New Acropora-Eating Flatworm

What Is the New Acropora-Eating Flatworm?

The so-called new Acropora-eating flatworm is a much smaller and darker flatworm than the classic AEFW most reef keepers are familiar with. It has been found living and feeding on Acropora corals and can be surprisingly difficult to spot because of its tiny size and fast movement. Adults are only a few millimetres long, with a dark body and a pointed rear end that can make them look almost like a tiny black tadpole or slug moving across the coral. Unlike the more familiar AEFW, these worms may spend time deep within the coral structure and around the corallites, which can make visual detection even harder. They appear to have a strong preference for Acropora, and infected colonies can develop rapid tissue damage and decline if the infestation is allowed to build. One of the biggest challenges with this pest is that we still know far less about its biology than we do about traditional AEFW. Its exact taxonomy, full life cycle and the most reliable treatment regime are still not well established. That means successful management depends heavily on careful observation, early detection and a disciplined treatment approach, rather than relying on one proven protocol.

Signs and Damage

The early warning signs can look very similar to those seen with classic Acropora-eating flatworms.

You may notice a loss of lustre, reduced polyp extension, dulling or darkening of the colony, and small areas of tissue damage that are difficult to explain.

The major difference is often how quickly things can progress.

Once the population increases, affected Acropora can deteriorate rapidly. Small areas of tissue loss may spread quickly, exposed white skeleton can begin to appear, and a colony that looked reasonably healthy only a short time earlier can decline dramatically.

Because these worms are so small and can hide deep within the coral structure, the amount of visible damage may initially seem out of proportion to the number of pests you can actually find.

In a heavy infestation, experienced reef keepers have reported very rapid tissue loss - potentially severe enough for individual colonies to collapse over a very short period.

For that reason, unexplained Acropora decline that appears to be accelerating should be investigated quickly, particularly when the usual causes such as water chemistry, lighting and flow do not explain what you are seeing.

Confirming the Pest

Because these flatworms are so small and fast-moving, confirmation can take a little patience.

Start with a close inspection of the affected Acropora, ideally using a jeweller’s loupe or magnifying lens. Look carefully around damaged corallites, exposed skeleton, branch junctions and shaded areas.

A night-time inspection can also be very useful. Check the colony with a torch before the main aquarium lights come on, as the worms may be more visible while they are active on the coral surface.

If you still suspect an infestation, gently baste the colony with a turkey baster or similar tool. Dislodged worms may become visible in the water column, where their small, dark, tadpole-like shape and rapid movement can be easier to recognise.

A close inspection after dipping can also help confirm the diagnosis. Examine both the coral and the dip water carefully for small dark worms and scattered eggs.

Immediate Containment

As soon as you suspect it, treat it as urgent. Don’t remove anything from the tank, don’t move corals or water to other systems, and pause new additions until you’ve got a plan. Basically, act within days, not weeks.

• Do not transfer coral, water, racks or wet tools to another system.
• Inspect every Acropora sharing the same water.
• Keep affected coral removable and accessible.
• Use magnification and night inspection rather than relying on a quick daytime glance.

Alternative Treatment Evidence

Published treatment information specifically for this newer pest remains extremely limited. Classic-AEFW data can therefore only be treated as a possible starting point, not proof of efficacy.

Full Reset / Quarantine Route

• Remove all affected Acropora from the display where practical.
• Cut away old bases and unnecessary encrusted or dead substrate.
• Keep fragments on egg-crate or racks so the whole coral remains inspectable.
• Reduce light while the coral is under repeated treatment, then restore it gradually.
• Keep alkalinity, calcium, magnesium, salinity and temperature stable and monitor consumption as stressed coral may use less.
• Discard badly infested branches where that is the safer route than repeatedly carrying protected pest habitat forward.

Clearance and Recovery

Clearance should mean no detectable worms after repeated inspection, basting and dipping, no fresh eggs and no new feeding damage.

Keep the Acropora removable and observe for another three weeks. Continue close inspection and occasional basting. If the observation window stays clean, perform one final confirmation dip before remounting and restore lighting slowly.

Signs the Treatment Is Working

Look for better coloration, stronger polyp extension, and a gradual normalization of major element consumption as growth stabilizes. With this pest, the absence of new rapid tissue loss and a continued reduction in worms and eggs are also important early signs of progress.