The diatom frustule provides partial protection against copepod grazing. Whether the defense is due to the cells being de-selected or handled for so long that the grazers lose time for foraging is unknown. The mechanism has implications for the population dynamics of both defended and co-occurring, undefended nutrient competitors. We use video-observations to demonstrate that thick-shelled diatoms captured by the copepod Temora longicornis were rejected more frequently than thin-shelled diatoms, irrespective of cell size. The thick-shelled cells of the larger diatoms were handled for much longer, and the time spent handling these limits the consumption of phytoplankton. This may be why many diatoms, even in the presence of dense grazer populations, reach bloom concentrations, and thus, facilitate aggregation and mass sedimentation. This has implications for both carbon sequestration and for securing a large population of cells at depth ready to colonize the pelagic, when growth conditions again become favorable.