In my years of reefkeeping, I’ve seen some odd stuff. But, back in 2013, I was stumped. My corals were receding like something was eating them, but no matter how long I watched the corals at 3 a.m., I couldn’t find the culprit. I had heard of Acropora “black bug” parasites, but I couldn’t find any pictures of them. I really wasn’t convinced they actually existed in the hobby, or if they did, that I had them.
Speck on Acropora Coral
One day, I noticed the tiniest gray speck of movement on an Acropora coral. I watched it for a while, but I wasn’t sure what I was seeing. It definitely wasn’t a red bug (Tegastes acroporanus), but it also did not seem like random detritus movement. I decided to err on the side of caution and dip the coral. (You see it, right??)
First, I started with Coral Rx, but the speck didn’t fall off. In fact, I couldn’t move the speck with a pick. When I grabbed a magnifying glass, I realized the tiny “pod” had lodged itself in the coral’s tissue and was still very much alive.
I then dipped the coral in a different dip, and the speck fell off. This is what I found under the microscope. Sorry for the poor quality – this was back in my sub-par microscope days.
Alteuthellopsis corallina or “Black Bug” Acropora Parasites:
Although I am not certain, I believe the parasites were Alteuthellopsis corallina (so we’re going to go with that assumption for the remainder of this post). These parasites are coral predators, and according to this article, they are known to infect Acropora, Merulina, Pocillopora, and other stony corals.
How to Identify:
While corals affected with red bugs tend to “brown out,” the corals infected with A. corallina appeared bleached and as though something was eating them. A. corallina are only a little larger than 0.5 mm, so they’re extremely difficult to see due to size and color. As mentioned above, this species seems to have an ability to lodge themselves into coral tissue. These parasites are more gray than black, but most reefkeeping websites reference odd pods as being black. In the hopes of providing the most information possible on Acropora parasites, A. corallina is associated with both gray and black bugs here.
In order to determine the presence of any sort of unusual pods, I typically use a magnifying glass to inspect the corals prior to placement in a quarantine system. After that, I observe the coral daily for a week. I also take a photo of the coral and zoom in to look for anything I missed.
If you suspect you have a pod problem, I recommend ruling out all other options (e.g., water quality, large pests, flow problems) Next, inspect the coral for movement since the human eye can spot movement somewhat better than a 0.5 mm stationary speck. Even if you can’t spot anything, a quick dip may indicate pest presence.
Treatment Options:
It appears likely that typical red bug treatments will work on the black/gray bugs, but I have been unsuccessful photographing or studying them further. Thankfully, in 14+ years of reefkeeping, I have only encountered these once.
Method 1: Control
Based on my experience with A. corallina, I can’t recommend natural controls. These pods can really lodge themselves into the coral tissue, and any pod predator would probably cause significant damage to the coral. Regardless, I advise against natural control methods for aquarists who plan to sell/trade their corals.
Method 2: Coral Dips
Coral dips often help stun the pests so they can be blown off gently with a pipette or powerhead. For live bearer parasites, this may be a one-shot treatment. Unfortunately, I don’t know how these parasites reproduce, so if you dip, I still recommend quarantine. Red bugs are live bearers, but I wouldn’t take any chances regarding applicability to A. corallina.
As mentioned above, CoralRx did not cause any noticeable harm to A. corallina at the manufacturer’s recommended dosage. It may work on other similar parasites, at higher dosages, or longer time, but I did not have enough pods to test this on. Other dips will likely work, as a different dip was successful for me.
Since a succession of coral dips work for Acropora-eating flatworms and red bugs, I dip all Acropora corals daily the first week. After that, I dip all Acropora corals every other day the second week, every third day the third week, and then once a week for weeks four through six. I have not lost a single coral with this method, and it saves me the hassle of treating with different medications for different parasites at different times.
Note: I have never found a pest after week two, but I still go the full six weeks, since the lifecycle of AEFW is quite long (and I always treat Acropora corals for both AEFW and red bugs). Also, some dips can be extremely damaging to smooth-skinned corals, so please use caution if you plan to utilize a coral dip.
WARNING: Some aquarists use Bayer Advanced Insecticide as a coral dip. Per Bayer Advanced Insecticide’s labeling, “It is a violation of Federal law to use this product in a manner inconsistent with its labeling.” Due to this warning, this website does not cover Bayer use as a coral dip.
Coral Dip Treatment Protocol for A. corallina Parasites:
Follow dip manufacturer instructions.
Method 3: Prescription Flea Medications
Hobbyists considered Interceptor (milbemycin oxime), a prescription canine flea medication, as the golden standard for red bug parasite treatment. At least, they did before the manufacturer discontinued it. Even then it was not the easiest solution. Aquarists had to remove all shrimp, crabs, and other crustaceans before dosing. This die-off would spike ammonia levels, so reefkeepers also had to prepare for a water change and to run carbon. Additionally, many people had difficulty obtaining Interceptor from their veterinarians, since this is an off-label use. Eventually, the Veterinary Information Network published information on red bug parasites for reference.
Once Interceptor was no longer available, reefkeepers began searching for a replacement. Currently, Milbemax is the most often recommended replacement. It is a prescription combination of milbemycin and praziquantel. The latter medication is commonly used for fluke treatment in reef tanks (e.g., PraziPro); however, it is a much lower dose than would be effective for common marine flukes.
Providing proof of infection to a veterinarian and asking him/her to look up red bugs on the VIN may be enough to justify the medication for gray/black bugs. Some veterinarians have recommended different medications with different treatment protocol (e.g., MilbeMite Otic), so those individual cases are not discussed here.
Unfortunately, many aquarists are finding some red bugs have a resistance to Milbemycin, so gray/black bugs may as well. If you try it with no success, work with your veterinarian to find an alternate medication, or consider one of the alternate methods described here. See the red bug page for more information on reference dosing protocols for both Interceptor and Milbemax.
Method 4: Temperature Adjustment
Many aquarists (including myself) have noticed red bugs die at lower temperatures, and this may be an acceptable treatment for gray/black bugs as well. A temperature of 72°F may be adequate to control/reduce the population. Use caution with this method as it can cause coral stress if the temperature drops too suddenly or if the oxygen level drops.
Due to a six-hour power outage in Utah, my quarantine tank dropped to 65 degrees. Afterward, there was no sign of red bugs on a known infested Acropora. Note: this is experimental, and there is not enough data to determine if this treatment works 100% or with gray/black bugs. Use with caution!
Conclusion:
It is truly terrifying and amazing what a 0.5mm speck of a pod can do to a coral. It is likewise amazing and rewarding how the Acropora coral can recover. With this sort of risk, a good quarantine is absolutely essential, but there are a few viable treatment options available.
