Gravitaxis of Bursaria truncatella: Electrophysiological and behavioural analyses of a large ciliate cell

Bursaria truncatella is a giant ciliate. Its volume of 3×10⁷ μm³ and a sedimentation rate of 923 μm s^?1 would induce the cell to rapidly sink to the bottom of a pond unless compensating mechanisms exist. The upward swimming behaviour of a cell population (negative gravitaxis) may be either a result of reorientations of the cells (graviorientation) and/or direction-dependent changes in propulsion rate (gravikinesis). The special statocyst hypothesis assumes a stimulation of mechanosensitive ion channels by forces of the cytoplasmic mass acting on the lower membrane. Here, we present basic electrophysiological data on B. truncatella. Investigation of the mechanosensitivity reveals a polar distribution of depolarising and hyperpolarising mechanosensitive channels at least on the dorsal membrane of the cell. Analysis of swimming behaviour demonstrates that Bursaria orients against the gravity vector (r_Oc=0.34) and performs a negative gravikinesis (?633 μm s^?1) compensating the sedimentation rate by 70%. Under hypergravity conditions gravitaxis in Bursaria is enhanced. Microgravity experiments indicate an incomplete relaxation of graviresponses during 4 s of weightlessness. Experimental data are in accordance with the special statocyst hypothesis of graviperception, as was demonstrated in other ciliates.