1. Diatoms account for 40 % of the ocean primary production and play a key role in the oceans’ ability to sequester carbon. The evolutionary success of diatoms and their role in ocean biogeochemistry are related to the siliceous shell that provide partial protection against grazing. 2. The structure and function of phytoplankton communities are governed by environmental constraints and organismal trade-offs. Defence mechanisms may help explain the high diversity of phytoplankton (incl. diatoms) in the ocean, but only if the defence comes at a cost. Defence costs have been notoriously difficult to demonstrate and quantify in marine phytoplankton. 3. Here, we demonstrate for seven species of planktonic diatoms that their shell thickens and their growth rate declines when cells are exposed to chemical cues from copepods, important predators of diatoms. The responses are proportional to the concentration of grazer cues, but are also highly variable, both between and within species. 4. At our standard experimental condition, the typical decline in growth rate is 10 %, and the typical increase in cellular biogenic silica is 16 %. The latter value corresponds to a decline in grazing mortality due to small copepods of 11 %. Thus, silification in response to grazers is exactly warranted. 5. The similar magnitude of the costs and benefits of silification suggests a flat fitness landscape along the competition-defence axis. This may help explain the high diversity of coexisting diatoms in the ocean. 6. The significant but variable contribution of diatoms to the downward flux of organic carbon in the ocean depends to a large extent on the silica content of the cells. This is due less to the ballasting effect of silica, but mainly to the different life histories of more or less defended cells that are governed by evolutionary adaptations and – as demonstrated here - plastic responses to grazers.