Things that Suck: Acropora Eating Flatworms

Perusing through the forums of Reef Central or other aquarium internet sites these days can make any reefkeeper queasy. In the last few years more and more hobbyists have found themselves battling one type of pest or another. As we’ve gotten better at keeping corals alive, we’ve found more and more little wee beasties that don’t necessarily want what we want. This is the first article in a series on things that not only suck to have in your tank, but literally suck the life out of your corals. These articles are based only on organisms that I have battled and treatments that I have done, witnessed or at least think I know something about. Furthermore, these articles will tend to be short, concise and to the point, as presently three impatient little tykes are demanding to be pushed in the “crazy” swing.

Two Types of Flatworms

Photo courtesy of Joseph Weatherson.

The first fun-filled creature I’ll be dealing with is the Acropora-eating Flatworm or AEFW (for those who think it’s cool to use acronyms to confuse those beneath us). These flatworms have probably been around since the first Acropora were kept in captive settings, but they probably went unnoticed for quite some time because we had a multitude of other ways to kill Acropora, most of which were far quicker than AEFW. Once the mysteries of keeping and growing Acropora were solved, the coral gods knew we needed something else to haunt our sleep - enter the evil Mr. Flatworm.

In my experiences at least two different types of flatworm seem to affect our acroporids although others probably exist. Because the taxonomy of flatworms isn’t quite on my agenda of things to learn, I’m not sure if the two types I have dealt with are two different species, or the same species exhibiting two different types of behavior. Of these two types, one seems to be a rather benign species that seems to do harm only when present in large numbers. This flatworm acts as an irritant and the corals hosting it keep their polyps withdrawn a majority of the time. I have never observed any tissue recession, bite marks or egg masses with this particular flatworm. If you get AEFW, pray to your God of choice that this is the one you get!

The other flatworm I’ve seen more than makes up for his wussy counterpart (photos above & below). Its main agenda is to cause you pain and mental anguish - in addition to also eating all of your Acropora. So the first thing we have to do is determine if this particular predator is eating your corals. As its name suggests, this flatworm has been found only on Acropora species. If all your corals are dying or recessing, the thing that sucks is probably you. There are a few species of Acropora that are usually not affected. In general, corals that produce copious amounts of mucous (e.g. the “Bali Slimer” Acropora sp.) tend not to be attacked. Corals that are being attacked tend to show a loss of coloration and growth and tissue recession from their base. If this describes one of your corals, take a closer look at the newly bared coral skeleton. If tiny brown egg sacs are found, I’ll give you a minute to ponder your bad luck. Because these flatworms are nearly impossible to see while on the coral in the water due to their camouflage by transparency, removing the coral from the water can provide a definite answer. While it’s out of the water, allow the coral to dry off a bit. Often the flatworms only become evident as the coral dries out. A short freshwater dip is yet another way to reveal these flatworms. Dip the coral in one of the various solutions discussed later in this article. If flatworms are present, they will begin to fall off the coral during one of these dips. Take the flatworms that have fallen off and remove them from the water. Smash and kill them in assorted creative ways! Although it will do nothing for the coral, it may make you feel a bit better.

Die, you Fiends!

If it has been determined that your corals are, in fact, parasitized, the next thing to do is eliminate the worms, although that's easier said than done. To my knowledge, no in-tank treatment exists that positively eliminates these flatworms and harms nothing else. Various fish have been tried as natural controls with varying degrees of success. I’ve added different types of wrasses to infected systems, but I’ve never seen a 100% removal of the flatworms. If a biological control is what you seek, try wrasses from the genera Halichoeres, Psuedocheilinus or other small inquisitive wrasses. Many times biological controls can keep a predator or pest population in check, but cannot eliminate them completely. Be mindful of this when trading your corals. Your corals maybe doing fine and look great, but may still have one or two flatworms lurking in their inaccessible areas. If trading your corals, at least mention to the person that you have had this problem and the ways you took to eliminate it. Give them the choice of whether to accept the coral based on all the facts.

Photo courtesy of Joseph Weatherson.

The most effective way to eliminate corallivourous flatworms is in a separate quarantine tank. Make sure this tank is well cycled and conditioned and is appropriate for housing Acropora corals. These corals are already stressed and compromised, so putting them in a suboptimal environment will hasten their death. The corals can be treated and then placed immediately back into the display tank. Though not ideal (keeping them isolated in a quarantine tank is ideal), I was successful. Whatever you do, make sure that all affected corals from the display are removed for dipping as you want to make sure all flatworms go through the dip. This might entail breaking off large encrusted colonies or chiseling off encrusted tissue from surrounding rocks.

Various treatments have been used to get rid of AEFW. I’ve used Salifert’s Flatworm Exit, levamisole hydrochloride, freshwater dips and iodine-based dips. Other treatments are available, most notably ones using Fluke-Tabs (Aquarium Products) and trichlorfon (Dylox 80 Bayer A.G.). My problems were eliminated before I ever had a chance to try these out, and I didn’t have much of a desire to re-infect my corals to do more trials.

Flatworm Exit™ tends to work in extremely high concentrations. These flatworms were unaffected until the dosage was increased to 30x the recommended dosage for the normal Convolutriluba sp. flatworms. I felt these Acropora flatworms were probably laughing as they died because they knew that I had just spent $20 trying to kill a few of them. Freshwater dips can also work, but can be pretty harsh on most Acropora. If you use this treatment, make sure the temperature and pH are similar to the water they are removed from, and then do a quick 20-30 second dip. A more successful method is to use iodine-based dips. There are a few of these on the market including the relatively new Tropic Marin Pro Coral Cure (the acronym is TMPCC for the cool people). Lugol’s iodine can also be used at 5ml/gal. Tropic Marin Pro Coral Cure tends to work well and doesn’t seem to bother most corals. Follow the manufacturer’s directions for the dip. The dip that I used with great success is levamisole hydrochloride. Levamisole is used for many things, but is used widely as a pig dewormer. It can be acquired through agricultural supply stores and is usually around $20 for a small bottle. A dip can be done at a dose rate of 40ppm (see sidebar below). The length of the dip is left up to the individual. Usually the flatworms fall off after a few minutes. Some of the larger “mongrel” worms can stay attached for up to 30 minutes. Either gently swish the corals around in the dip water, or take a small power head (e.g. Maxi Jet 250, Marineland Inc.) and blow all the coral’s crevices and branches to dislodge any stubborn flatworms. While doing this, carefully inspect the coral for any sign of eggs. If you see small brown egg sacs, either cut off the area, or scrape these eggs off of the coral. Sometimes it is best to cut off all the dead parts of the coral, leaving a healthy, flatworm- free coral. After a few minutes with no flatworms falling off the coral, it can be removed from the dip and placed back into the display or, ideally, a quarantine tank. None of the above treatments harms the flatworm’s eggs, so to be sure to get them all; multiple dips are necessary. I dipped once per week for four weeks, though three treatments should be adequate.

Any of the above dips can negatively affect compromised corals. Levamisole can cause corals to be more susceptible to bleaching after multiple exposures to the drug. Some corals also react negatively to dosages above 40ppm or to prolonged dips. It is also best to use levamisole at the first sign of the infestation. Dipping corals in the drug once they have become severely compromised, can lead to their death. At the Omaha Zoo, the flatworms were discovered fairly soon and before their populations grew too large. Though some of the Acropora did show some lightening during the later dips, none of the corals were lost and the flatworms were eliminated and have not returned for two years. If your corals are in an advanced stage of infestation, they might not survive the dips. Catching the flatworms before their populations become overwhelming to corals is the key to a quick victory.

Quarantine, Quarantine, Quarantine!

One way to ensure that these predators never get into your display tank is the one thing that is rarely done:  quarantine. Many aquarists have the mistaken idea that you have to spend a lot of money on an elaborate quarantine system. A quarantine system can be a simple 20-gallon tank with a sponge filter, powerhead and a dual bulb shoplight. Use water from your display tank for initial setup and water changes. Keep the coral in quarantine for at least a week. During this quarantine period do a couple of investigative dips with your dip of choice. A close inspection with a magnifying glass can also expose some unwanted guests. For a little over $100 and a bit of your time, a lot of heartache, stress and coral loss can be avoided. Of course, quarantining fish and corals has been trumpeted for quite some time, and still a vast majority of hobbyists do not follow this simple protocol. The ones who do quarantine are usually those who have gone through a tank wipe out by one of the various predators. If you have no plans for a quarantine tank, at least dip your coral or thoroughly inspect it once before adding it to your tank. Keep in mind that eggs are not affected by dipping, and if one egg gets by a dip or inspection, misery will ensue.

Dealing with Acropora-eating flatworms can be a nightmare to deal with, but they can be defeated. By using a quarantine process, you’ll likely never have to deal with them on a grand scale. If they are found in your main tank, take a deep breath, do some research and plot out a plan of action. It will take some time and some hard work, but eventually these coral munching predators can be defeated.

Oh, and did I mention the thing about quarantine?