Motile behaviour of the free-living planktonic ciliate Zoothamnium pelagicum (Ciliophora, Peritrichia)

•Among the peritrich ciliates, Zoothamnium pelagicum is the only free-floating species that develops its complete life cycle in the pelagic realm.•The motile behaviour of a free-floating peritrich ciliate is for the first time characterised.•The pseudocolony changes its contour from lenticular to spherical in three milliseconds, attaining a peak speed up to 350 mm s−1.•The contraction induces a forward jump of 1-2 mm that attained 110 mm  s−1 (∼250 pseudocolony lengths s−1) in five milliseconds after the onset.•This medusa-like locomotion at low Reynold numbers allows to escape from predators and to exploit new patches of food resources.

Zoothamnium pelagicum is the only free-floating species among ∼1000 peritrich ciliates that develops its complete life cycle in the open ocean. In the NW Mediterranean Sea, Z. pelagicum was usually associated with ectobiotic bacteria, while in the South Atlantic Ocean was sometimes fouled by the diatom Licmophora. Each colony constituted a radial branch that joined at its base with other colonies to form a lens-shaped pseudocolony of up to 400 zooids. The cilia beat slowly, propelling the expanded pseudocolony in the direction of the concave face. Contraction was triggered by external stimuli (threat) or occurred spontaneously. Frame-by-frame analyses of high-speed camera sequences revealed that during contraction the pseudocolony reduced its diameter 70-75% in 3-3.2 ms with peak velocity up to 350 mm s−1. The contraction induced a forward jump of 1-2 mm that attained a peak speed of 110 mm s−1 (∼250 pseudocolony lengths s−1) in 5 ms after onset. This medusa-like locomotion at low Reynolds numbers allowed the pseudocolony to exploit new patches of food resources, as well as to escape from predators. Zoothamnium pelagicum has been able to proliferate in the oligotrophic open ocean, while its sessile counterparts are restricted to eutrophic environments.