Why Do The Tips Of My Acro Bleach?

It’s a common concern and occasionally, a nightmare for SPS lovers of all levels of experience: the tips of a prized Acropora frag or colony have suddenly started to lose their color. For novice coralheads, this can trigger alarm and self-doubt. Is my water chemistry close to optimal? Have I done something wrong? Is that $150 Acropora frag about to become worthless? Experienced reefkeepers look at whitening Acro tips through a different lens, based on intuition and years of observation: What are my WQ params right now and what are the trends over the last few days and weeks? What, if anything, have I changed recently? Have I had an outbreak of Acropora-Eating Flatworms (AEFWs?) The fact is, it could be all, none, or a combination of these and other factors.

Here are a few of the reasons why the tips of your Acropora frags or colonies may be bleaching:

• Are you certain that they’re bleaching? – The whitening tips of your Acro frag or colony might actually be a very good sign. As Acropora coral grows, the calcium carbonate skeleton and the thin veneer of living tissue covering the tips of the branches often appear whitish, as the photosynthetic zooxanthellae which give the coral its color have not yet migrated into the new growth. It’s possible that what you perceive as bleaching is actually a very happy Acro colony reaching toward the light.
• Damage from neighboring corals – Look closely. Is the bleaching just affecting the tips or are other areas of the frag or colony that also appear spotted? It could be that your Acro is getting stung by the mesentery filaments or sweeper tentacles of a neighboring colony of another coral species. These tentacles can extend much further from a coral colony than you might think, gathering food and protecting the living space of the host colony from encroachment by other corals. Before moving the affected coral, consider placing some type of barrier between it and an aggressive neighbor or neighbors for protection.
• Recent swings in carbonate hardness – Whatever you prefer to call it, carbonate hardness (KH,) or alkalinity, corals in general and Acropora and other SPS species in particular, are not tolerant of swings in this and other water chemistry parameters. A sudden drop or rise in alkalinity can shock an Acro frag or colony, resulting in bleached tips. This might kill your Acro frag altogether or stun it, causing a long convalescence if it survives at all. Moral of the story: keep your WQ parameters as stable as possible at all times.
• Lighting changes – A recent change in lighting intensity or in light fixtures can shock the symbiotic algae living in the tips of your Acros and cause death and expulsion of these critically important symbionts. If you’ve recently made a lighting adjustment, you may have cranked the brightness a skosh too much. Consider restoring it to its previous setting and see if your Acros and other corals color up again. This can take weeks, so be patient.
• Rapid Tissue Necrosis (RTN) – Most research indicates that RTN is caused by a bacterial infection, which can strip the living tissue off a coral skeleton with frightening speed. This infection may or not follow insults caused by ciliates, flatworms, or other known SPS parasites. Likewise, it may or not follow physical injuries from coral stings, recent fragging, an accidental bump while scrubbing algae, etc. While much research into RTN is ongoing, the only thing we know for sure is that this infection moves quickly and indiscriminately and strikes fear into the hearts of all SPS coral keepers.
• Coral pests and nibblers – There are a host of invertebrates that like to snack on Acropora tissue: The aforementioned AEFWs, parasitic copepods (red bugs,) and perhaps other hitchhikers too small to spot with a casual glance before they are introduced to your reef. There are also a whole slew of corallivore fishes and crustaceans that are either obligate coral eaters or simply opportunistic critters with a taste for coral tissue on the newest growth on the tips. The question then becomes: Have I introduced something unseen on a new coral frag? Is that gorgeous fish I just bought reef-safe? Is that crab I bought to control algae also snacking on my corals?
• Rapid changes in water chemistry – The old adage about SPS coral success states: stability, stability, stability. Are the WQ parameters in your system stable or have there been some recent shifts in these values? This is when regular water quality testing and tracking the results becomes invaluable. Keeping some kind of log allows you to spot trends in your params that can give you keen insight into one reefkeeping problem or another. Automation and other maintenance shortcuts can save time and make reefing more fun but if something gets stuck or a probe gets fouled by biofilm or algae, it can result in disastrous and rapid changes in water chemistry. Test your params and chart the results! It takes a lot of the guesswork out of troubleshooting.
• Idiopathic tissue loss – Idiopathic is a medical term occasionally used when a disease or other condition happens from an unknown origin. Sometimes, the tips of Acros and other coral bleach and die for no discernable reason at all. Maybe it affects one colony of Acropora or it affects them all. Maybe the Acro frags and colonies in one area of your reef are affected but other areas are not. It’s frustrating and terrifying in equal measure but this happens to hobbyists and professionals alike.

As you can see, Acropora tip bleaching can happen for a variety of reasons. Some of these reasons are predictable and preventable, while others are pretty darn inscrutable. If you see the tips of your Acros bleaching and you’re certain that it’s not simply new growth without pigmentation, there are no fast, easy answers or fixes. Cutting unto healthy tissue below the bleaching may work or it may expedite the loss of the rest of the tissue. Removing the affected frag or colony to a dedicated system for treatment may save it or it may not. Like many aspects of coral husbandry, do your research and due diligence on finding a cause, do what you can to remedy the situation, and rely on your intuition and experience as a coralhead. At the end of the day, it’s how we all learn and get more accomplished with reefkeeping. Good luck and thanks for reading!