Vertical distribution of planktonic ciliates - an experimental analysis of swimming behaviour

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Abstract
Many marine planktonic ciliates accumulate close to the water surface or around the pycnocline. Similar vertical distribution patterns are repeated if natural or cultured populations allowed to swim in an experimental water-column. Turbulence, light, pycnocline or chemical gradients do not seem to be necessary to explain the accumulation close to the surface. It is suggested that gravity is the ultimate cause for directed swimming towards the surface, although no evidence could be foudn for a statocyst-like mechanoreceptor. Experimental analysis of ciliate models in glycerol and manipulations of ciliate density as well as medium density indicate that the negative geotaxis results from interactions between sinking velocity, swimming velocity, tumbling rate and shape or density asymmetry of the cell. This is supported by the lability of the geotaxis which can be reversed by changing temperature or light intensity which correlate with changes in swimming patterns. Measurements of vertical velocities show that the ciliates studied can move between 1 and 2.5 m h-1 and this may determine, together with water turbulence, the vertical distribution of planktonic ciliates. A simple stochastic model was constructed in an attempt to study the combined effects of vertical swimming velocity and water turbulence. This model explains the different distribution patterns observed in the field as a function of the relative magnitude of vertical swimming velocity and water turbulence rate. The ecological significance of a possible ability of planktonic ciliates to influence their vertical distribution is discussed.