Richard Ling (CC BY-SA 2.0)
Polyclad flatworms (clam / Euphyllia eaters)
Order Polycladida (families Pseudocerotidae and others; Euphyllia-eating and clam-eating species are rarely identified to species in the hobby)
Big nocturnal predators that can empty a Tridacna clam overnight, wipe out snail crews and steadily eat Euphyllia heads while hiding in rock all day.
Identification
Large, very flat, smooth worms with a ruffled, rippling margin and a pair of small pseudotentacles at the front. Size runs from about 1 cm for Euphyllia-eating forms to 8–14 cm for the big clam-eating and leopard-patterned species. Colors range from translucent white and mottled brown leopard patterns to vivid black, yellow and orange banded Pseudoceros. They hide in rock and under shells by day and roam at night. On Euphyllia, look for worms flattened against the flesh between polyp and skeleton and clusters of tiny grey eggs around the heads; on clams, look for a worm wrapped over the shell or mantle at night and sudden snail losses.
Often confused with
Harmless small planaria and red planaria — those are a few millimeters long and sit on glass and rock in the light; polyclads are far larger, mostly nocturnal and hide in rock by day. The colorful Pseudoceros species also get mistaken for nudibranchs, but they have a completely smooth, flat body with a ruffled margin and no gills or cerata.
What it targets
Depends on species group: clam- and snail-eating polyclads hollow out Tridacna clams and consume snails and other bivalves, while Euphyllia-eating polyclads feed on torch, hammer and frogspawn tissue and hide in the corallites. Other polyclads prey on ascidians, barnacles and assorted sessile invertebrates.
How it spreads
Hitchhikes in live rock, on clam shells and inside Euphyllia skeletons from wild and maricultured stock. Nocturnal and rock-dwelling, so a worm can hunt a tank for months while the keeper only sees the losses.
Control (safest first)
Manual removal is the core method: hunt at night with a flashlight, siphon or net worms off glass, rock and coral, and use a bait trap — a piece of fresh clam in a clip or bottle trap set after dark and checked before lights-on, a method one Reef2Reef keeper stumbled onto when a clam left out for a copperband was stripped overnight and the worm was found cleaning the shell in the morning. For Euphyllia-eating worms: lift every Euphyllia colony (treating one colony alone fails), inspect flesh and skeleton with a magnifier, scrape off the grey egg clusters, and dip weekly for about a month. Keeper reports favor Revive-type dips plus vigorous turkey-baster flushing to blast worms out of corallites; Flatworm Exit results are disputed, with some keepers reporting control from high-concentration dips and others reporting it barely affected their worms. Remove and inspect clams and suspect rock outside the tank; a worm hiding inside rock can be coaxed out by holding the rock in a container of tank water, and keepers also report flushing rock with soda water or salinity swings — harsh methods that kill other rock life and belong outside the display. Predators: no reliable reef-safe predator is established for large polyclads. Some wrasses pick at small flatworms, but do not buy fish expecting them to clear a clam-eating polyclad. Prevention: quarantine and dip new Euphyllia and clams, and inspect clam shells and byssal openings before adding them — by the time snails start disappearing, a worm is usually well established.
Natural predators
No natural predators are listed.
Field notes
A Reef2Reef keeper's hammer closed and nearly died; a dip shook loose flatworms, but the other Euphyllia were left undipped and a torch declined next with the same worms. Other keepers in the thread reported control after about a month of weekly dips for every Euphyllia plus scraping grey egg clusters found around the heads under magnification. Opinions split on chemistry in that same discussion: one keeper dosed Flatworm Exit dips and scraping and got there, while another tried all the standard dips and found Revive clearly the most effective, saying Flatworm Exit hardly affected the worms at all. A second Reef2Reef case shows the clam side of this pest: a hand-sized leopard polyclad in a 20-gallon tank ate every snail in the system and hid inside rock by day, leaving its keeper weighing night bottle traps baited with grocery-store clam against pulling and flushing the rockwork.