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 down-ward direction
to prevailing in the upward direction. A cellular density of 1.036
g cm-3 was
determined mea-suring 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 ( 1.002
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
vertical locomotion in gliding and swimming cells.