Triangular-headed planarian
Dugesia gonocephala
Dugesia gonocephala is a widespread flatworm that prefers living under stones in cool, fast-flowing streams. The species is known for its characteristic triangular head with two clearly visible eyes and lateral auricles. It is a predator feeding on small invertebrates and plays an important role in the food web of running waters. Due to its sensitivity to pollution, it serves as a bioindicator for high water quality.
Details
Identification
Distinctly triangular head, two eyes with pale halos, usually brownish to olive-green coloration, flattened body shape.
Social behavior
Mostly solitary, but forms resting aggregations under stones for protection against current and light.
Diet
Feeds on small crustaceans (e.g., amphipods), insect larvae, small snails, and carrion.
Hunting strategy
Active tracking of prey using chemoreceptors; prey is fixed with mucus and digested via the protrusible pharynx.
Spawning substrate
Stalked egg capsules (cocoons) are attached to the underside of stones or aquatic plants.
Overwintering
Overwintering as an adult or in the egg stage within the sediment of the stream bed.
Ecology
Ecological role
Important predator in the benthic food web, regulating populations of small invertebrates.
Natural predators
Fish (e.g., trout, sculpins), larger predatory insect larvae (dragonfly larvae, caddisfly larvae).
Competitor species
Other planarian species such as Polycelis felina or Crenobia alpina.
Ecosystem service
Contributes to the biological self-purification of water bodies by consuming carrion.
Threats
Water pollution, eutrophication, thermal stress, and physical destruction of stream habitats.
Scientific profile
Profile
Distinguishing features
Characteristic is the distinctly triangular head with two laterally protruding auricles (sensory lobes). The head features two dark pigment-cup eyes, each situated within a pale, pigment-free patch. The eyes are positioned relatively close together compared to other Dugesia species.
Diet
Small oligochaetes, chironomid larvae, small crustaceans (e.g., gammarids), and fresh carrion.
Reproduction
Hermaphroditic. Reproduction occurs sexually through the deposition of stalked cocoons (egg capsules) onto the substrate or asexually through transverse fission followed by regeneration.
Role in food web
Secondary consumer; plays a role in regulating benthic insect populations and serves as prey for larger predators.
Protection & threats
Status not on standard scale
Main threats
River engineering, eutrophication, thermal stress due to climate change (warming of streams), and pesticide input from agriculture.
Conservation measures
Preservation and restoration of near-natural stream structures, protection of riparian vegetation for shading (temperature control), and reduction of diffuse nutrient/pollutant inputs.