How to Identify and Treat Acropora Eating Flatworms

Acropora Eating Flatworms (Prosthiostomum acroporae) was detected in your aquarium? This is what you need to know.

Why Are Acropora-Eating Flatworms a Problem?

Acropora Eating Flatworms (AEFW, Prosthiostomum acroporae) are highly destructive, obligate coral predators that specifically target Acropora species. These flatworms belong to the Polycladida order and are known for their cryptic nature, making them difficult to detect until significant coral damage has occurred. Native to various reef environments, they have become a major concern for reef aquarists due to their ability to rapidly decimate Acropora colonies.

AEFW are flat, oval, and translucent brownish-tan/yellow in color, often camouflaging against the coral’s surface. Their presence is typically indicated by tissue loss, faded coral coloration, and white feeding scars along coral branches. Over time, this can weaken and stress the coral, making it more vulnerable to secondary infections, disease, ability to adapt to lighting changes, and colony death. This is especially relevant in aquariums where there may be no natural predators to keep them in check. 

They reproduce through egg-laying, depositing small, circular clusters along bare coral skeletons (not on live tissue). The eggs hatch into free-swimming larvae that later settle on corals to continue their life cycle. 

We have observed that during the day, AEFW tend to hide on shaded parts of Acropora branches making visual detection even harder.

Impact:

AEFW infestations can cause severe damage to Acropora, leading to:

• Tissue Necrosis: These flatworms consume coral tissue, leaving behind characteristic white patches where polyps have been devoured.
• Reduced Growth & Health: Infested corals show slowed growth, reduced polyp extension, and loss of coloration due to stress and tissue damage.
• Secondary Infections: Damaged tissue can become susceptible to bacterial, fungal and protozoan infections, accelerating coral deterioration.
• Colony Death: Without intervention, infestations can lead to the complete collapse of Acropora colonies, affecting overall reef tank stability.

 

How do AEFW get in my tank?

A small study conducted in the great barrier reef found AEFW occurring in 7 out of 10 Acropora colonies sampled in the wild, having 1-5 AEFW per infected colony. This implies that AEFW are more abundant than we would like to think.

Due to their stealthy nature, AEFW are introduced into aquariums through new coral additions. Without proper treatment, inspections and quarantine, they can rapidly spread, leading to widespread coral losses.

 

Life Cycle: How Fast Do They Multiply?

One of the reasons these flatworms are so hard to get rid of is their rapid life cycle. The eggs can hatch in just 11 days, and they reach adulthood in about 35 days. This means a new generation can appear every 46 days. Warmer temperatures speed up their reproduction, leading to more severe infestations. Even worse, each egg can contain multiple embryos and newly hatched flatworms can swim and survive for a few days without a host coral. This gives them a chance to rapidly spread to other parts of your tank infecting other Acropora colonies.

Here you can see the AEFW embryos moving around in the eggs. Each egg contains 1-4 embryos.

Below is a fluorescent microscopy of AEFW egg cluster. Notice the operculum (the circle in the center of the egg where they hatch from) is still intact so they haven't hatched yet. The egg capsule is strong and is impenetrable and unaffected by the currently available remedies on the market.

AEFW always lay eggs on bare skeleton and surrounding substrates, not live tissue. If you have a colony with dead tissue and exposed skeleton, have a look closely for eggs. If you see eggs scrape them off or squash them with a hard object. Damage to the eggs will allow microorganisms from your aquarium to enter and eat the larvae from the inside. You can see if the eggs have hatched based on the presence of the operculum (egg cap), use a x10 magnifying glass or your phone on macro settings. If you can see inside the eggs, the larvae have hatched and the next generation is out and about. But they can be killed/removed by dipping.

We noticed that some colonies are more vulnerable to infestation than others due to factor we don't yet know. Either AEFW prefer to feed on specific Acropora species or some Acropora species have defense mechanisms to deter them or be less enticing.

 

Acropora Eating Flatworms Treatment Methods

As there is no one treatment option guaranteed to eradicate AEFW infestation, multiple approached should be used simultaneously. This is going to take some time and effort, but not as involving as Ich or Velvet treatments.

You will need to setup a separate Acropora tank and remove all colonies, frags and encrusting material from the main tank. The main tank should be free of Acropora for 60-70 days to allow the existing flatworms and their next generation (eggs) to starve to death. 

1. Manual Removal & Dipping 

Regular coral dipping is an effective method for reducing AEFW populations:

• Coral Dips: Commercial dips containing potassium chloride, iodine-based solutions, BioScrub (combining both potassium salts and iodine), CoralRX, Revive and Dip-X are effective in dislodging and killing flatworms.
• Dipping Procedure:
1. Prepare a dip solution following manufacturer guidelines.
2. Submerge the infected coral in the solution for 5–10 minutes, agitating with a turkey baster (or similar) to dislodge flatworms. They do attach to corals well, but the dip weakens them and they will eventually fall off. With prolonged exposure to the dipping solution, they will eventually turn cloudy and degrade. 
3. Rinse with clean saltwater before returning the coral to the QT tank.

Infested Acropora was placed in a diluted BioScrub dipping solution and blasted with a turkey baster to detach the flatworms. The flatworms were dead within 5 minutes of exposure and started to disintegrated at10 min of exposure. 

 

In your coral QT tank, you can have all Acropora corals on a removable platform for easy removal and dipping.

Limitation: Coral dipping solutions remove flatworms but they do not remove or kill the eggs. You therefore need to repeat the dip every 7 days. Furthermore this approach is simple to perform on frags or colonies on plugs as they can be easily removed from the tank. But large and established colonies would be more challenging to dip. If you have larger colonies prepare a potassium salt based dip solution (e.g., BioScrub) and spray on the colonies. After 10-15 min, rinse the colonies with clean aquarium water before placing back in the tank.

We recommend to also proceed with method 2 and 4.

2. Biological Control (Natural Predators)

Adding the right predators to your tank can help keep flatworm numbers down. Two reef-safe species that have been tested and proven to eat them are:

• Sixline Wrasse (Pseudocheilinus hexataenia) – These useful little fish will consume flatworms and to a lesser extent, their eggs. In controlled experiments, sixline wrasses wiped out 100% of adult flatworms from infested Acropora.
• Peppermint Shrimp (Lysmata vittata) – Not only do they eat adult flatworms, but they also munch on flatworm eggs, limiting the next generation and reducing the chances of reinfestation.

Additional natural predators to consider

• Wrasses (Genus Halichoeres): Yellow Coris Wrasse (Halichoeres chrysus) is known to eat flatworms.
• Mandarins & Dragonets: Synchiropus splendidus was reported to eat AEFW and contribute to controlling populations. Though we have not tested them and they prefer to stay at the lower portions of the tank, not where Acropora are usually placed.

Limitation: Predator effectiveness varies, and they may not completely eradicate AEFW.  We suggest you add them to your coral QT tank and to not over feed them so they will actively hunt the AEFW and the eggs. 

3. Main Aquarium Treatments

• Flatworm-Specific Treatments: Some reef-safe products, such as Flatworm Exit (Salifert), target various flatworms (Acoel Flatworm and red planaria). However, effectiveness against AEFW specifically is debated.

4. Egg Removal & Long-Term Control

• Scraping Egg Clusters: Since dips do not kill eggs, manual removal using a fine brush or scalpel is necessary.
• Quarantine New Corals: Keeping new Acropora frags in quarantine (4–6 weeks) with regular inspections can prevent AEFW introduction. Adding natural predators in the main tank and quarantine tank would be beneficial.

Treatment recap

For active infestations, we recommend the following approach:

1. Perform an initial dip to remove most adult flatworms.
2. Manually inspect corals for egg clusters and remove them.
3. Introduce natural predators if possible.
4. Repeat dips every 5–7 days for 4 weeks to target new hatchlings.
5. Monitor closely for reinfestation.
6. For severe or reoccurring infestations, we recommend removing all Acropora from the main tank including encrusted pieces and relocate them to a separate quarantine tank. Leave the main infested tank Acropora free for at least 60 days to ensure that the adults and the new generation (from eggs) would starve to death. In the meantime, treat the corals in the quarantine tank as mentioned above dipping, biological controls and manual egg removal.

By combining multiple treatment methods, you can effectively manage and prevent Prosthiostomum acroporae outbreaks, ensuring the long-term health of you Acropora colonies.