The microcosm is a laboratory method frequently used in ecological studies related to population and food web interactions and environmental dynamics. It simultaneously brings into interaction different species in the same controlled laboratory experimental area and provides an opportunity for modeling and reconstruction of relationships in the natural biocenoses and ecosystems. We applied that approach to determine and improve our understanding of predator–prey interactions in different freshwater environments. The inhabitants of the microcosms were isolated endosymbiotic microalga Desmodesmus subspicatus (Chlorophyceae) (Chodat) Hegewald et Schmidt (CZ), green hydras, freshwater turbellarians, and large water fleas. Experiments were performed in five replicates, at 25 °C and 13.5 °C, with fed and hungry predators, respectively. Herein, we proposed a mechanism for microalgal hunting net formation in the freshwater microcosm. Ultrastructural visualization of the endosymbiotic microalgae revealed rod-like structures on the cell wall surface, structures that could possibly fit together and interconnect, suggesting the possibility of microalgal hunting net formation. Interspecific cooperation between isolated microalgae and turbellarians resulted in stronger hunting net formation in preying upon water fleas. This study contributes to the diversity of species interactions and shows the producers as a top link, as opposed to what is generally considered as a basic link in the food web, and presents the microalgae as triggers of the dynamics in the freshwater microcosm.

The freshwater water flea (Daphnia magna Straus, 1820) is prey for numerous predators. Yet it possesses a wide range of strategies to defend itself against predation. The aim of this work is to investigate the defensive mechanisms employed by D. magna to reduce predation by the coelenterate Hydra viridissima, and two planarians, Polycelis felina and Dugesia gonocephala. To do this, we used a freshwater microcosm. An additional aim is to investigate interactions with the presence of the isolated endosymbiotic algae from green hydra, thus combining and observing the interaction of the zooplankton and microalgal component. Each experiment included five replicates (13.5 °C, 25 °C), in crystallizing glass containers (60 mL volume, 60 mm diameter, 35 mm height), including satiated (fed with larvae of Artemia salina) and starved predators, respectively (one or five individuals of a particular predator species in one microcosm). As the isolated microalgae are unique, we tracked the following three mechanisms of Daphnia defense for the first time including precisely this microalgal component: (i) grouping (visual magnification), i.e., two or more Daphnia holding together; (ii) the phenomenon of overproduction, i.e., any number of Daphnia in one container above the 10 initially added individuals; and (iii) accelerated movement (“bullet movement”), i.e., high-speed movements in particular microcosms. The results provide new information for a better understanding of the interspecific relationships in systems that include both zooplankton and microalgal components.

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.