Hitchhikers make their way into home aquariums by stowing away on pieces of live rock and coral. Some are relatively benign such as the occasional feather duster worm. There are even highly sought after hitchhikers such as commensal hermit crabs that live in some stony corals. This article though is about the top 5 worst pests that I have come across.

If you do this hobby for long enough, it becomes routine to manage the low level pests that show up here and there. Things like Aiptasia and bristle worms in the grand scheme of things are not nearly as bad as they seem. Pesky perhaps, but manageable. The sort of pests in the top 5 are actually a result of good pest management.

Over time, new techniques and new products come along to help mitigate the risk of hitchhikers. All that is great! After a while though, only the worst pests remain. Often the pests that remain are resistant to all sorts of measures and are capable of causing significant damage to our reef aquariums.

#5: Acropora Red Bugs

“Red Bugs” are small crustaceans that infest Acropora colonies. I liken them to fleas that irritate the coral and eventually causes polyps to stop extending fully. A greater concern is the potential for die-off which can lead to the loss of the whole colony. Acropora are a sensitive species and the colony can go down hill in a hurry.

Red bug issues are difficult to diagnose because of their size. It is very difficult to see one of these specimens unless you know what to look for. They appear as small red dots on the skin of the Acropora colony.

Red bugs are resistant to many of the commercially available coral dips on the market, which is odd because dips in general seem highly effective against most other crustaceans. Some hobbyists report having success using a dip in either Interceptor, a prescription pill for heart worm disease in cats and dogs or Bayer Advanced Insect Killer which you can find at a hardware store.

This dip is great when you can remove the infected colony from the tank however in some cases the colony is too large to remove safely or has grown onto the rocks. In this case, you might consider using a product called levamisole, a commercial pig dewormer on the entire aquarium. Please note that this is a drastic measure. It is absolutely brutal on many of the tank’s inhabitants. It will kill all the beneficial inverts in the aquarium and it is going to stress out fish. It may be a good idea even to try and catch all the fish and inverts and relocating them to a quarantine system for a few days.

Luckily, there is another method of dealing with red bugs in an established tank where the Acropora cannot be removed. Dragonface Pipe Fish are a close relative of sea horses and act as a natural predator of the crustaceans. They are surprisingly adept swimmers and constantly pick off microorganisms.

#4: Acropora Eating Flat Worms

Sticking with Acropora pests, we have Acropora eating flatworms. These guys can be tricky to see, so often it’s not possible to catch the problem early. One day, you might see these white speckles show up on Acropora and then you know you have an infestation. Those white speckles are the bite marks from where the flatworms were eating the coral, and there is a good likelihood that they are all over the coral.

There is some good news and some bad news. The good news is, these flat worms can be removed by dipping; the flatworms let go of the host coral. In some particularly bad cases of flatworm infestation, what you thought was a brownish-tan colored Acropora was actually a white Acropora that was covered with hundreds of flatworms. Yeah… that was the good news. The bad news is, removing the adults probably won’t eliminate the infestation because they lay eggs and those eggs survive almost every dip. It may take weeks of diligent dipping to fully cure a colony.

#3: Zoanthid Eating Nudibranchs

Zoanthids on one hand are one of the easiest corals to keep in the reef keeping hobby. On the other hand, they are also one of the most susceptible to a wide range of pests. One such pest is the Zoanthid Eating Nudibranch. This variety of Nudibranch was particularly well disguised because as they eat the Zoanthids, they take on the color and fluorescence of the polyps. After a while, they look just like another head in the colony.

These nudibranchs can be removed by dipping in commercially available dips but like the Acropora eating flat worms, their eggs survive and can be very difficult to remove. The eggs form a spiral pattern and have a thick gel coat making them difficult to remove with metal scrapers and brushes. What seems to work better is a dry paper towel. The paper towel dries out the gel coat and are coarse enough to pull them off the polyp.

Despite the effectiveness of dipping to kill the adults and physical removal of the eggs, an infestation of Zoanthid Eating Nudibranchs is still difficult to manage because unlike many pests that only stick to the coral, these pests are perfectly comfortably roaming the tank looking for new colonies to eat. Even if the hobbyist was diligent and cleaned every single zoanthid colony, it is likely these sea slugs are still hiding somewhere in the rock work and can reemerge later.

#2: Zoanthid Eating Spiders

If the nudibranchs weren’t bad enough there is a second pest that plague Zoanthids and is the stuff of nightmares. There is a variety of sea spider gobbles up Zoanthid colonies. If you are familiar with the Alien movie franchise, these sea spiders are the face hugger aliens of the reef aquarium hobby. They latch on to the Zoanthids at the mouth of the polyp and lay eggs inside the polyp. Later as the spiderlings hatch the polyp disintegrates in to a grey mess.

These sea spiders can be physically removed with tweezers and dental tools. Commercially available pest control dips are also effective at killing them if given enough time in the bath. The eggs however are completely unaffected by either method of removal because they reside deep within the tissue of the polyp. It takes several rounds of removal to get rid of a spider infestation and can be a frustrating process especially because during this whole time the Zoanthid collection looks like it is dissolving before your very eyes.

#1 Montipora Eating Nudibranchs

The Montipora Eating Nudibranch is one of the most difficult pests to manage. They are so bad in fact that I now consider discarding a coral infested with them rather than trying to eradicate the nudibranchs. The Montipora Eating Nudibranch is white in color and tiny compared to the Zoanthid-eating variety. They can be difficult to spot at first because they tend to start working on the bottom of the colony.

Removal of this variety of nudibranch is more challenging than any other I have had the displeasure of dealing with. First off, they are resistant to dipping. The adults do succumb in time to an aggressive dose but can shrug off most regular concentrations of coral dip. The eggs are difficult to remove and require scraping of the coral’s skeleton to dislodge. Even with careful extraction of the eggs, they are small enough that many go undetected only to hatch later. Given their persistence and considerable damage they are capable of, the Montipora Eating Nudibranch is a top tier pest in the hobby today.


Hitchhiker pests unfortunately are a reality in the reef keeping hobby. As a conscientious hobbyist, one can never ever assume that a coral or rock is clean because it comes from a seemingly good system. Developing a systematic approach to new coral introduction is worth its weight in gold if it successfully prevents just one outbreak. Some combination of preventative dips and quarantine will help immensely as will purchasing from aquacultures sources as those corals are more likely to have undergone dipping themselves.

Read More

The Batman Problem, by Than Thein

The Batman Problem, by Than Thein

Learn more about these pests and the methods useful for their control in Than Thein’s latest article, The Batman Problem, in the November/December 2015 issue of CORAL Magazine.

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