Freshwater flatworms are identified by their soft, elongate, and flattened bodies, with a triangular “head” formed by a slight restriction of the body and often 2 eyespots that give the animal a cross-eyed appearance. The body is not segmented and ranges in length from <1mm-30mm. Flatworms are hermaphroditic and can reproduce sexually or asexually. Because flatworms undergo no metamorphosis as they age, juveniles can only be distinguished from adults by their smaller size.
Freshwater flatworms are actually part of a larger phylum of flatworms called Platyhelminthes. Most of the flatworms in this phylum are internal parasites, but the freshwater flatworms are primarily predators and scavengers of various soft-bodied invertebrates. Species of freshwater flatworms have evolved to exist under a wide range of environmental conditions. Despite having species in almost all aquatic habitats, flatworms are most commonly found on the surfaces and undersides of rocks in slow moving water.
Most species of freshwater flatworm are at least moderately tolerant of polluted conditions. Tolerant species tend to live in areas where decaying organic matter causes the water to contain only a small amount of dissolved oxygen, which they absorb along the entire surface area of their bodies. In healthy ecosystems, flatworms usually exist in low to moderate numbers. If an aquatic macroinvertebrate community is dominated by flatworms, it is likely an indicator that the area is polluted by organic materials, such as soil and fertilizer.
Photo: A microscopic view of a flatworm (Turbellaria)
Source: BioMedia Associates
Voshell, Jr., J. Reese; illustrated by Amy Bartlett Wright. 2002. A Guide to Common Freshwater Invertebrates of North America. Blacksburg (VA): The McDonald & Woodward Publishing Company. 442 p