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 hydro- mechanical 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-pro- pelled organism, u ∝ a 2 Ur -2 . In contrast, at high Reynolds numbers, a self-propelled organism gen- erates a forward hydromechanical disturbance that has the form u ∝ a 3 Ur -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.