Chemical and hydromechanical components of mate-seeking behaviour in the calanoid copepod Eurytemora affinis

The ability of pelagic copepods to remotely detect and encounter potential sexual partners is a critical issue in zooplankton ecology. In calanoid copepods, mate-finding behaviour involves either chemical or hydromechanical cues. Here, I first provide a detailed description of the three-dimensional swimming behaviour of Eurytemora affinis, males, non-ovigerous females and ovigerous females swimming freely in the absence of chemical and hydromechanical cues and discuss potential reasons for the discrepancies observed in the literature. I further demonstrate that E. affinis males modify their swimming behaviour in response to a diffuse background of pheromone concentrations, which can be thought of as an adaptation to increase the encounter rate and follow trail mimics containing female scent. I finally describe the mate-seeking behaviour of E. affinis and show how males swimming freely in a three-dimensional environment modify their direction of motion upon encountering the pheromone trails of females and subsequently follow female trails over distance and duration of nearly 15 cm and 36 s until they hydromechanically sense the female and lunge at her. The relevance of these results is discussed in the general framework of search pattern strategies and the relative contribution of chemical versus hydromechanical cues in relation to the turbulent nature of the surface ocean.