All animals face the problem of producing, detecting, and correctly interpreting signals intended for communication. In any species, effective signals must be appropriately tuned to the sensory systems of receivers, but well-designed signals must also contend with the environment within which they are transmitted. This is particularly true for visual signals that are used underwater in marine or freshwater environments. Water is a particularly difficult medium within which to use visual signals. This is because particles in the water, and the water molecules themselves, tend to scatter light, degrading images (and signals) even at relatively short distances, making long-distance transmission impossible. Furthermore, water transmits light within a restricted spectral band, primarily at blue wavelengths in clear marine systems (but varying among water types; for example, in freshwater, green or even yellow may be best transmitted). This limited spectrum, which becomes ever narrower with increasing depth, makes colour signals nearly useless in waters more than a few metres deep and favours patterns that stand out well under blue illumination.