Biology - Nutrition and Digestion

Department of Natural Resources and the Environment

College of Life Sciences and Agriculture

University of New Hampshire



Polyclads are carnivores. Favorite prey items include bivalve molluscs such as oysters, clams, and mussels. But polycalds cannot just swallow entire mussels or clams. Instead, they slip between the valves of the oyster and once inside, push out their pharynx (feeding tube). Because of this, they are considered pests by oyster farmers worldwide. The way in which they enter their prey is still not known. Some scientists think that the worms just slip in when the valves are slightly gaping, whereas others speculate that the worms produce some toxins that act as either anaesthetics or irritants.

two flatworms in an oyster shell
Two specimens of Imogine mcgrathi snacking on an oyster.

On coral reefs, polyclads like to feed on sedentary colonial animals such as ascidians. Worms can glide over the surface of the colony and suck out individuals one at a time, or many all at once. For example, the pharynx of Pseudoceros is branched into sections and so the worm can feed on many ascidian individuals at once.

flatworm with extended branched pharynx flatworm with pharynx in an ascidian
Pseudoceros bifurcus extending its branched pharynx and feeding on ascidians.
Note the everted pharynx of a Pseudobiceros sp. within the housing of an ascidian. Photo courtesy of M. Neubig©


Once food enters the pharynx it moves into a highly branched cavity. In fact, polyclads get their name from this branched digestive system ("poly" = many; "clade" = branches). These branches reach throughout the body to deliver digested food to the remainder of the animal. Most flatworms do not have an anus, so what goes in the mouth and cannot be digested and pass on to the gut cells, is expelled again via the mouth.

schematic diagram of polyclad digestive system flatworm showing highl branched digestive system through body wall
  Aquaplana sp. showing multibranched intestine through the body wall. Photo by RF Bolland©, used with permission.

Digestion is partially extracellular and partially intracellular. Nutrients can diffuse into the body and may be moved around by cells in the parenchyma. Polyclads, like all flatworms, have no circulatory or respiratory systems. So, transport within their bodies is via diffusion. Food reserves are stored as fats and starving polyclads will use up these reserves. If starvation continues, the worms gradually regress in size because they are mobilizing their own tissues for food. Polyclads can survive up to 6 months without feeding.

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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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