Advertisement and Concealment in the Plankton: What Makes a Copepod Hydrodynamically Conspicuous?

Abstract

Euchaeta rimana, a pelagic marine copepod, roams a 3-dimensional environment and its antennular setal sensors are oriented to detect water-borne signals in 3 dimensions. When the copepod moves through water or moves water around itself, it creates a fluid dis- turbance distinct from the ambient fluid motion. As it swims and hovers, the copepod's laminar feeding current takes the unstable nature of small-scale turbulence, organizes it, and makes the local domain a familiar territory within which signals can be specified in time and space. The streamlines betray 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 copepod receptor-giving the copepod early warning of the approach of a prey, predator, or mate. The copepod reduces the complexity of its environment by fixing information from a turbulent field into a simpler, more defined laminar field. We quantitatively analysed small-scale fluid deformations created by copepods to document the strength of the signal. Physiological and behavioral tests confirm (a) that these disturbances are relevant signals transmitting information between zooplankters and (b) that hydrodynami- cally conspicuous structures, such as feeding currents, wakes, and vibrations, elicit specific responses from copepods. Since fluid mechanical signals do elicit responses, copepods shape their fluid motion to advertise or to conceal their hydrodynamic presence. When swimming, a copepod barely leaves a trace in the water; the 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. Prey caught in a flow field expose their location by hopping. These escape hops shed jet-like wakes detected by copepod mech- anoreceptors. Copepods recognize the wakes and respond adaptively.