GRAVIRESPONSES IN PARAMECIUM CAUDATUM AND DIDINIUM NASUTUM EXAMINED UNDER VARIED HYPERGRAVITY CONDITIONS

The swimming behaviours of two species of ciliates characterized by different mechanosensory and ciliary motor properties were investigated under hypergravity up to 5.4 g. The experiments were designed to examine large numbers of cells using video recording, digital data processing and statistics for the documentation of the rates and orientations of swimming. The gravikinetic responses (change in active swimming rates) were calculated from (1) the velocities of vertical swimming in the gravity field, (2) sedimentation of Ni2+-immobilized cells and (3) the intrinsic rate of propulsion, independent of gravity. Propulsion was determined from the intersection of regression lines of the gravity-dependent upward and downward swimming velocities. The rates of swimming and sedimentation, and consequently the gravikineses, were linear functions of gravitational acceleration. Comparisons of cell populations from different cultures suggest that there is an age-dependent change in gravikinesis. In starved Paramecium caudatum (7-day cultures), the kinetic responses antagonizing sedimentation (negative gravikinesis) increased with acceleration. In Didinium nasutum, negative gravikinesis was documented at 1 g in downward-swimming specimens only, which agrees with the mechanosensory organization of this cell. Hypergravity induced the gravikinesis of Didinium to change sign. In both species, and at all accelerations tested, a neutral gravitaxis was documented. Such behaviour incorporates distinct acceleration-dependent orientational and velocity responses, keeping populations of cells stationary in the gravity field (taxis coefficients close to zero).