Primary production of phytoplankton can be seriously impaired by defensive strategies they use to reduce mortality to a competitive level, as any protection requires allocation of energy to metabolically inactive matter. Thus, in context with the extreme variety of heterotrophic planktivorous organisms, the sphere of action of a specific defense strategy must be restricted. This, in turn, leaves room for a high diversity of species with individual defence strategies. Besides the chemical approach, mechanical armours, consisting both of purely organic and of biomineralised structures, are effectively used.Geometry and material properties are the decisive factors for the mechanical properties of any physical structure. Therefore, we performed virtual and real loading tests (using Finite Element Analysis, calibrated glass microneedles and micropipettes) and show that geometry and material properties can make diatom frustules (Thalassiosira punctigera and Fragilariopsis kerguelensis) mechanically very strong, hence resistant to large physical forces. The Phaeocystis colony skin will be presented as a non-mineralised, less rigid, yet equally tough complement to the diatoms frustules. Still, the silica used by diatoms, which proved to be surprisingly tough and can, due to its amorphous nature, assume almost any conceivable shape, may have been the motor for the evolution of the high diversity of diatom frustule architectures. The results are beautiful but nonetheless strictly functional structures.