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

The swimming behaviours of two species of ciliates characterized by di fferent mechanosensory and ciliary motor properties were investigated under hypergravity up to 5.4g. The experiments were designed to examin e large numbers of cells using video recording, digital data processin g and statistics for the documentation of the rates and orientations o f swimming. The gravikinetic responses (change in active swimming rate s) were calculated from (1) the velocities of vertical swimming in the gravity field, (2) sedimentation of Ni2+-immobilized cells and (3) th e intrinsic rate of propulsion, independent of gravity. Propulsion was determined from the intersection of regression lines of the gravity-d ependent upward and downward swimming velocities. The rates of swimmin g and sedimentation, and consequently the gravikineses, were linear fu nctions 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 gravikinesi s was documented at Ig in downward-swimming specimens only, which agre es with the mechanosensory organization of this cell. Hypergravity ind uced the gravikinesis of Didinium to change sign. In both species, and at all accelerations tested, a neutral gravitaxis was documented. Suc h behaviour incorporates distinct acceleration-dependent orientational and velocity responses, keeping populations of cells stationary in th e gravity field (taxis coefficients close to zero).