text: Immersed in water, extremely water repellent (super-hydrophobic), structured surfaces trap air between the structures and this air-layer is maintained for a period of time. A silvery shine, due to the reflection of light at the interface of air and water, is visible on the submerged surfaces.
  Long lasting air layers also occur   in aquatic arthropods which breathe via a physical gill (plastron) e. g. the water spider (Argyroneta) and the saucer bug (Aphelocheirus) Air layers are presumably also conducive to the reduction of friction in fast moving animals under water, as is the case for the back swimmer Notonecta.
  The best known examples for long term air retention under water are the floating ferns of genus Salvinia. About ten species of very diverse sizes are found in lentic water in all warmer regions of the earth, one widely spread species (S. natans) found in temperate climates can be even found in Central Europe. The ability to retain air is presumably a survival technique for these plants. The upper side of the floating leaves is highly water repellent and possesses highly complex and species-specific very distinctive hairs. Some species present multicellular free-standing hairs of 0.3–3 mm length (e. g. S. cucullata) while on others, two hairs are connected at the tips (e.g. S. oblongifolia). S. minima and S. natans have four free standing hairs connected at a single base. The Giant Salvinia (S. molesta), as well as S. auriculata, and other closely related species, display the most complex hairs: four hairs grow on a shared shaft; they are connected at their tips. These structures resemble microscopic eggbeaters and are therefore referred to as “eggbeater trichomes”. The entire leaf surface, including the hairs, is covered with nanoscale wax crystals which are the reason for the water repellent properties of the surfaces. These leaf surfaces are therefore a classical example of a “hierarchical structuring“.
  The egg-beater hairs of Salvinia molesta and closely related species (e.g.  S. auriculata) show an additional remarkable property. The four cells at the tip of each hair (the anchor cells), as opposed to the rest of the hair, are free of wax and therefore hydrophilic; in effect, wettable  islands surrounded by a super-hydrophobic surface. This chemical heterogeneity, the Salvinia paradox, enables a pinning of the air water interface to the plant and increases the pressure and longtime stability of the air layer.
  The air retaining surface of the floating fern does not lead to a reduction in friction. The ecological extremely adaptable Giant Salvinia (S. molesta) is one of the most important invasive plants in all tropical and subtropical regions of the earth and is the cause of economic as well as ecological problems. Its growth rate might be the highest of all vascular plants. In the tropics and under optimal conditions, S. molesta can double its biomass within four days. The Salvinia effect, described here, most likely plays an essential role in its ecological success; the multilayered floating plant mats presumably maintain their function of gas exchange within the air-layer.