Analysis of flow fields associated with freely swimming calanoid copepods

High-resolution video observations of freely swimming adult female copepods were utilized to characterize the flow fields produced by Centropages velificatus, an omnivore with strong tendencies toward carnivory, and Paracalanus aculeatus, a herbivore. Horizontal and vertical components of the translational velocity of the copepod were subtracted from the corresponding components of flow field velocity, allowing fluid velocities to be calculated from an external frame of reference and without the need for mechanical restraints on the copepod. Large differences in the geometry of flow fields were found between species. The flow field geometry of P. aculeatus is similar between individuals, and lower in velocity than the flow field of C. velificatus. The now field geometry of C. velificatus is variable between individuals and seems to be dependent on copepod body orientation and swimming trajectory. Evidence is presented that a third factor, fluid distortion around the moving copepod, also affects the flow field geometry. The ecological implications regarding behavioral plasticity and the influence of flow field velocity and geometry on the feeding success of copepods in the field are discussed.