Mapping the free-swimming attack volume of a planktonic copepod, Euchaeta rimana

The ability of planktonic copepods to detect and pursue remote prey is well documented, but there are no empirical descriptions of their three-dimensional (313) sensory fields. In this study, the attack volume of females of Euchacta rimana Bradford, a planktonic calanoid copepod, was mapped by plotting the positions of attacked prey within a standardized 3D coordinate system defined by the body axes of E. rimana, This analysis was performed using videotaped observations of predatory interactions between free-swimming E. rimana and smaller copepod species. Attack by E. rimana was an oriented response, accurately directed toward rot-note prey within an ellipsoidal volume anterior to its paired first antermules. This attack volume enveloped the large mechanosensory setae projecting anteriorly from the first antermules, with attack distances averaging 1.5 mm, or less than one body length of the predator. E. rimana attacked a larger prey species, Acartia fossae, at significantly longer distances than it attacked a smaller species, Acrocalanus inermi, reflecting prey-specific perceptive volumes. Such perceptual biases may underlie the selective feeding patterns observed in E. rimana and other copepod species. These observations are consistent with mechanosensory mechanisms of prey identification and localization, suggesting that fluid disturbances provide the releasing and directing stimuli for E. rimana during predatory interactions. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-001-0735-z.