The fluid physics of signal perception by mate-tracking copepods

Within laboratory–induced swarms of the marine copepod Temora longicornis , the male exhibits chemically mediated trail–following behaviour, concluding with fluid mechanical provocation of the mate–capture response. The location and structure of the invisible trail were determined by examining the specific behaviour of the female copepods creating the signal, the response of the male to her signal, and the fluid physics of signal persistence. Using the distance of the mate–tracking male from the ageing trail of the female, we estimated that the molecular diffusion coefficient of the putative pheromonal stimulant was 2.7 times –5 cm 2 s –1 , or 1000 times slower than the diffusion of momentum. Estimates of signal strength levels, using calculations of diffusive properties of odour trails and attenuation rates of fluid mechanical signals, were compared to the physiological and behavioural threshold detection levels. Males find trails because of strong across–plume chemical gradients; males sometimes go the wrong way because of weak along–plume gradients; males lose the trail when the female hops because of signal dilution; and mate–capture behaviour is elicited by suprathreshold flow signals. The male is stimulated by the female odour to accelerate along the trail to catch up with her, and the boundary layer separating the signal from the chemosensitive receptors along the copepod antennule thins. Diffusion times, and hence reaction times, shorten and behavioural orientation responses can proceed more quickly. While ‘perceptive’ distance to the odour signal in the trail or the fluid mechanical signal from the female remains within 1 to 2 body lengths (< 5 mm), the ‘reactive’ distance between males and females was an order of magnitude larger. Therefore, when nearest–neighbour distances are 5 cm or less, as in swarms of 10 4 copepods m –3 , mating events are facilitated. The strong similarity in the structure of mating trails and vortex tubes (isotropic, millimetre—centimetre scale, 10:1 aspect ratio, 10 s persistence), indicates that these trails are constrained by the same physical forces that influence water motion in a low Reynolds number fluid regime, where viscosity limits forces to the molecular scale. The exploratory reaches of mating trails appear inscribed within Kolmogorov eddies and may represent a measure of eddy size. Biologically formed mating trails, however, are distinct in their flow velocity and chemical composition from common small–scale turbulent features; and mechanoreceptive and chemoreceptive copepods use their senses to discriminate these differences. Zooplankton are not aimless wanderers in a featureless environment. Their ambit is replete with clues that guide them in their efforts for survival in the ocean.