Paramecium is a common freshwater ciliate, a unicellular swimming eukaryote that feeds on bacteria. Since the discovery that its motility is controlled by action potentials, which earned it the nickname of "swimming neuron", many studies have documented the effect of various stimuli on its behavior, as well as their physiological basis, highlighting the richness of adaptive behavior in this unicellular organism. The logic of its autonomous behavior has received comparably less attention. For example, when Paramecium encounters a surface, it may slide or avoid it by a directional change, triggered by an action potential, or it may immobilize on the surface, a behavior often called thigmotaxis. Here we ask why and how Paramecium immobilizes on surfaces. With long term behavioral tracking, we show that this variable behavior is a component of Paramecium's foraging behavior in the presence of bacteria, where the organism alternates between exploring and stopping on surfaces to feed. This immobilization is physiologically controlled by a calcium influx. Using mutant strains, high speed imaging, particle image velocimetry and hydrodynamics simulation, we further show that surface immobilization occurs by an inhibition of locomotor cilia, while oral cilia continue beating strongly. These findings open the perspective of studying the physiological control of foraging in a unicellular organism, a complex ecologically relevant behavior potentially involving sensory processing, memory, decision making, and motor control.
Hosseini, A., Boyden, W., Fosse, C., Stimberg, M., Guy, R. D., Brette, R.
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