Tracking response of the freshwater copepod Hesperodiaptomus shoshone: Importance of hydrodynamic features

Abstract

Using three-dimensional Schlieren-based videography, males of the freshwater alpine species Hesperodiaptomus shoshone (Wyoming) were found to follow both conspecific females and conspecific males, remaining 0.45 ± 0.13 cm (male) and 0.56 ± 0.13 cm (female) from the lead copepod for 0.91 ± 0.35 seconds (male) and 0.84 ± 0.46

seconds (female). Trail following is initiated when the male makes a rapid reorientation. Chemical pheromones either were not produced by the female or were not

detected by the male because males would follow trail mimics composed of female-conditioned water. Using unconditioned water, males were found capable not only of

following trail mimics but they showed a preference, quantified as a higher follow frequency, of trails running at speeds matching that of their female mate.

Remarkably, the male copepods always followed upstream, micro-casting between the edges of the trail to remain on track. Trails flowing at speeds matching their

mate’s swimming speed were followed for a longer period of time and at greater gross distance. As the flow speed of the trail mimic increased, the distance the

copepod would advance would decrease until the threshold speed of 2.30 cm/sec at which it would not follow a trail and only station hold. Station holding has never

been observed before for copepods and may represent an adaptive behavior to avoid being washed out of their resident alpine pond. At speeds greater than that evoking

station holding, the stream seemed to push the copepod out of the flow even though the copepod would make repeated efforts to swim up the stream. This research

revealed a behavior not documented before: instead of relying on discrete pulses of flow left by hopping copepods, this high alpine lake copepod followed smoothly swimming mates or continuously flowing thin streams, relying only on sensing hydrodynamic cues.