Department of Natural Resources and the Environment

College of Life Sciences and Agriculture

University of New Hampshire


With the exception of a few species, polyclads are marine animals and mostly free-living, i.e., they are not parasitic. Most polyclads are found in the intertidal zone, on the crest of coral reefs, on coral reef slopes, on mangrove roots, as part of fouling communities on dock pilings, or in seagrass beds. However, they also have been collected at Pacific Ocean seamounts from depths of 2500m. A few species are pelagic, floating in the open ocean or hitiching a ride attached to Sargassum seaweed.

Species found in muddy and sandy habitats tend to be interstitial. Many benthic forms hide under rocks, in crevices, and under coral rubble. These species are cryptic, meaning that they hide from light and predators. It is mostly at night when they leave their protective environs to forage for food.

pink flatworm crawling over rocks
An acotyelan polyclad crawling along the sea bottom.

Flamboyantly colored species may be seen on the reef in the open even during day light. Their bold colors advertise either a real or perceived toxicity to potential predators thus, protecting them (see the biology section on Color and Mimicry).

colorful flatworm cralwing over sponges
Pseudobiceros gloriosus crawling over a sponge on the reef.

Although polyclads are not actually parasitic, they may form intimate associations with other invertebrates. For example, species of Apidioplana live attached to the branches of corals, and may be a problem for people keeping saltwater aquaria. Imogine zebra can be found living within shells occupied by hermit crabs. Living inside reproductive cavities of brittle stars and sea urchins are several species of Discoplana. There the worm may feed on the gonads and eggs of the echinoderm, and then lays its own egg capsules containing as many as 1000 young. This is probably the closest that polyclads come to parasitism.

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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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