Feeding behavior of <i>Calanus pacificus</i> in mixtures of food particles1

Abstract

The hypothesis was proposed that apparent size‐selective feeding by copepods is not behaviorally determined. The hypothesis was tested by feeding adult females of Calanus pacificus pairs of single‐celled diatoms of different sizes and comparing predicted and observed clearance rates on the two sizes of cells. In 24 experiments, representing a broad range of relative proportions of two sizes of cells, individual abundances of two sizes of cells, and absolute abundance of food, C. pacificus invariably filtered the large cell at a higher rate than the small cell. This suggested a strong passive or mechanical component to apparent size‐selective feeding in C. pacificus . Measurements of clearance rates of the copepod on different size‐classes of single diatom species and of feeding behavior of copepods in suspensions containing several sizes of plastic spheres and diatoms supported this interpretation. It is concluded that C. pacificus differentially filters particles of different sizes and that the mechanism is purely mechanical, reflecting variable retention efficiency of the second maxillae for particles of different sizes. That is, the copepod indiscriminantly filters suspensions of food particles, but captures, handles, and ingests large particles more efficiently than small particles.