Turbulence as an Environmental Determinant of Relative Growth in Daphnia

Specific Turbulence- and Chaoborus-Induced Morphotypes Affect the Streamlining Properties of Daphnia cucullata

Inducible defenses are a wide-spread defensive mechanism in Daphnia. For example, D. cucullata is known to form different adaptive morphologies under changing environmental conditions. In this species, predator presence elicits defensive helmets. Defended animals perform better and survive more frequently in predation attempts. Another peculiarity of D. cucullata is that they develop these defenses also in response to turbulence. The mechanisms that underly this defensive effect are so far unclear. Therefore, we investigated the three-dimensional morphology of the typical, the turbulence- and Chaoborus-exposed morph and analyzed their surface and volume alterations. We furthermore measured the swimming velocities of these morphs. With the determined shapes and the measured velocities, we simulated the drag force under different angles of attack and determined the energy necessary to move through the water. In the light of previous findings and hypotheses we here discuss biomechanical mechanisms that improve D. cucullata’s fitness in the investigated environments.

Predation risk and the structure of freshwater zooplankton communities

Many predators inflict substantial mortality on their prey. The prey respond to these selective pressures with changes in their spatial and temporal overlap with the predator (density risk responses), or with changes in their vulnerability to the predator (prey vulnerability responses). Here we develop a simple predation model that permits quantification of the basic response types of the prey in nature. We then test the hypothesis that prey response will be proportional to the intensity of the predation mortality relative to all other sources of mortality and decreased natality acting on the prey. A significant regression relationship is obtained for the prey vulnerability response but not for any of the density risk responses. The individual response values and regression statistics are used to interpret the relative importance of the different response types and to assess the predator's influence on prey community structure.

STUDIES ON THE EVOLUTION OF THE OPHRYOSCOLECIDAE (CILIATA: OLIGOTRICHA): I. A NEW SPECIES OF ENTODINIUM WITH "CAUDATUM," "LOBOSO-SPINOSUM," AND "DUBARDI" FORMS, AND SOME EVOLUTIONARY TRENDS IN THE GENUS ENTODINIUM

A new species of Entodinium—E. simulans—with an intraspecific variability similar to that of E. caudatum, is described. A parallel variability in E. rectangulatum is reported. It is shown that sets of the forms of the "caudatum," "loboso-spinosum," and "dubardi" types are produced by each of these three species of Entodinium. It is demonstrated that within the "caudatum–laterale" group of Entodinium there exists an evolutionary trend to the transformation of the left lateral groove into a narrow furrow, situated between the nuclear apparatus and the contractile vacuole.