Remote prey detection in Oithona similis: hydromechanical versus chemical cues

We quantified prey encounter rates and prey reaction distances in the ambus h-feeding cyclopoid copepod Oithona similis by video recording freely swimm ing copepods at different concentrations of prey, the dinoflagellate Gymnod inium dominans. Prey encounter rate increased with prey concentration, and a maximal clearance rate of 0.42 +/- 0.10 ml h(-1) was estimated. The avera ge distance (from the antennules) at which O.similis reacts to prey is 0.01 4 +/- 0.007 cm. A simple prey encounter model was used to combine observed predator and prey velocities and prey reaction distance, and yielded a clea rance rate similar to that estimated directly from prey encounter rates. Th e observed prey reaction distance was consistent with that estimated from a published model of hydromechanical prey perception. The possibility of rem ote chemodetection was examined by modeling the distribution of solutes lea king 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 sta tionary when perceiving prey, it is the width rather than the length of the trail that matters. Owing to advection, the chemical signal vanishes almos t instantaneously off the sides of the swimming flagellate, and solute conc entrations are below any likely detection threshold within 40-50 mu m from the flagellate. Our observations are thus inconsistent with remote chemodet ection in O.similis. The considerations are generalized, and it is conclude d that ambush-feeding copepods, unlike cruisers and suspension feeders, can not utilize chemical signals for the detection of individual prey, but rely on either hydromechanical detection or direct interception of prey.