Flatworms — Hitchhiker IDs

Small transparent acoels; including Amphiscolops-like forms

Many free-living forms cause little trouble; clear colour does not establish a safe identification.

Quick ID

A very thin, transparent or whitish animal gliding smoothly over glass or rock. Internal contents may be visible; use a macro photograph to inspect its outline.

Typical size

Often only a few millimetres long. Size and colour overlap with other flatworms.

Usually found

Aquarium glass, rock, algae and sheltered surfaces, sometimes near concentrations of small pods.

Commonly confused with

Pale coral-eating flatworms, juvenile polyclads and tiny slugs. The absence of brown colour does not rule out a pest.

Worth knowing

Many feed on small organisms rather than coral tissue. Acoels are commonly called flatworms in reefkeeping but belong to a different lineage from true polyclad flatworms.

What should I do?

Observe a small stable population and obtain an ID. If animals sit on damaged coral, isolate the coral for closer inspection rather than assuming they are harmless.

Research sources

• Original aquarium observations and acoel identification
• Melev’s Reef: clear and red aquarium flatworms

Convolutriloba spp.; hobby name “red planaria”

Large populations can cover surfaces, and mass die-offs can harm livestock.

Quick ID

Small reddish or rust-brown flattened bodies gliding on surfaces. Convolutriloba typically has three lobes at the rear; magnification helps.

Typical size

Usually a few millimetres; the photographed C. retrogemma is approximately 3 mm long.

Usually found

Light-exposed rock, substrate and glass; dense populations may also spread over corals.

Commonly confused with

Waminoa, other brown acoels and detritus. Red, rust and brown planaria are overlapping hobby names, not three separate diagnoses.

Worth knowing

These are not the same as Acropora-eating flatworms. Asexual reproduction can build numbers quickly; released toxins are a particular concern when many die together.

What should I do?

Siphon accessible animals and inspect new arrivals in quarantine. For a heavy infestation, plan management carefully: avoid an uncontrolled mass die-off or unverified chemical dosing.

Research sources

• Experimental aquarium acoel study

Waminoa spp.; coral-associated acoels

Heavy coverage can interfere with coral feeding and light; they are distinct from tissue-eating coral flatworms.

Quick ID

Small tan or brown flattened discs resting on living coral. The outline is often rounded or notched, rather than the three-tailed Convolutriloba shape.

Typical size

Typically millimetre scale; a close photograph is more useful than colour alone.

Usually found

Living coral surfaces, including bubble and other fleshy corals. Look for individual discs moving across the tissue.

Commonly confused with

Red planaria, coral pigmentation and tissue-eating flatworms. Host location alone cannot confirm the genus.

Worth knowing

Waminoa can consume coral mucus and intercept food. Research documents reduced host feeding efficiency; this is different from directly consuming coral tissue.

What should I do?

Monitor coverage and coral response. Isolate affected new coral and seek a coral-appropriate removal plan; avoid scraping living tissue or treating the entire tank casually.

Research sources

• Research: Waminoa and host feeding
• Acoel identification study

Polycladida; leopard-type and other larger flatworms

Some species prey on snails or bivalves; a spotted pattern does not establish diet.

Quick ID

A broad, very thin leaf-like animal with a rippling edge, gliding beneath rock. Some have leopard-like spots; others are plain or differently patterned.

Typical size

From small juveniles to several centimetres long, depending on the species. They can stretch and fold dramatically.

Usually found

Live-rock undersides, crevices, empty shells and areas near molluscs, especially after dark.

Commonly confused with

Nudibranchs, acoels and harmless-looking mottled film. A flatworm lacks a slug’s typical raised body and obvious external appendages.

Worth knowing

Polyclads have varied diets. Some consume bivalves, while others specialise on different prey; do not label every colourful marine flatworm a clam eater.

What should I do?

Isolate an unidentified larger specimen for photographs. If molluscs are disappearing, inspect rock and clam surroundings at night and remove confirmed predators to a separate system.

Research sources

• Original study: polyclads associated with bivalves
• Review: polyclad mollusc predation