Flatworm

Scientific Name: Turbellaria

Number of Species/distribution
There are about 3,000 known species. They can be found living in North America and Europe.

Diet/Feeding
Flatworms can be carnivors, scavengers or herbivours.
  • Carnivorous predators and scavengers feed predominantly on bacteria, protozoans, other small invertebrates, and other available animal matter.
  • Some have special mouth structures that enable them to swallow their prey whole, in pieces, or just to suck the body fluids out.

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Flatworm

  • Some species switch from herbivory to carnivory as they mature.
  • Most food is partially digested and reduced to a soupy consistency prior to swallowing.
  • A few species are herbivorous (eating only plant matter), feeding on microalgae.
Habitat
Most freshwater flatworms are free-living and can be found in ponds, lakes, streams, ditches, and temporary puddles.
  • They live under rocks, plants, and debris to avoid direct sunlight. 
  • They can be found on hard and soft substrates, but are more common on the hard surfaces.
Life Cycle
Flatworms are hermaphroditic (having both male and female sex organs) and they typically reproduce both sexually and asexually.
  • The majority of sexual reproduction is through cross-fertilization (where both individuals fertilize each other).
  • The fertilized eggs are frequently stored for a period of time within the flatworm, and are either retained in the parent or laid as egg masses.
  • Some freshwater flatworms produce special overwintering eggs, which are retained within the parent until spring.
Asexual reproduction occurs under environmentally stressful conditions or when mating partners are scarce. This usually occurs by one of two methods;
  • The first method is through budding; the buds grow out along the length of the parent's body, forming chains until they are ready to separate into new individuals.
  • The second method is called transverse fission; the posterior half of the worm attaches itself to the substrate while the anterior half continues to move forward until the two halves pull apart. Each half regenerates to form a complete worm. A few species can actually fragment themselves into several pieces, each of which regenerates to form several small worms.

The flatworm's lifespan is uncertain, but in captivity members of one species lived from 65-140 days.

Role in Food Chain
Flatworms may regulate population dynamics of zooplankton in ponds.
  • They are also consumers of protozoans, rotifers, and algae, and help to regulate populations of these organisms.
Interesting Facts
  • If a flatworm is cut through the middle of the body (as it is in normal transverse fission), each half will regenerate the corresponding lost part. However, if the animal is cut transversely near one end, say, separating a small piece of the tail from the rest of the body, the larger piece will grow a new posterior end, but the piece of tail lacks the capability to produce an entire new anterior end.
  • Flatworms usually have one pair of direct or inverted cup eyes for detecting movement.
  • Scientists can train flatworms to do simple tasks, such as finding water in a maze. When these flatworms are then cut in half, the new halves learn faster than the original.
  • Small flatworms use waves of ciliary action for locomotion to glide over surfaces, whereas larger species use muscular movements of their entire body to creep, swim, twist or somersault along the substrate.
  • If a flatworm is starved it is capable of shrinking to hatching size and when fed it has the ability to grow back to its original size.

Sources
Animal Life Resource (2010). Turbellarians. Available here.
Brusca, R. C, and Brusca, G. J., 2003. Invertebrates, 2nd ed. Sinauer Associates, Inc., Publishers, Massachusetts. Pages 295, 296, 303 and 305.
Canada's Aquatic Environments (2002). Turbellaria. Available here.
Miami University (2000). Platyhelminthes and Parasitism. Available here.
Soil and Water Conservation Society of Metro Halifax (2006). Class Turbellaria. Available here
Susan A. Bergeron (2007). Class Turbellaria (flatworms). Available here.
The Marine Life Information Network (2010). Class Turbellaria - flatworms. Available here
Waterwatch South Australia (2004). Sponges, Hyrdas and Worms. Available here.

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