Cue hierarchy and foraging in calanoid copepods: ecological implications of oceanographic structure

Fine-scale water column structure was mimicked in a laboratory plane jet flume to examine responses of the calanoid copepods Temora longicornis and Acartia tonsa to layers consisting of a velocity gradient, density gradient, phytoplankton exudates and food (phytoplankton). Copepods were exposed to isolated layers and combinations of cues as defined by in situ conditions. Behaviors elicited by the velocity gradient and chemical exudate layers included increased swimming speed and turn frequency consistent with excited area-restricted search behavior, which led to increased proportional residence time in the layers. Both species had significant responses to isolated layers of velocity gradients and chemical exudates, with T. longicornis responding more intensely to chemical cues than velocity gradients and A. tonsa responding equally to both. Combined fluid mechanical and chemical cues elicited species-specific responses. For T. longicornis, chemical presence induced responses that strengthened or cancelled initial reactions to the velocity gradient. These results suggest a cue hierarchy where a velocity gradient acts as an initial cue for narrowing search regions, and chemical cues and food presence determine consequent responses. For A. tonsa, combining velocity gradient and chemical cues had the same effect on copepod behavior as the individual cues, which suggests both cues are equal sources of information but are not closely associated. In both species, physical contact with particles or cells initiated feeding behavior, resulting in lower swimming speeds. Fluid density had a potentially aversive effect, as neither species responded with an area-restricted search response and individuals rarely crossed a strong density gradient. Observed behaviors may lead to aggregation, especially when superimposed on rhythmic movements such as diel vertical migration.