Formation of Microalgal Hunting Nets in Freshwater Microcosm Food Web: Microscopic Evidence

Abstract

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.

How Daphnia magna Defends Itself against Predators: Mechanisms and Adaptations in a Freshwater Microcosm

“…In this research, we used well-known organisms from freshwater ecosystems: H. viridissima, P. felina, D. gonocephala, D. magna, and the isolated endosymbiotic microalga D. subspicatus. These and related species are used in microcosm research [27][28][29][30][31][32][33][34]. The results of our study point to the defense mechanisms and adaptation of D. magna in freshwater microcosms: grouping, overproduction, and accelerated movement.…”

“…A lower incidence of grouping was observed when the microalgae were present in the system along with predators, in comparison to other microalgae-free setups. It is possible that grouping is hindered by the aggregation of microalgae and the formation of the microalgal net, with water fleas trapped in the microalgal hunting net [30]. In conclusion, with the presence of predators in the microcosm, individuals of D. magna defended themselves by grouping in such a way that two or more living or dead individuals were grouped together.…”

“…In fish, individuals of Pacific salmon in larger groups have lower predation risk [42]. In addition, the microalgae form aggregations and nets to protect themselves from predation attacks [30]. One of the important drivers of grouping is the detection of predators, so exposure to the chemical cues released by fish and by invertebrate predators (and a substance released from homogenized Daphnia) induces a tendency to form aggregations in Daphnia [43].…”