Acropora-eating flatworm

DestructiveCapable of taking a whole genus, or a whole tank.

A flat, oval worm that eats Acropora tissue and nothing else. It is transparent and takes on the colour of the coral it is eating, which is why it is usually found after the damage rather than before — researchers describing it from the wild called it easy to overlook even when they were looking for it. Left alone it will strip an Acropora collection.

Make sure that's what it is

Do not look for the worm, look for the evidence: circular pale bite scars roughly a millimetre across, and oval egg clutches laid on bare skeleton at the base of branches. Then take the colony out and baste it hard with a turkey baster over a white container — they detach and become obvious against the white. A colony that is pale at the base and retracting with no water-chemistry explanation is worth basting.

What it looks like

No photos of Acropora-eating flatworm yet. If you have it in your tank, a clear shot here is what the next person will compare theirs against — it's the most useful thing you can add to this page.

Getting rid of it

In the order we'd try them. Every method cites where it comes from, and where the reason a treatment works isn't actually established, it says so.

Dip on a schedule, and scrape the eggs

By hand

Why it works

The single most important fact about treating this pest is that dips do not kill the eggs. The flatworm lays multiple egg batches directly on its Acropora host, and hatchlings emerge at around 250-300 micrometres — so a coral dipped once, declared clean and returned to the display is a coral that reseeds itself days later. That is why every successful protocol is a schedule rather than an event: repeated dips spaced to catch each hatch before it matures, combined with physically removing the clutches, which is the only step that addresses the eggs at all.

What to do

Remove the colony, dip it, and baste it hard while it is in the dip so detached worms are visible and can be discarded with the water. Scrape visible egg clutches off the bare skeleton at branch bases. Return it and repeat on schedule rather than on observation, because seeing nothing is the expected result between hatches. Treat every Acropora in the system, not just the one showing damage.

Careful: Never judge success by looking — this species is described in the literature as cryptic, coloured to match its host and easy to overlook even by researchers examining colonies deliberately. No product is linked here: the coral dips used are off-label chemistry with no aquarium dosing standard.

Source: Discovery of the Corallivorous Polyclad Flatworm, Amakusaplana acroporae, on the Great Barrier Reef (PLoS ONE, 2012)

Keeping it out

Quarantine and inspect every incoming Acropora, and treat the eggs as the real problem — a colony can look clean and carry a clutch.