Even on a smaller scale than global reef decline, the ability to assess coral health is essential. I am often asked how I know where to even start to rescue corals. In my opinion, determining how healthy the coral is, or what the problem might be, is usually the first step. This is an introduction on assessing coral health to determine what might be causing the coral stress.
Assessing Coral Health for Stung Corals:
Corals stung by another coral are probably the most easy to save. These corals are healthy overall, but a portion of them is damaged. As long the coral avoids infection and no further damage occurs, then healing is rather quick. When a coral “stings” another coral, the stung coral is sometimes actually “digested.” When corals have to compete for territory, they expel their digestive organs, called mesenterial filaments, and use them to digest a nearby coral. This typically results in one area of severe damage and no damage elsewhere. A coral stung this coral shown, but the remaining tissue was very healthy. You can read more about the Trachyphyllia’s recovery here.
Lack of Proper Husbandry:
A lack of proper husbandry is typically a slow process. It can result in the tissue between polyps receding first. As shown here, portions of recent die-off right are beside healthy tissue and coralline-encrusted skeleton. With these corals, good water parameters are essential. Removal of invasive algae is also critical. These corals typically do not need much additional care. You can read more about this Lobophyllia’s recovery here.
Assessing Poorly-Fragmented Corals:
When a coral is improperly fragmented, various outcomes are possible depending on damage. Sometimes the tissue rips apart improperly, or the skeleton is crushed underneath intact tissue. As long as there is about half the polyp left, the coral typically survives with the natural healing process. This Euphyllia coral unfortunately became infected and died.
Corals with Pest Damage or Parasites:
Attempting to rescue an infected coral is risky, as it may infect other corals. This type of rescue requires a proper quarantine tank and medications on hand. Typically, once the pests and any eggs are removed, the coral will heal quickly on its own. Here, this infected Acropora has both Acropora-eating flatworms and red bugs (Tegastes acroporanus). With proper treatments to remove both pests, this coral made a full recovery.
Assessing Coral Health due to Infections:
Attempting to rescue infected corals can be dangerous, complex, and expensive. Any attempts require a dedicated hospital tank, proper supplies (including a microscope), and extensive knowledge in coral diseases and treatments. Furthermore, keeping a low expectation for coral survival is key. These infections vary by cause (e.g., bacterial, viral, fungal, ciliate, etc.), so the owner must tailor treatment to the cause. Determining the cause can be nearly impossible without the proper equipment. Any attempts also require the utmost caution, as some infections are transferable to humans. This coral shown had “brown jelly disease.”
Bleached Corals:
Coral bleaching also has a variety of causes, and knowledge of the cause can help determine possible solutions. Assessing a bleached coral’s health can be challenging as high light, chronic low light, high temperature, low temperature (rare cases), lack of oxygenation, and some infections can cause bleaching. Since a bleached coral is lacking its typical zooxanthallae population (the symbiotic algae that lives inside the coral’s tissues and produces food for the coral), it will need regular feedings. Unfortunately, in my experience, sometimes these corals never thrive again, even with coloration return.
The photo below shows several corals that were poisoned with household bleach. An angry customer poured it into a local fish store’s tanks, which killed all the livestock. I attempted to save everything that still had tissue, but even these corals eventually died.
Starved Corals:
Although starvation is more commonly seen in non-photosynthetic corals (NPS), it is still possible in photosynthetic corals as well. This is more likely to occur in in ultra-low nutrient systems, also known as ULNS. Here, a Dendrophyllia coral is starving, shown by the algae-encrusted skeleton. Regular feedings are essential in these cases.
Poisoned Corals:
Poisoned corals usually have retracted tissue with areas of white skeleton showing. Tissue colors are usually brown or white, depending on the toxin. Any other typical colors (e.g., blues, reds, greens) are muted. This coral was poisoned by either the flatworm treatment a LFS used, or by the flatworm toxins themselves. You can read more about this coral’s amazing recovery here.
As shown, coral health has a multitude of causes, and only a few examples were shown here. If you would like to continue learning about coral pathology, a more technical source is available on NOAA.
