Biology - Color, Patterns and Mimicry

Department of Natural Resources and the Environment

College of Life Sciences and Agriculture

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Some polyclad flatworms may appear almost transparent and their organs can be clearly seen through the body wall.

transparent flatworm


An almost transparent species of Bulaceros.

More commonly though, their dorsal side is colored, and the ventral or belly side appears cream, gray, or even white in color. The dorsal colors are rather dependent on where the worms are found. In temperate waters, the colors mostly match the background color of the substrate, and so creamy to dark browns and grays, even blacks, are common.

mussel shell with flatworms
The acotylean polyclad, Pleioplana atomata, a common species in the Gulf of Maine, feeding on blue mussels.

Although there are some brown and gray colored worms in warm tropical waters, many polyclads exhibit a rainbow palette of vibrant colors and color patterns. Colors may range from red to blue to green to yellow, orange, pink, and purple. In addion, the colors may appear in patterns of bands, spots, stripes, blotches, and lines; and in many cases are even combined in a single species.

schematic drawing of different flatworm patterns
A schematic representation of different color patterns that can be found in polyclad turbellarians.

The following photographs will give you an indication about the wide variety of color and color patterns that can be found in polyclads.

Pseudoceros flatworm
Pseudoceros flatworm
Maritigrella flatworm
Pseudoceros bimarginatus
Pseudoceros confusus
Maritigrella fuscopunctata
Pseudoceros flatworm
Thysanozoon flatworm
Pseudoceros flatworm
Pseudoceros scintillatus
Thysanozoon sp.
Pseudoceros imitatus

The color in these worms can have several different sources. In some worms, the cells of the epidermis contain pigment granules, in others the granules may be located in deeper layers, such as the muscle layers or even deep within the animal itself. In some cases, the pigment granules are located within the gut and have actually been taken in when the worms were feeding upon a colored prey. Many tropical polyclads feed on colorful sponges and sea squirts (see section on the digestive system and nutrition). It is not unusual for the same species of worm to either be red when feeding on a red prey, green when feeding on a green sea squirt, or black when the prey is also black.

Why are some worms so brightly colored? From experiments, we know that fish will not eat polyclads. When offered a brightly colored worm, they may taste it but then immediately spit it out again. Research has shown that the tissues of some of these worms contain toxins similar to to the toxin found in fugu (puffer fish). It is highly likely that some of the rhabdoid granules located in the skin of the worms contain these toxins. Polyclads then use their flamboyant colors to warn off potential predators; they are advertising their toxicity. This is called aposematic coloration, and it is quite common among animals (e.g., poisonous snakes, insects).

Some animals have learned to take advantage of this advertising strategy without having to produce toxins themseleves. They copy or mimic a toxic species by flashing similar colors and color patterns at predators. You are probably familiar with the mimicry between the Viceroy and the distasteful Monarch butterfly, and between the poisonous Coral and the harmless Scarlet King snake.

viceroy butterfly
Monarch butterfly
Viceroy Butterfly
Monarch Butterfly
King snake
Coral snake
Scarlet King Snake
Coral Snake

The above butterfly and snake examples are known as Batesian mimicry. In this type, the mimic adopts the colors or shapes of a noxious or poisonous model. In another type of mimicry, Müllerian mimicry, both the model and the mimic may be harmless and edible, but they adopt colors or patterns commonly found in toxic animals.

flatworm and nudibranch of similar color pattern on reef
This is an example of Batesian mimicry between a palatable polyclad and a noxious sea slug.

Some polyclads exhibit a different kind of mimicry called cryptic mimicry. In this type, the worms adopt colors and patterns that help them blend into the background and thus, camouflage them against visual predators.

flatworm on similar colored background
A well camouflaged callioplanid polyclad is difficult to detect against its background.

These pages are for educational purposes only. If you use any of the information or the images in your classes, please credit these web pages. © MK Litvaitis, 2005

Photo credits to Leslie Newman, Andrew Flowers, or Anne DuPont, unnless otherwise indicated.


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