Hydromechanical signals in the plankton

The distance at which plankters can detect and thus interact with each other depends on their sensitivity, size, and motion, as well as the hydrodynamic characteristics of their behaviour. Through a simple consideration of the distribution of forces exerted on the ambient fluid by different plankton behaviours, it is possible to deduce the spatial scale over which the associated hydromechanical disturbance propagates. At low Reynolds numbers, for passive sinking or for a feeding current, the associated hydromechanical velocity, u, attenuates with distance, r, as u ∝ aUr –1 where a is the length scale of the organism and U is its velocity relative to the fluid. Similarly, for a self-propelled organism, u ∝ a2 Ur –2. In contrast, at high Reynolds numbers, a self-propelled organism generates a forward hydromechanical disturbance that has the form u ∝ a3Ur –3. Within this context, observed planktonic interactions, particularly for copepods, were analysed and showed reasonably good support for the theory. The remote detection of inert particles by feeding-current-generating and free-swimming copepods was found to be feasible for known copepod sensitivities. Directional information and signal timing for flow disturbances and vortices provided a means of locating active organisms. Finally, the effect of turbulence was considered, as it can impair a copepod’s detection ability. A simple analysis of ambush-feeding copepods detecting swimming ciliates under turbulent conditions showed good agreement with previously reported observations.