Clutch-size variability in Daphnia: Body-size related effects of egg predation by cyclopoid copepods

Cyclopoid copepods had unexpected effects on four species of Daphnia in an enclosure study. Small copepodites entered the brood pouch of large Daphnia and preyed on eggs. The resulting low clutch sizes could erroneously be ascribed to food limitation, but a clear difference between copepodite and food-limited populations could be seen when clutch sizes were plotted vs. body lengths. Food limitation resulted in consistently lower egg numbers for all individuals; egg predation resulted in large variability of clutch sizes because individuals were randomly affected by the copepods. Only small copepodites could enter brood pouches, and only large daphniids were affected. Daphniids below a body size of 2.25 mm were immune to egg predation regardless of species or size at first reproduction. This unusual mechanism of size-selective invertebrate predation (affecting large cladocerans) may explain unexpected results found in field studies.