Food Size Selection Among Copepods

The marine copepod Acartia tonsa was fed small plastic beads (7—70 microns) in a variety of size—frequency distributions and a concentration of 3,000—4,000 beads/ml. Both selective and non—selective feeding were observed. Selective feeding was intense over a narrow size range (selected particle sizes over 20 times more abundant in the gut than in the water); the actual sizes selected depended on the size frequency distribution offered, associating always with the largest abundant particles. A hypothesis for the mechanism of food size selection is that the animals "scan" the size distribution by capturing particles larger than the previous one ingested more efficiently than those that are smaller; the copepods then "locate" an abundance peak and halt the process just to the right of it.