  In unicellular flagellates, growing evidence suggests control over a complex
repertoire of swimming gaits is conferred intracellularly by ultrastructural
components, resulting in motion that depends on flagella number and
configuration. We report the discovery of a novel, tripartite motility in an
octoflagellate alga, comprising a forward gait ($run$), a fast knee-jerk
response with dramatic reversals in beat waveform ($shock$), and, remarkably,
long quiescent periods ($stop$) within which the flagella quiver. In a reaction
graph representation, transition probabilities show that gait switching is only
weakly reversible. Shocks occur spontaneously but are also triggered by direct
mechanical contact. In this primitive alga, the capability for a millisecond
stop-start switch from rest to full speed implicates an early evolution of
excitable signal transduction to and from peripheral appendages.
