ABSTRACT
In 1973, Katona depicted the mate-Þnding response of copepods as occurring when the male copepod detects the edge of the diffusing cloud of pheromone emanating from the female at a distance of 4 mm. Here, the process of diffusion is needed to
transport
the signal molecules to the sensors of the male copepod to alert him of the presence and location of a female copepod. However, using the equation for the characteristic diffusion time (Dusenbery, 1992;
t
=
r
/4
D
, where the diffusivity coefÞcient
D
= 10
cm
/s for small chemical molecules), it would take approximately 45 min for the pheromone to diffuse this distance. It is unlikely the female copepod would remain in the same three-dimensional (3D) position in the ocean for that length of time. Instead, in 1998, we (Doall et al., 1998; Weissburg et al., 1998; Yen et al., 1998) reported that male copepods detect discrete odor trails left in the wake of the swimming female copepod (Figure 10.1). Within this low Reynolds number regime, viscosity limits the rate of diffusion of the odor trail to molecular processes. Diffusion does not transport the pheromone to the male and, instead, acts to
restrict
odor dispersion. The scent persists as a coherent trail, with little dilution of signal strength, and hence remains detectable for a period that gives enough time for the male to encounter it. In the case of the copepod
Temora longicornis
, this aquatic microcrustacean could Þnd trails that were less than 10.3 s old. Trails were followed for distances as long as 13.8 cm (~100 body lengths), greatly extending the encounter volume of the copepod (Gerritsen and Strickler, 1977). Past studies showed that copepods could detect signals only within a few body lengths (see Haury and Yamazaki, 1995, for a review). Our Þndings show the perceptive distance can be 10 to 100 times greater.
