while swimming, which means the top of the coral is typically the most damaged. Therefore, the lesion locations are broken up into basal/peripheral (base or around the perimeter), medial/central (middle/center), and apical (top). Of course, severe or extreme cases could present as combination locations.
Basal/Peripheral Lesion Location:
Basal or peripheral lesions are ones that originate at the base or outer perimeter of the coral. As pictured, this coral has a lesion that originated at its outer perimeter along the base. (I found this coral while scuba diving, and I do not know its long-term outcome.)
Medial/Central Lesion Location:
Medial or central lesions occur around the middle of the coral. The coral colony pictured does have a few lesions that would fall under the “apical” category, as they are along the top of the coral, but hopefully this is still a sufficient example of centrally-located lesions. (I found this coral while scuba diving, and I do not know its long-term outcome.)
Apical Lesion Location:
Apical lesions are found at the top of the coral. The Acropora coral pictured has a few branches with lesions at the tips, caused by fireworms. (I found this coral while scuba diving, and I do not know its long-term outcome.)
Combination Lesion Locations:
There are lesions covering these coral colonies, for a combination presentation (more common in severe or extreme cases of lesion severity). The lesions are along the base, at the top, and all across the center. (I found this coral while scuba diving, and I do not know its long-term outcome.)
Coral Disease Assessment – Lesion Size
Documenting the lesion size is, in theory, fairly straightforward, as it is just the measurement of affected tissue. However, coral structure, obstructions, and indistinct margins can complicate the measurement. And, of course, the measurement units must be consistent in documentation (e.g., using centimeters or inches).
Coral Disease Assessment – Lesion Duration:
Simply observing the coral’s skeleton can provide a good indication of how long the lesion has existed, and, if monitored over time, how fast the lesion is progressing. White skeleton indicates recent necrosis (about a few days), while a light green skeleton may indicate necrosis of a week or more ago. Skeletons populated with multiple algal types indicate necrosis in the more distant past (possibly months or more).
Acute Lesion Duration:
The left-center of this coral shows an Acute Lesion Duration, with the white skeleton showing no algal colonization. Notice the far-left side of the coral skeleton, where there is a film algal colonization along with filamentous algal growth.
Subacute Lesion Duration:
No, you aren’t seeing double (at least not unintentionally). The far-left side of the coral’s skeleton represents a Subacute Lesion Duration, where filamentous algae is beginning to colonize on the coral. This is in contrast to the left-center of the coral, where there is an Acute Lesion Duration showing the white skeleton.
Chronic Lesion Duration:
Although there is a portion of an Acute Lesion Duration to the center-left of the coral shown with white skeleton, the majority of the remaining coral skeleton shown represents a Chronic Lesion Duration. There is a gradation of algal colonization, with caulerpa, bubble algae, and coralline algae growing across the coral’s skeleton.
Coral Disease Assessment – Lesion Severity:
Lesion severity is the percentage of affected coral and ranges from mild to extreme. Interestingly, I had a difficult time finding mild and moderate examples to show, as most of the corals I’ve rehabilitated fell into the severe or extreme categories. Typically, I do not attempt to salvage mild or moderately affected corals, as they tend to heal without any intervention.
Mild (<10%) Lesion Severity:
This Echinophyllia (chalice) coral had a mild lesion on one side, where the lesion size was less than 10% of the coral size. This is an example of a mild (<10%) lesion severity.
Moderate (10-24%) Lesion Severity:
This Homophyllia (“Scolymia”) coral had a moderate lesion (let’s assume this one is 24%, although it is close to bumping up into the “severe” lesion severity category) due to a coral sting. This is an example of a moderate (10-24%) lesion severity. To learn more about this coral and its recovery, click here!
Severe (25-49%) Lesion Severity:
Similar to the Homophyllia coral, another coral severely stung this Trachyphyllia coral. Notice how the lesion was just shy of half of the coral. This is an example of a severe (25-49%) lesion severity. This was a severe case, but the coral made a full recovery. To learn more about this coral’s journey, click here!
Extreme (50%-100%) Lesion Severity:
This Lobophyllia coral had multiple issues, with lesions covering more than half of the colony. Despite such a bad condition, this coral recovered in only four months. This is an example of extreme (50-100%) lesion severity. Read more on this coral here!
Coral Disease Assessment – Lesion Color:
Lesion colors are important to coral disease assessment, as they may indicate potential disease to consider (e.g., white patch disease). Lesion colors are highly interesting to me, and I haven’t found enough research to support what I’ve been seeing with my corals. I’ll add more here as I learn more.
Purple Lesion Color:
The lesions depicted are purple (and are a combination of annular/irregular with a coalescing distribution). This was the first example I had seen of purple lesions aside from Aspergillosis fungus in Gorgonian corals and may be an example of Dark Spot Disease (DSD). (I found this coral while scuba diving, and I do not know its long-term outcome.)
White Lesion Color:
Although these lesions are not entirely white, white lesions are areas of bleached tissue. This coral had two large areas of bleached tissue, but it made a full recovery. To learn more about this coral’s recovery, click here!
Pink Lesion Color:
Distinguishing a pink lesion from coralline algae may be a bit tricky, but in this case, the pink coloration is possibly originating as a defense mechanism from the coral. Despite the coral’s poor condition, it made a full recovery. Click here to learn more!
Blue Lesion Color:
This coral has both blue and white lesions; the blue coloration is not natural, as it occurred post-bleaching during recovery. In my experience, sometimes corals with bleached lesions will heal back with blue coloration. However, these corals rarely survive long-term after the initial recovery, in my experience (my guess is that the blue coloration is from hosting a particular zooxanthallae population that is insufficient to sustain the host coral – it is almost like they become non-photosynthetic corals – or at least insufficiently photosynthetic). This is such a recurring challenge for me that I call it “Bleached-Blue-Death” given the progression from bleaching to blue coloration to death. When I have rescued corals like this, I continue spot feeding them long-term. This seems to work so far as a way to keep them alive. (I found this coral while scuba diving, and I do not know its long-term outcome.)
Yellow Lesion Color:
This Gorgonia coral has Aspergillosis, which starts with deep purple lesions that can expand and develop yellow centers.
Coral Disease Assessment – Lesion Pattern:
Lesion patterns, especially when combined with other lesion characteristics, can help provide a holistic picture for coral disease assessment. These patterns help determine if the cause may be bacterial, due to corallivore predation, or just due to poor water quality.
Annular Pattern:
This isn’t the best example of an annular lesion pattern, but it’s the best I have. An annular lesion is shaped like a ring, with healthy tissue typically on both sides of the ring. Unfortunately (and surprisingly), this coral did not recover.
Irregular Pattern:
An irregular shaped lesion has edges without any sort of pattern or form. The lesion edges are not smooth or straight. Although this Homophyllia had multifocal and coalescing lesions, it still survived and was one of my favorite rescues. Click here to learn how all this decay still recovered!
Linear Pattern:
Linear lesion shapes are in a straight line (or straight but curving along the skeleton in rounded or massive bouldering corals). This Micromussa (Acanthastrea) coral likely had “Brown Jelly Syndrome” (the coral is pictured in a dip, with the brown jelly-like substance already removed); this damage entirely occurred overnight. Thankfully, I was able to stop the infection and salvage the coral.
Circular Pattern:
Similar to the “annular” pattern, both are round-shaped. However, the “annular” shape is more of a ring-shaped lesion with a healthy center, whereas the “circular” lesion pattern is entirely the lesion.
Oblong Pattern:
The “oblong” pattern is similar to the “circular” pattern, in that the entire lesion is in an oblong shape, rather than a circle. Crazy, I know. I’m sure I have a photo for this, but I feel like it wouldn’t add anything that your imagination hasn’t already addressed.
Coral Disease Assessment – Lesion Distribution:
How the lesion presents across the coral can also provide key insights to the coral disease assessment process. If the lesion is across the entire coral, then it could be a waterborne issue; whereas, a focal lesion could indicate physical interference or corallivore predation.
Diffuse Lesion Distribution:
Diffuse lesions are spread across the coral, without a central location. This lesion presented as tissue decay across the highest points of the coral, with only tissue in the lower points surviving. Despite the coral’s terrible condition, it still fully recovered. Click here to learn more!
Focal Lesion Distribution:
Focal lesions are very defined, with healthy tissue surrounding a central point. Initially, I thought something fell on the coral to cause the damage (e.g., a heater); however, I found flatworms were consuming it. Removal of the flatworms and the damaged portions resulted in the coral’s recovery.
Coalescing Lesion Distribution:
Although this is likely a combination presentation with diffuse and coalescing lesions, it is still a good example of lesions across the coral growing and merging together – coalescing into larger lesions. Only a very small piece of this coral survived.
Linear Distribution:
Linear lesion shapes and linear lesion distributions are practically synonymous. The lesion progresses in a line across the coral (even if it is a bit of a curved line). (I found this coral while scuba diving, and I do not know its long-term outcome.)
Multifocal Lesion Distribution:
Oh, this little fellow is a troublemaker for the host coral. Notice all of the white lesions around the hole (and the hole is a growth anomaly, as well). These lesions are from fish bites, so they are multifocal (more than one location). Although multiple fish bites in a central location could form a coalescing distribution appearance, a multifocal lesion description is more appropriate given that the lesions themselves are not growing.
Coral Disease Assessment – Lesion Edges/Border:
Personally, I tend to mix up the lesion edges/border with the lesion margin/band. The lesion edge/border is the entire length of the lesion across the coral. This length should be much longer than the lesion margin/band, which is the distance from the apparent healthy coral to the lesion or coral skeleton.
Lesion edges (or the border of the lesion) are typically considered distinct/discrete or indistinct/diffuse, as shown below, to support in coral disease assessment. There is also a category for “annular,” but given that it’s already a lesion shape, I don’t see the value in having it describe the border as well. (I also don’t have a photo for an example, so maybe I’m just already biased against it.)
Distinct or Discrete Lesion Edges/Border:
The sharp contrast between the brown tissue and the white skeleton creates a distinct lesion edge (notice how some polyps are split between half-healthy and half-skeleton). Although this Cyphastrea coral only had a small portion of healthy tissue remaining, it recovered and eventually grew to one of the largest corals in my aquarium. Click here for the before and after photos!
Indistinct or Diffuse Lesion Edges/Border:
Contrasted with the previous Cyphastrea with distinct lesion edges, the indistinct lesion edges on this Cyphastrea create difficulty distinguishing healthy tissue and the remaining skeleton. Thankfully, this coral made a full recovery. Read more about its journey here!
Coral Disease Assessment – Lesion Margin/Band Type:
Lesion margins are the lines separating healthy tissue from unhealthy tissue or the coral’s skeleton. These are also frequently called “bands.” As previously mentioned, I tend to mix up the lesion margin/band with the lesion edge/border. The lesion edge/border is the entire length of the lesion across the coral. This length should be much longer than the lesion margin/band.
Smooth Margin/Band:
As previously shown, this lesion progresses in a smooth line across the coral (even if it is a bit of a curved line). (I found this coral while scuba diving, and I do not know its long-term outcome.)
Serrated (or Irregular) Margin/Band:
This isn’t the best example of a serrated lesion, but when I think of serrated lesions, I think of coral predation, like corallivorous starfish or Acropora-eating flatworms. This poor Acropora was dying from a bit of everything but did make a full recovery.
Serpiginous (or Irregular) Margin/Band:
A serpiginous lesion edge reminds me of a snake (i.e., serpent), where the edge winds tightly about the coral. Again, this coral made a full recovery.
Undulating (or Irregular) Margin/Band:
Similar to the serpiginous lesion margins, the undulating lesion margins wind about the coral but in a less tight manner. (I encountered this coral while scuba diving, so I do not know its outcome.)
Coral Disease Assessment – Margin/Band Thickness:
The lesion margin/band size is the measurement of the edge tissue, which, due to the narrow shape, may be difficult to obtain. And, in indistinct or diffuse lesions, the margin/band may be impossible to discern. Once again, the measurement units must be consistent in documentation (e.g., using centimeters or inches).
Coral Disease Assessment – Margin/Band Color:
Lesion margin/band colors vary and provide good indicators for coral disease assessment. In fact, several named coral diseases reference the band color in the name – it is the primary indicator of the disease. Some of these include Black Band Disease, Yellow Band Disease, Red Band Disease, and White Band. Conversely, other syndromes and diseases have no distinct colored band, such as the Caribbean white syndromes. While I wish I had my own photos of these diseases to include here for educational purposes, I’m thankful that I have not yet encountered many obvious examples.
This coral has a purple margin or band color, surrounding the lesion in the center. (I photographed this coral while scuba diving and do not know the outcome of the colony.)
This staghorn Acropora coral may have “White Band Disease.” Notice the zip tie placement near these bands toward the base of the coral. This physical indicator helps reef managers track the lesion growth and determine its growth rate. Also, there is a damselfish hiding in the top center of the photo. Damselfish nest in these corals and create algal beds. (I photographed this coral while scuba diving and do not know the outcome of the colony.)
Conclusion:
We’ve only begun to scratch the surface of coral diseases and how they may present, especially as Western Atlantic and Indo-Pacific creatures cohabitate within our aquariums. As you find new information on coral disease assessment, please reach out or share in the comments below. For additional information, please consider the following references.
References:
- “Coral Disease Handbook: Guidelines for Assessment, Monitoring & Management,” Global Environment Facility, June 2008. Website
- Hawthorn, Aine, et al. “An introduction to lesions and histology of Scleractinian corals.” Veterinary Pathology, vol. 60, no. 5, 31 July 2023, pp. 529–546. Website
- “Lesion Terminology and Visualization.” Coral Disease & Health Consortium, 13 Nov. 2023. Website
- Edited by Cheryl M. Woodley et al., Diseases of Coral | Wiley Online Books, Wiley Online Library.