This review briefly describes four physical and two physiological hypotheses that have been proposed on the mechanism of negative gravitaxis or geotaxis of Paramecium. In the hydrostatic hypothesis (Verworn, 1889), the density of the rear part of Paramecium is greater than that of the rest of the organism so that the body axis is tilted anterior upward (Fig. 1,A). Of the three hydrodynamic hypotheses, the sedimentation velocity hypothesis (Roberts, 1970) ascribes the tilting to a slower sedimentation velocity of the anterior part of the body caused by its smaller cross-sectional area as compared with that of the other parts (Fig. 1, B). The sedimentation gyration hypothesis or the propulsion-gravity hypothesis (Winet and Jahn, 1974) supposes inbalance between the counter torque to a downswing of the anterior part and the enhancing torque to an upswing in the gyration of the cell body (Fig. 1,C). In the sedimentation drag hypothesis or the lifting force hypothesis (Nowakowska and Grebecki, 1977), the angle of attack between the average direction of the body axis and the swimming direction, which is shifted by sinking, generates a force which lifts the anterior during non-vertical swimming of the organism (Fig. 1,D). The physiological models include the velocity hypothesis and the orientation hypothesis. The former predicts that the swimming velocity depends on the tilt angle of the body axis (Fig. 1,E). The latter assumes that the upward tilting is caused by differential acceleration of ciliary beat on the upper and the lower sides of the organism (Fig. 1,F). Discussion is made on a possible mechanism by which gravity affects the swimming behaviour of Paramecium indirectly through mechanoreception and membrane potential changes possibly mediated by vibration of the medium. A special attention is paid to an antagonism between the facilitatory effect of hyperpolarization and the inhibitory effect of depolarization on the gravitactic behaviour. The implications of gravitaxis in such biological activities as food collection are also discussed.

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