Does Acartia clausi (Copepoda: Calanoida) use an area-restricted search foraging strategy to find food?

Abstract

The swimming behavior of Acartia clausi within small aquaria containing phytoplankton was videotaped, along with unfed controls. From these videos, aspects such as swimming speed, distance traveled, and 2D (horizontal, X, and vertical, Z) headings were then measured from over 75000 X Z positional data in order to quantify their swimming functional response. There was generally no significant difference in swimming behavior between different food levels, although there was a large difference between feeding and unfed controls. Because feeding bouts moved the copepods a short distance at a slow speed, net vertical displacement decreased as feeding-bout frequency increased. Non-feeding individuals showed longer periods of sinking, interspersed with longer-distance, high-speed jumps, which displaced them farther distances than filter-feeding copepods over the same time interval. For both fed and unfed control copepods, a 1D unbiased random walk was adequate to describe their displacement over these very short time periods (10 s). This behavior is consistent with an area-restricted search foraging strategy. The net vertical displacements predicted by combining the experimental data with a 1D random walk model suggest that A. clausi would be retained within even small ∼10 cm phytoplankton patches long enough to fill their gut. However, copepods outside of phytoplankton patches must rely on other means to find patches of food greater than ∼50 cm apart. Potentially, knowledge of a copepod's swimming functional response could enable predictions about the typical spatial and temporal patchiness of its food in situ.