Black Bugs & Gray Bugs…Invisible Killers

In my years of reefkeeping, I’ve seen just about everything.  But, not too long ago I was stumped.  My corals were receding like they were being eaten, but no matter how long I watched the corals at 3 a.m., I couldn’t find the culprit.  I had heard of black bugs bothering coral, but in all my research I couldn’t find any pictures of them.  I really wasn’t convinced they actually existed.

Speck on Acropora Coral

Speck on Acropora Coral

One day, I noticed the tiniest gray speck of movement on an Acropora coral in the photo above.  I watched it for a while, but I wasn’t sure what I was seeing.  It definitely wasn’t a red bug (Tegastes acroporanus), so I decided to err on the side of caution and dip the coral.  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 Bayer insecticide dip, and the speck fell off.  This is what I found under the microscope:

Alteuthellopsis corallina

Alteuthellopsis corallina

A. Corallina

A. Corallina

A. Corallina

A. Corallina

Alteuthellopsis corallina is a coral predator, and according to this article on Advanced Aquarist, it is known to infect Acropora, Merulina, Pocillopora, and other stony corals.  Although I don’t have photographs of other bugs known to infect chalice corals, acans, etc., they seem to respond similarly to treatments for red bugs.  The Psammacora in the photo below kept receding, and I couldn’t find the cause.  Once I dipped it in Bayer, it immediately started to recover.

Psammacora that Recovered Immediately after Dipping

Psammacora that Recovered Immediately after Dipping

How to Identify:

Most “black bugs” are only about 0.5 mm, so they’re extremely difficult to see.  As I mentioned above, some species seem to have an ability to lodge themselves into coral tissue.  LPS corals can hide these bugs better than SPS due to their additional tissue.  If you suspect you have a pod problem, I recommend ruling out all other options (water quality, large pests, flow problems, etc.)  Next, inspect the coral for movement since the human eye can spot movement somewhat better than a 0.5 mm speck.  Even if you can’t spot anything, a quick dip may indicate pest presence.


As I mentioned above, CoralRx did not cause any noticeable harm to A. corallina at the manufacturer’s recommended dosage.  It may work on other pest “bugs”, at higher dosages, or longer time, but I did not have enough bugs to test this on.

It appears that typical red bug treatments will work on the black/gray bugs, but I have been unsuccessful photographing or doing *real* studies on them.

Method 1:  Control

Based on my experience with A. corallina, I can’t recommend natural controls.  These bugs can really lodge themselves into the coral tissue, and any bug predator would probably cause significant damage to the coral.

Method 2:  Coral Dips

I also don’t recommend CoralRx based on the manufacturer’s recommended dosage/time.  Bayer Advanced Insecticide worked on A. corallina, and it fixed whatever was ailing corals with similar problems.  I don’t know how these pests reproduce, so if you dip, I still recommend quarantine.  Red bugs are live bearers, but I wouldn’t take any chances.

Method 3:  Interceptor

Interceptor is an in-tank treatment typically used for red bugs, but some red bugs are Interceptor-resistant.  Additionally, Interceptor can wipe out the beneficial invertebrate population.  This may be a good option for mature tanks with a black/red bug problem.  See the red bug page for more information.

Method 4:  Lower Temperature

Many aquarists (including myself) have noticed red bugs die at lower temperatures, and this may be an acceptable treatment for black bugs as well.  Advanced Aquarist suggests 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 power outage, my quarantine tank dropped to 65 degrees for about 6 hours.  Afterward, there was no sign of red bugs.  Note:  this is experimental, and there is not enough data to determine if this treatment works 100%.  Use with caution!


Although I don’t have any real answers in this article, hopefully it will increase awareness in the reefkeeping community about other coral pests.  And, hopefully, it’ll continue to pound home the idea that a good quarantine procedure is absolutely essential.  Happy reefkeeping!



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  1. Brad

    Hey is there any way i can send you a photo and get your feedback if you think this is the same little demon?

    1. admin

      Of course! Email the photo to reefdup@gmail.com.

  2. Aaron

    What was your treatment concentration of Bayer dip?

    1. admin

      Hi Aaron,

      I sent this to your email as well, but I wanted to make sure others had the info:

      I assume that speck moves around occasionally? It does look about the same size as the ones I had.

      I only used 0.5ml of Bayer to 2 cups (16 oz) tank water for a 15 min dip. I would start there, and if that doesn’t work, only then would I increase. There are tons of people who have supposedly used several times that dosage, but I can’t confirm that they had the same pest (they had no close-up photos of the pest).

      I hope that helps. Let me know how it goes.

  3. Brad

    Hey did your increase in Interceptor work? i did a regular interceptor treatment and nothing happened. Bayer dip at 20 ml per cup works but only after 30 minutes. is there any systemic tank treatment?

  4. admin

    For “black bugs”? I didn’t use Interceptor on them since the Bayer worked as a dip. I’ve had luck against red bugs by lowering the tank temp. That might work for the whole system, but it’s not without risk. Let’s discuss over email.

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    […] http://www.reefdup.com/2013/01/25/black-bugs-gray-bugs-invisible-killers/ I have been battling them as well. They are attacking acropora and eating it like doughnuts. I have just treated with a 250% interceptor dosage and it did not touch them!! I have now treated another 250% dosage again this morning with no water changes or carbon added yet so basically a 500% dosage of interceptor in a 12 hour period. We decided to increase the dosage initially after reading posts of how hardy these pests are. If anyone has had success getting rid of them please chime in… For those wondering interceptor can still be ordered on net from overseas. Check out my new build here: http://www.reef2reef.com/forums/large-aquariums-180g/154333-lasers-1000-inwall-starfire.html Reply With Quote […]

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