Evidence of central and peripheral gravireception in the ciliate Loxodes striatus

Effects of the density of the external medium on gravireception in Loxodes striatus were investigated using Percoll solutions. With increasing density, the swimming rates changed from prevailing in the downward direction to prevailing in the upward direction. A cellular density of 1.036 g cm−3 was determined measuring direction and speed of sedimenting immobilized cells at different accelerations and medium densities. Viscosity increases by Percoll were measured and taken into account. At 30% air saturation Loxodes maintained a negative gravikinesis of approximately −27 μm s−1 at external densities corresponding to cellular density (±0.02 g cm−3). Negative gravikinesis decreased gradually to −9 μm s−1 with the density difference rising from 0.020 to 0.036 g cm−3 (=normal). The data indicate the existence of central gravireception, presumably by the Müller organelle, to generate in swimming Loxodes a constant value of gravikinesis and a bimodal gravitaxis. Peripheral gravireception occurs, in addition to central gravireception, when the transmembrane density difference exceeds 0.02 g cm−3. Peripheral gravireception can neutralize, in part, gravikinesis as raised by the central gravireceptor. We hypothesize that both central and peripheral gravireception of Loxodes guide vertical locomotion in gliding and swimming cells.