Hammer coral Euphyllia ancora with anchor-shaped tentacle tips

Not All Flatworms Are Coral Killers: How to Tell Red Planaria from Real Coral Pests

You spot a tiny flatworm on a coral in your tank and your stomach drops. Is it eating the coral? Do you need to pull everything and start dipping tonight?

Not necessarily. Reef tanks host several very different animals that all get called "flatworms" or "bugs." Some are harmless hitchhikers, some are a nuisance, and a few are true coral predators. Treating a harmless one can stress your coral for no reason, and missing a real predator can cost you a colony. Identify first, then decide how to treat.

1. Common "Red Planaria" (Convolutriloba)

Red planaria flatworm Convolutriloba retrogemma on aquarium glass, about 3 mm long
Convolutriloba retrogemma, the classic "red planaria," on aquarium glass (about 3 mm long). Photo: AquariaNR, CC BY 4.0.

These are the flatworms most reefers meet first: small, rust-red to brown, often with a notched, lobed tail. They gather on glass, rock, sand and coral surfaces in well-lit spots, and they can multiply quickly into a visible carpet.

Do they eat coral? Generally, no. They are not coral predators. The problem is numbers: a heavy population can coat corals, block light and irritate tissue, so corals may stay closed. It is a nuisance, often linked to excess nutrients, not an attack.

A related acoel flatworm you may see on LPS corals is Waminoa, a small round acoel flatworm that rides on the surface of bubble corals and other stony corals:

Small round Waminoa acoel flatworms covering a bubble coral
Waminoa acoel flatworms on a bubble coral (Plerogyra). They live on the coral's surface; heavy coverage can shade it. Photo: Samuel Chow, CC BY 2.0.

2. Euphyllia-Eating Flatworms (EEFW)

Torch coral Euphyllia glabrescens in a home reef aquarium
A torch coral (Euphyllia glabrescens). Torches, hammers and frogspawn are the EEFW's targets. Photo: Josuevg, CC BY-SA 3.0.

This one is a real coral predator. EEFW attack the Euphyllia family: hammers, torches and frogspawn. They live underneath the polyps, against the hard surface of the colony, and at the base of the heads, so most of the time you won't see them.

What they look like: pale beige to light brown, and often close to see-through. Compared with harmless aquarium flatworms they are larger (about 6 to 10 mm), narrower and noticeably thicker-bodied. Now and then one crawls out onto the glass.

What the damage looks like: the coral looks unwell for no clear reason. Tentacles may still come out, but the polyp stays short and close to the skeleton instead of fully extending. Over weeks the coral slowly declines, tissue pulls back from the skeleton, and heads can be lost one after another. The stressed tissue is also more open to infections such as brown jelly.

How to check: take the coral out and hold it in a clear container of tank water against a light-colored background. EEFW are easiest to spot when they move. Look at the base of each head and into the crevices of the skeleton, and shine a flashlight down into the polyps.

Important: where a worm sits, or whether you can see it, does not prove what it is. Judge by its color and shape and by what the coral is doing. If you find any flatworm on a hammer, torch or frogspawn and are not sure, inspect it closely and treat it as a possible EEFW until you know otherwise.

Why they are hard to beat: a dip stuns the adults, but some retreat into the polyp mouths, and a stunned worm can stay stuck to the coral. While the coral is still in the dip, blow it off with a turkey baster, then throw the dip water away. The eggs are the bigger problem: they are glued firmly to the skeleton and survive dipping, so they have to be found and removed by hand. One clean dip is never proof that a coral is clear. Repeating the dip weekly for at least 6 to 8 weeks, removing eggs and observing closely gives you the best chance.

3. Acropora-Eating Flatworms (AEFW)

Acropora-eating flatworm camouflaged on an Acropora branch, and the same flatworm on its own
Left: an Acropora-eating flatworm camouflaged on an Acropora branch. Right: the flatworm on its own (scale bar 1 mm). From Hume et al. 2014, Coral Reefs, CC BY 2.0.

AEFW (Prosthiostomum acroporae, described in earlier papers as Amakusaplana acroporae) are a serious SPS pest. They eat Acropora tissue, and at high numbers they can kill whole colonies. Researchers have found that they take on pigment from the coral they eat, which helps them blend in, and they keep their bodies flat against the branch. Unless the light hits them right, they are very hard to see.

Look for: "bite marks" where tissue is missing, patches of bare white skeleton, and clusters of eggs on the skeleton near the base of the colony, often where tissue has already died back.

What the research says about natural predators: in a 2020 study, a sixline wrasse ate every adult flatworm on infested Acropora fragments but none of the egg capsules, while the peppermint shrimp Lysmata vittata ate about 82% of adults and about 64% of egg capsules within 24 hours. Predators can help, but neither one is a complete fix on its own, especially against the eggs.

Our own AEFW routine (repeat dips, then daily observation) is in How TIA Handles Pest Control.

4. Coral-Eating Nudibranchs

Not flatworms at all, but just as important to recognize. Nudibranchs are sea slugs, and some species specialize in one kind of coral.

Montipora-eating nudibranchs Phestilla subodiosus and their egg masses on a damaged Montipora coral
Montipora-eating nudibranchs (Phestilla subodiosus, arrows) and egg masses (circles) on a Montipora. Note the bare white skeleton where tissue has been eaten. From Wang et al. 2020, ZooKeys, CC BY 4.0.
  • Montipora-eating nudibranchs (Phestilla subodiosus) are small, white and frilly, only a few millimeters long. Hobbyists reported them as a pest for about two decades before scientists formally named the species in 2020. They hide under the colony where it attaches to rock or a frag plug, and they breed fast. A Montipora losing tissue from the edges with nothing visible on top deserves a look underneath.
  • Zoanthid-eating nudibranchs are brown, frilly aeolids, up to about half an inch, that blend in with the zoa colony they are eating. Zoanthid polyps that stay closed or disappear one by one are the usual clue.

And here is the "identify first" lesson again: not every nudibranch is bad news. Berghia is sold on purpose to reef keepers because it eats only Aiptasia, the pest anemone:

Berghia nudibranch laying a spiral of eggs on aquarium glass
Berghia stephanieae laying its spiral egg mass on aquarium glass. It eats Aiptasia anemones, not coral. Photo: Timwijgerde, CC BY-SA 4.0.

How to Protect Your Reef

  • Inspect every new coral: the top, the underside, the base and plug, and down into the polyps. Use a flashlight and a magnifying glass.
  • Dip new corals with a reef coral dip, following the product's directions.
  • Look for eggs. Dips remove exposed adults, but eggs survive. Check the skeleton and base for egg clusters.
  • Quarantine and observe when possible. Pests that survive a dip, or hatch afterward, show up during about a month of observation.
  • Identify the pest before you treat. Red planaria call for nutrient control and patience; EEFW, AEFW and coral-eating nudibranchs call for isolating the coral and repeated treatment.

The Takeaway

Seeing a flatworm does not automatically mean your coral is being eaten. Correct identification matters. The same goes for any honest coral seller: no one can promise a coral is 100% pest-free, because some pest stages are too small to see. What matters is a careful process and a reef keeper who knows what to look for.

Browse our Euphyllia corals, Acropora and Zoanthids, and read How TIA Handles Pest Control for our own quarantine routine.

Frequently Asked Questions

Do red planaria eat coral?
Generally, no. Red planaria (Convolutriloba) are not coral predators, but large populations can cover corals, block light and keep them closed. They are often linked to excess nutrients.

How can I tell red planaria from Euphyllia-eating flatworms?
Red planaria sit in the open on glass, rock and coral surfaces and multiply into visible groups. EEFW are larger (about 6 to 10 mm), narrower, thicker-bodied and pale beige to light brown. They hide underneath the polyps and at the base of the heads of hammers, torches and frogspawn, and the coral stops extending fully and slowly declines. Seeing a worm in the open does not rule out EEFW.

Does a coral dip kill flatworm eggs?
No. A dip removes exposed adults, but eggs survive. Repeat dips, check the skeleton and base for egg clusters, and keep observing the coral.

Do sixline wrasses eat Acropora-eating flatworms?
In a 2020 study, sixline wrasses ate all adult AEFW on infested fragments but none of the eggs. Peppermint shrimp (Lysmata vittata) ate most adults and about two-thirds of the egg capsules.

Are all nudibranchs pests?
No. Montipora-eating and zoanthid-eating nudibranchs are pests, but Berghia eats only Aiptasia anemones and is kept on purpose as pest control.

Sources

Photo credits: Featured image: hammer coral (Euphyllia ancora), Anilao, Philippines, by Diego Delso, Wikimedia Commons, CC BY-SA 4.0. Red planaria by AquariaNR, Wikimedia Commons, CC BY 4.0. Waminoa on Plerogyra by Samuel Chow, Wikimedia Commons, CC BY 2.0. Torch coral by Josuevg, Wikimedia Commons, CC BY-SA 3.0. AEFW panels from Hume, B.C.C. et al. 2014, Coral Reefs, doi:10.1007/s00338-013-1101-6, via Wikimedia Commons (left, right), CC BY 2.0; panels combined and enlarged. Montipora-eating nudibranchs from Wang, A. et al. 2020, ZooKeys, Figure 1, doi:10.3897/zookeys.909.35278, CC BY 4.0. Berghia stephanieae by Timwijgerde, Wikimedia Commons, CC BY-SA 4.0. Some images resized, cropped or combined.

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