The fluid physics of signal perception by mate-tracking copepods

Abstract

Within laboratory-induced swarms of the marine copepod Temora longicornis, the male exhibits chemically mediated trail-following behaviour, concluding with £uid mechanical provocation of the mate-capture response.The location and structure of the invisible trail were determined by examining the speci¢c behaviour of the female copepods creating the signal, the response of the male to her signal, and the £uid physics of signal persistence. Using the distance of the mate-tracking male from the ageing trail of the female, we estimated that the molecular di¡usion coe¤cient of the putative pheromonal stimulant was 2.7 Â10 À5 cm 2 s À1 , or 1000 times slower than the di¡usion of momentum. Estimates of signal strength levels, using calculations of di¡usive properties of odour trails and attenuation rates of £uid mechanical signals, were compared to the physiological and behavioural threshold detection levels. Males ¢nd 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 £ow 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. Di¡usion 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 £uid mechanical signal from the female remains within 1^2 body lengths (55 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 À3 , 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 in£uence water motion in a low Reynolds number £uid 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 £ow velocity and chemical composition from common small-scale turbulent features; and mechanoreceptive and chemoreceptive copepods use their senses to discriminate these di¡erences. Zooplankton are not aimless wanderers in a featureless environment. Their ambit is replete with clues that guide them in their e¡orts for survival in the ocean.

Zooplankton use of chemodetection to find and eat particles

“…It has been demonstrated qualitatively for only 2 species, a planktonic shrimp (Hamner & Hamner 1977) and a planktonic copepod (Kiørboe 2001), and it is known that several copepods can follow pheromone trails to find mates (Tsuda & Miller 1998, Yen et al 1998. Such observations are difficult to make.…”

Chemical Communication in a Multimodal Context

“…9). Ultimately, in both groups, the final leap of the male (just before capturing the female) is thought to be mediated by hydrodynamic cues (Yen et al 1998).…”

Analysis of wind events in a coastal area: a tool for assessing turbulence variability for studies on plankton

“…At smaller scales, turbulence and the shear derived from it affect the flux of solutes to and away from microscopic particles (Lazier and Mann, 1989;Karp-Boss et al, 1996), and the encounter probability between particles (Rothschild and Osborn, 1988). Thus, it influences nutrient uptake (Karp-Boss et al, 1996;Maar et al, 2002), particle feeding processes (Marrasé et al, 1990;Saiz and Kiørboe, 1995;Mackenzie and Kiørboe, 2000), mating (Yen et al, 1998), and aggregation/disaggregation processes (Kiørboe, 1997).…”