Motility patterns and mate encounter rates in planktonic copepods

Mate encounter rates in pelagic copepods depend on the characteristics of the mate detection mechanism as well as on the motility patterns and concentrations of males and females. We describe male and female motility patterns in two species (Centropages typicus and Pseudocalanus elongatus) in which the females excrete male-attracting pheromones and in one species (Acartia tonsa) in which the mates are detected by hydromechanical signals.

For both pheromone-producing species, male and female motilities differ strongly; males are more directionally persistent and swim as fast as or faster than the females, and only at scales exceeding several centimeters do the male motilities resemble random walk. This prevents resampling of the same volume at the scale of encounter and maximizes the rate at which males encounter female signals.

In contrast, female motility patterns are more similar to random walk, even at small scales. For the species depending on hydrodynamic cues and in which detection is practically symmetrical between the mates, male and female motilities are similar and can both be described as random walk, also at small scales.

We develop simple encounter models and utilize information on motility patterns and mate signaling from our own observations and the literature to estimate search volume rates for pelagic copepods ranging between 0.6-3.0 mm in length. We show that pelagic copepods are capable of searching tens to thousands of liters of ambient water for mates daily, that search capacity increases approximately with the cube of copepod length for both chemical and hydrodynamic signalers, and that these impressive mate search volume rates are sufficient to sustain populations at typical adult densities.