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 disease and even killing entire colonies—especially in aquariums where there may be no natural predators to keep them in check. They also reduce the coral’s ability to adapt to lighting changes, further harming the colony’s health.

They reproduce through egg-laying, depositing small, circular clusters along bare coral skeletons, which hatch into free-swimming larvae that later settle on corals to continue their life cycle.

Impact:

AEFW infestations can cause severe damage to Acropora species, 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 damage.
• Secondary Infections: Damaged tissue can become susceptible to bacterial and fungal 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 they get in my tank?

A small study conducted in the Great Barrier Reef found AEFW occurring in 7 out of 10 Acropora colonies sampled with 1-5 AEFW per colony. This implies that AEFW are more abundant than we would like to think.

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

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

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. At around 27°C, their eggs hatch in just 11 days, and they reach adulthood in about 35 days. This means a new generation can appear every 38 days—which is pretty fast! Warmer temperatures speed up their reproduction, leading to more severe infestations. Even worse, newly hatched flatworms can swim and survive for a few days without a host coral, giving them time to spread to other parts of your tank.

Treatment Methods

As there is no one treatment option guaranteed to eradicate AEFW infestation, multiple approached should be used simultaneously.

1. Manual Removal & Dipping Treatments

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

• Coral Dips: Commercial dips containing potassium chloride, iodine-based solutions, CoralRX, Revive, or proprietary blends like Bayer Advanced Complete Insect Killer are effective in dislodging flatworms.
• Dipping Procedure:

1. Prepare a dip solution following manufacturer guidelines.
2. Submerge the infected coral in the solution for 5–10 minutes, agitating gently to dislodge flatworms.
3. Rinse with clean saltwater before returning the coral to the tank.

Limitation: While dips remove adult flatworms, they do not kill eggs, requiring repeated treatments. 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. We therefore recommend to also proceed with method 2.

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 proven to eat them are:

• Sixline Wrasse (Pseudocheilinus hexataenia) – These colorful little fish love snacking on flatworms. In controlled experiments, they wiped out 100% of adult flatworms.
• 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 (Mandarin Dragonet) can also contribute to controlling populations.

Limitation: Predator effectiveness varies, and they may not completely eradicate AEFW.

3. Targeted 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.
• Hydrogen Peroxide Baths: Some reefers report success using a mild hydrogen peroxide dip to weaken and remove AEFW.

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 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.

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