INTRODUCTION

Protists are predatory or photosynthetic single-celled eukaryotes (in the nanometre to millimetre range) that have physiological capabilities comparable to those in metazoans where they are distributed amongst specialized cells. Since the protists have colonized solid and fluid environments, they exhibit a wide range of behavioural patterns. To some extent pelagic species compensate for the tendency to sink by using regulatory buoyancy mechanisms. They are also capable of responding to external factors and physical parameters by adjusting their movements and swimming behaviour. The physiological activities involved in buoyancy regulation and generation of coordinated movements use a complex system of protein filaments, the so-called cytoskeleton, that extend throughout the cytoplasm (actin, microtubules, non-actin nanofilaments and their associated proteins) and a sophisticated machinery involving signalling molecules, cell-surface receptors, motor molecules and ions (Amos & Amos, 1991).

The present chapter reviews adaptations of individual cells and colonial forms of marine planktonic protists to buoyancy and swimming. The environmental factors guiding the generation of concentration/convection patterns and the collective dynamics of micro-organisms that have recently been reviewed are not described here (Kessler, 1985,1989; Levandowsky & Kaneta, 1987).