Two typical elementary behaviors of Paramecium are usually reported in the literature: the avoiding reaction and the escape reaction. In the avoiding reaction, Paramecium touches an obstacle with its anterior end, or enters a repelling medium, and sensory transduction triggers an action potential that in turn triggers a reversal of ciliary beating. As a result, Paramecium swims backward for a short while, then turns around and swims forward again. In the escape reaction, something touches the posterior end, and Paramecium swims forward faster. This is mediated by mechanotransduction to a hyperpolarizing current, followed by calcium entry.

Here the authors show two new behaviors, triggered by the heat of a laser. When the heat is low, Paramecium shows the escape reaction. When it is a bit higher, the escape reaction is transiently accelerated. To understand this, one needs to know that Paramecium swims by coordinated beating of its cilia, forming a so-called “metachronal wave” progressing over the cell (coordination mediated by hydrodynamic interactions). This ensures highly efficient swimming with a continuous flow. Here the cilia beat synchronously in response to the heat stimulus, which produces a strong transient push, and then a pause (as in butterfly stroke). Thus, the average speed is not enhanced but the organism can react very quickly.

Then when the heat is strong (5-10 °C increase), something quite spectacular happens. Paramecium literally throws little spears calls trichocysts, which are stored near its membrane, in the direction of the stimulus. This makes Paramecium “jump” very quickly (5 ms) sideways, at about 10 mm/s, something that would be impossible with cilia beating. Trichocyst exocytosis is also controlled by calcium.