Who Ate Whom—Competition and Predation in a Freshwater Microcosm

Rapid environmental change is reshaping freshwater ecosystems, influencing food availability and predator–prey dynamics. This study examined interactions among four freshwater invertebrates—the cnidarian Hydra viridissima (HV), the turbellarians Polycelis felina (PF) and Dugesia gonocephala (DG), and the cladoceran Daphnia magna (DM)—under controlled microcosm conditions. We investigated the effects of temperature, light regime, and predator satiation on predation intensity, prey survival, and interspecific behavior during the 24 h period. DM served as a universal prey, with survival strongly affected by both temperature and predator feeding state. Predation was generally higher at 25 °C and among hungry individuals. HV proved to be the most efficient predator and competitor, whereas DG dominated among planarians by preying on PF and adopting its dark pigmentation—a potential camouflage strategy enabling mimicry of both prey and habitat. PF responded by forming defensive groups, highlighting species-specific behavioral adaptations. PF simultaneously exhibited traits of both predators and prey. These findings demonstrate that microcosm experiments can reproducibly capture natural freshwater interaction patterns. Moreover, this study provides the first evidence of a planarian predator exhibiting both prey mimicry and environmental camouflage, revealing a novel behavioral strategy in flatworm ecology.