Deformation of the Active Space in the Low Reynolds Number Feeding Current of Calanoid Copepods

Abstract

The active space is the zone around an alga where chemical exudates are sufficiently concentrated to promote a behavioral response in a grazing copepod. Chemosensory perception of algae by copepods is investigated with a numerical model of the effects of advection in copepod feeding currents on the chemical diffusing from algae. Recent observations of encounters between algae and calanoid copepods define length and speed scales for a doubly sheared, low Reynolds number flow. These scales and shears are incorporated in an analytical description of the feeding current. Advection of the chemical in the laterally sheared flow changes the distribution of the exudates to create a deformed active space. An initially spherical active space is elongated and the edge of the active space is projected an order of magnitude further in the direction of the flow than the undisturbed radius. The advance warning received by a copepod of an approaching alga is a function of the radius of the active space, the detection threshold, the position of the alga in the stream field, and the molecular diffusivity of the exudate. An example of the relation between radius, detection threshold, and diffusivity for a detection distance of 1000 μm demonstrates that the model is well behaved for widely varying parameter values and loosely defines some bounds for the controlling parameters.