title: Motile behaviour of the free-living planktonic ciliate Zoothamnium pelagicum (Ciliophora, Peritrichia).
abstract: 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 3-3.2ms with peak velocity up to 350mms-1. The contraction induced a forward jump of 1-2mm that attained a peak speed of 110mms-1(∼250pseudocolony lengthss-1) in 5ms 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.
authors.affiliation: Carmen Campos Panisse 3, E-11500 Puerto de Santa María, Spain. Electronic address: fernando.gomez@fitoplancton.com.
authors.forename: Fernando
authors.initials: F
authors.lastname: Gómez
mesh_terms: D001267:Atlantic Ocean; D009068:Movement / Q000502:physiology; D016802:Oligohymenophorea / Q000166:cytology* / Q000502:physiology*; D010781:Photography; D010933:Plankton / Q000166:cytology / Q000502:physiology
keywords: Escape behaviour; Flow pattern; Peritrichous ciliates; Predator avoidance; Protist locomotion; Stalk contraction