Remote prey detection in Oithona similis:: hydromechanical versus chemical cues

We quantified prey encounter rates and prey reaction distances in the ambush-feeding cyclopoid copepod Oithona similis by video recording freely swimming copepods at different concentrations of prey, the dinoflagellate Gymnodinium dominans. Prey encounter rate increased with prey concentration, and a maximal clearance rate of 0.42 +/- 0.10 ml h(-1) was estimated. The average distance (from the antennules) at which O.similis reacts to prey is 0.014 +/- 0.007 cm. A simple prey encounter model was used to combine observed predator and prey velocities and prey reaction distance, and yielded a clearance rate similar to that estimated directly from prey encounter rates. The observed prey reaction distance was consistent with that estimated from a published model of hydromechanical prey perception. The possibility of remote chemodetection was examined by modeling the distribution of solutes leaking out of a swimming cell. The cell leaves a long slender chemical trail in its wake. However, since the ambush-feeding O.similis is essentially stationary when perceiving prey, it is the width rather than the length of the trail that matters. Owing to advection, the chemical signal vanishes almost instantaneously off the sides of the swimming flagellate, and solute concentrations are below any likely detection threshold within 40-50 mu m from the flagellate. Our observations are thus inconsistent with remote chemodetection in O.similis. The considerations are generalized, and it is concluded that ambush-feeding copepods, unlike cruisers and suspension feeders, cannot utilize chemical signals for the detection of individual prey, but rely on either hydromechanical detection or direct interception of prey.