The three upper images above are of a relatively large (about one inch long) flat worm recently seen in a reef tank.  The original image is on the left, and I have added some labels to show structures on the image to the right.   The middle image is taken looking along the aquarium wall, and shows that these animals are almost two-dimensional.  Compare this image to the image of the "red planaria" Convolutriloba retrogemma below the other images.  The red planarian was about one eighth inch long and discussed in a bit more detail below.  Many different flatworms are found in our tanks, but seldom are they as obvious as the large white one illustrated above.  Most of these little worms are harmless or beneficial.  However, the red planarian may be a severe nuisance, and true coral-eating flatworms (see below) may be occasionally encountered.  The lower right image is a composite of three photos of a small white flatworm, which eats some of the brown flatworms found in many reef tanks.  It may also eat the red planarian, but this is uncertain, and experiments are being done to confirm this.

Contributed by Guy Comstock, this pair of images shows the feeding mode of a carnivorous flatworm


This flatworm is a predator on small crustaceans such as copepods which it envelops with the expanded front of the body, as shown in the upper image above. The enveloped prey are enclosed in the "bag" and eaten by the mouth, which is located in the middle of the animal.

Polyclad Flatworms

Polyclad flatworm are larger animals than those pictured above, and have a branched gut that is often visible inside the animal.  The name "polyclad" means "many branches and refers to the many visible branches off of the central part of the gut.  These worms may exceed several inches in length, although those seen in aquaria are not generally over a couple of inches long.  Polyclads are generally thought to be predatory, often on small worms, crustaceans, but also occasionally in aquaria, snails and clams.

Copyright by M. Taylor, these images show the top view (left) and bottom view (right) of a polyclad that was found in his aquaria.  Click on the images to make them full sized to see the gut in the center of the animal.

Red Planaria = Convolutriloba retrogemma

One of the more common pest animals in reef aquaria are small flatworms, often referred to as "red planaria," which is a truly excellent aquarium name for them as they are neither red nor planarians.   These small acoel worms, (all "true" flatworms including planarians belong to a different group) have a brownish, tank, or pink coloration.  They also contain zooxanthellae whose own color modifies the animals color in sometimes peculiar ways.  They are small from about 1 mm to 3 mm long.  One good identifying characteristic is the presence of three lobes at the tail end of the animal.

The red planarian, Convolutriloba retrogemma.

These little worms can rapidly reproduce asexually and reach huge populations in some tanks.  They can literally smother some sessile animals.  In small numbers, however, they are harmless.  Presently there is no real defined cure for infestations with the possible exception of one species of head-shield slugs, Chelidonura varians , which eats them.

Coral-Eating Flatworms

There are large number of flatworms that are found in reef aquaria.  Most of these are harmless or, at worst, nuisances that do not directly harm most of the animals in the aquaria.  However, in nature, coral-eating flatworms abound and occasionally they find their way into our systems, probably entering on wild collected or captively-grown corals.

These images, contributed by Jim Nastulski, show some coral-eating flatworms that were found in his aquarium.


This image, contributed by Minh Nguyen, shows several polyclad flatworms on Alveopora gigas. These worms are likely ectoparasitic on the coral eating tissue or coral mucus.


Ron Shimek