Pheromone Trail Following in Three Dimensions by the Freshwater Copepod emHesperodiaptomus shoshone/em

Finding mates can pose a particular problem for obligately sexual planktonic organisms, resulting in a variety of adaptations to ensure sufficient mating. Several types of mate-finding behavior have been observed in marine copepods, but the one most effective at low population density, following a pheromone trail, has not been observed in freshwater copepods. Using three-dimensional (3D) videography, we show that males of the large-bodied alpine species Hesperodiaptomus shoshone follow pheromones in the female's trail. Using a trail mimic comprised of female-conditioned water, we found that males followed female scent without the presence of the female. This behavior was reduced when the female scent was diluted, suggesting that the male's behavior can be modified by the intensity of the chemical signal. Analyses of the 3D trajectories of copepods that formed mating pairs indicate that the male does not make a direct approach to the female, as might be expected if he relied purely on hydrodynamic or visual cues. Instead, males that are >0.5 cm from females react to crossing female trails by making an abrupt turn and spending more than 2 s following the female's trail. Furthermore, flow field analysis showed that at this distance it was unlikely that copepods could distinguish the hydrodynamic signal from the background flow. This is the first demonstration of chemical trail following in a freshwater copepod and has important implications for encounter rates and viable population densities in similar species.