ADVERTISEMENT AND CONCEALMENT IN THE PLANKTON - WHAT MAKES A COPEPOD HYDRODYNAMICALLY CONSPICUOUS

Euchaeta rimana, a pelagic marine copepod, roams a 3-dimensional envir onment and its antennular setal sensors are oriented to detect water-b orne signals in 3 dimensions. When the copepod moves through water or moves water around itself, it creates a fluid disturbance distinct fro m the ambient fluid motion. As it swims and hovers, the copepod's lami nar feeding current takes the unstable nature of small-scale turbulenc e, organizes it, and makes the local domain a familiar territory withi n which signals can be specified in time and space. The streamlines be tray both the 3-dimensional spatial location (x, y, z) as well as the time (t) separating a signal caught in the feeding current and the cop epod receptor-giving the copepod early warning of the approach of a pr ey, predator, or mate. The copepod reduces the complexity of its envir onment by fixing information from a turbulent field into a simpler, mo re defined laminar field. We quantitatively analysed small-scale fluid deformations created by copepods to document the strength of the sign al. Physiological and behavioral tests confirm (a) that these disturba nces are relevant signals transmitting information between zooplankter s and (b) that hydrodynamically conspicuous structures, such as feedin g currents, wakes, and vibrations, elicit specific responses from cope pods. Since fluid mechanical signals do elicit responses, copepods sha pe their fluid motion to advertise or to conceal their hydrodynamic pr esence. When swimming, a copepod barely leaves a trace in the water; t he animal generates its flow and advances into the area from which the water is taken, covering up its tracks with the velocity gradient it creates around itself. When escaping, it sheds conspicuous vortices. P rey caught in a flow field expose their location by hopping. These esc ape hops shed jet-like wakes detected by copepod mechanoreceptors. Cop epods recognize the wakes and respond adaptively.