Influence of spatial heterogeneity on predation by the flatworm Mesostoma ehrenbergii (Focke) on calanoid and cyclopoid copepods

Through laboratory experiments, we analysed the influence of spatial heterogeneity on predation by Mesostoma ehrenbergii on the calanoid Boeckella gracilis and the cyclopoid Acanthocyclops robustus in four horizontal and two vertical spatial arrangements. This spatial heterogeneity simulated that of Juncaceae stems, a major macrophyte in the natural environment of these zooplankton. Our results indicated that M. ehrenbergii preyed differently on these copepod species. The rate of predation of M. ehrenbergii on A. robustus females reached saturation in all the treatments with and without horizontal spatial heterogeneity, but lower predation rates were observed in the medium heterogeneity treatment. Predation rates of Mesostoma on B. gracilis increased with the increase in prey abundance in the treatment without heterogeneity, while predation rates reached saturation in the treatments with horizontal spatial heterogeneity. Mesostoma ehrenbergii consumed males and females of B. gracilis in each of the arrangements tested. In natural habitats, interaction between extent of macrophyte development and intensity of predation by M. ehrenbergii on copepods species may be expected. We suggest that the structural complexity given by macrophytes in Patagonian fishless habitats provide a bottleneck for M. ehrenbergii predation.