Acropora-eating flatworms (AEFW)

Identification

Small flat oval worms about 2–5 mm (some guides say larger, up to roughly twice red-planaria size) that glide on Acropora tissue and take on the coral's color by ingesting its tissue and zooxanthellae, making them almost invisible in the tank. Practical signs come first: patchy bite marks where color and polyps are missing, loss of color and polyp extension, basal tissue recession, and clusters of reddish-brown eggs on bare skeleton at the colony base. A dip in a white bucket reveals worms that were invisible on the coral.

Often confused with

Red planaria (Convolutriloba) — those are rusty-red, live on rock, glass and sand rather than glued to Acropora tissue, and form visible mats. Also confused with red bugs (tiny moving specks best seen in macro photos, no bite-mark fields or egg clusters on skeleton) and with ordinary STN from water chemistry, which shows no eggs and no worms in a dip.

What it targets

Acropora spp. only, with smooth, short-polyp species reported as more susceptible; Acropora nana is repeatedly named as a favorite. Other corals are not eaten.

How it spreads

Hitchhikes on wild and maricultured Acropora colonies and on frags from infested systems, most often as eggs on the base or plug that survive dips. Spreads colony to colony in the display as adults move and lay.

Control (safest first)

Quarantine dips on a cycle: move affected Acropora to quarantine, dip in Revive or Lugol's-type solution for 5–10 minutes while basting with a turkey baster to blast worms loose (per Reefs.com pest guide), rinse in clean saltwater, and repeat every 4–7 days until dips come out clean. Scrape off every egg cluster you find — no dip kills the eggs. Bayer-type and CoralRx dips in quarantine are widely used the same way; freshly hatched worms starve quickly off the host, so a clean frag rack in quarantine plus repeat dipping breaks the cycle without treating the display. In-display basting: blasting colonies with a baster every few days knocks worms into the water column where wrasses eat them. Keepers who tried to manage AEFW this way long-term report it suppresses damage for a while but rarely eradicates, and colonies slowly decline. In-tank products marketed for AEFW (KZ Flatworm Stop, Fauna Marin AEFW X) are sold as reef-safe, multi-week treatments that block larval development and feeding rather than killing adults outright; keeper reviews are mixed and treatment runs 4–6 weeks minimum per Fauna Marin guidance. Flatworm Exit is designed for red planaria — experienced keepers warn it does not reliably kill AEFW, and mass flatworm die-off in a display releases toxins, so do not dose a display expecting an AEFW cure. Last resort in heavy infestations: frag healthy tips well above damage, discard infested bases and eggs, and hold frags through the dip cycle in quarantine while the display runs without Acropora.

Natural predators

• Melanurus wrasse→
• Six-line wrasse→
• Yellow coris wrasse
• Target / spotted mandarin

Field notes

A long Reef2Reef eradication thread includes a keeper urging frag backups before any treatment, arguing that basting and in-tank products can maintain AEFW for a while but the worms eventually take a toll and ruin the fun of the tank — and recommending dipping a frag rack in Bayer every five days in quarantine to break the life cycle while encrusted display colonies get epoxied over. A MARSH forum keeper first blamed post-hurricane stress for a colony's decline, then found AEFW eggs on inspection and seriously considered throwing the colony away rather than risk passing the pest on; replies corrected the plan to dose Flatworm Exit, noting that product targets red planaria and barely touches AEFW or their eggs. CoralRx's guide captures the shared experience behind most threads: the infestation is usually discovered from bite marks and brown eggs at the base long after arrival, because the worms themselves match the coral and stay hidden on undersides.