On the blue coloration of vertebrates^†

Abstract

Summary Although the various vertebrate classes, from fishes to mammals are each distinctive, they possess many common features making it important to understand their comparative biology. One general feature that has long commanded interest is the integumental pigmentary system. Thus, much is known about particular pigment cells; however, the basis for some specific colors, such as blue, has escaped the scrutiny of the comparative approach. Regardless of Class, blue is almost always a structural color based upon incoherent or coherent scatter of blue wavelengths from the animal surface. The source of scatter may be intracellular or extra‐cellular. A main intracellular scatterer is the surface of reflecting platelets of iridophores of lower vertebrates. Extra‐cellular scatter is widespread and thought to occur from ordered dermal collagen arrays in primitive fishes, birds and mammals including humans. Among birds, feather structures provide major means for extra‐cellular light scatter. There is only one known example of blue color deriving from a blue pigment found within a pigment cell. For amphibians, reptiles and birds, the scatter of blue wavelengths, together with the presence of yellow pigmentation, is fundamental for the expression of green coloration.

Iridescence: a functional perspective

“…In many vertebrates, iridescence is produced by specialized cells that are also called iridophores, although there are structural and functional differences between these and invertebrate iridophores (Bagnara et al 2007). These cells usually contain a basal melanin layer and stacked reflecting platelets that are several layers thick (Bagnara et al 2007). Iridescence produced by iridophores is common in fishes and can result in a variety of colours, including the silvery iridescence produced by many species and some of the brilliant colours displayed by reef fishes (e.g.…”

Neural control of tuneable skin iridescence in squid

“…Iridescence plays an important role in animal communication and camouflage as it provides the body with the ability to vibrantly reflect any colour, something pigments cannot match in intensity or colour range [6,7]. For example, the blue colours in invertebrates, birds, fishes, lizards and amphibians are almost always iridescence based [6][7][8][9][10]. Some species can alter their structural coloration as a result of various environmental stimuli while others possess physiological control over their iridescence.…”

A nanostructural basis for gloss of avian eggshells

“…It includes colours that humans can and cannot see [1], colours that are matte or glossy [2] and colours that are perceived differently as the angles of observation and illumination shift (iridescence [3,4]). Colour can be produced through selective absorbance of light at particular wavelengths by pigments, by nanoscale structures that interact with light (structural colour) or by the interaction of pigments and nanoscale structures [5][6][7][8]. For example, a basal layer of melanin in Steller's jay (Cyanocitta stelleri) feathers absorbs incoherently scattered light, thereby enhancing the blue coloration that is produced by a quasi-ordered nanostructure of keratin and air [5].…”