The colour literature contains a large body of work on the physic

The colour literature contains a large body of work on the physics and chemistry of colour production and blue colours have received considerable research attention (Goodrich & Reisinger, 1953; Dyck, 1971; Veron, 1973; Rohrlich, 1974; Byers, 1975; Filshie, Day RGFP966 in vitro & Mercer, 1975; Kazlauskas et al., 1982; Blanquet & Phelan, 1987; Wilson, 1987; Goda & Fujii, 1995, 1998; Brink & Lee, 1999; Vukusic

et al., 2001; Kinoshita, Yoshioka & Kawagoe, 2002; Bulina et al., 2004; Prum et al., 2004; Prum & Torres, 2004; Vukusic & Hooper, 2005; Watanabe et al., 2005; Doucet et al., 2006; Bagnara, Fernandez & Fujii, 2007; Simmonis & Berthier, 2012). This research attention may reflect our curiosity about brilliantly blue-coloured animals and the potential that colour-producing mechanisms have for biomimetic industrial applications. Besides special cases, such as that of male satin bower birds Ptilonorhynchus violaceus who collect natural and artificial blue objects for display in courtship (Borgia, Pruett-Jones & Pruett-Jones, 1985), animals must produce their blue colours or sequester them from other animals. Except for the striking abundance and diversity of bioluminescent marine animals (Widder, 2010) and the firefly Amydetes fanestratus

that is bioluminescent at a blue-shifted wavelength (538 nm) (Viviani et al., 2011), colour production mechanisms are classified for into BAY 57-1293 cost two main categories: pigmentary and structural. While this dichotomous classification scheme seems convenient, it is potentially misleading, as it does not well represent the underlying biology of colour because pigments and structures often work in concert (Shawkey, Morehouse & Vukusic, 2009). Pigments are important directly or indirectly in the production of most colours (Shawkey & Hill,

2006; Amiri & Shaheen, 2012). Pigments can be generally defined as molecules that selectively absorb light at various wavelengths. Those wavelengths of light not absorbed are reflected, and it is these that result in the colour. A blue pigment, therefore, absorbs light at wavelengths across the whole visual range with the least absorption in the blue wavelengths (450–490 nm). Pigmentary molecules can be present in an organism in one of two ways: in an extracellular matrix (living or dead, e.g. feathers) or within a cell. Intracellular pigments are contained within the chromatosomes (pigment-containing organelles) of chromatophores (chromatosome-containing cells). Chromatophores of particular colours are named for their hue [e.g. cyanophores are cells containing blue chromatosomes (Goda & Fujii, 1995)]. Animals’ red, orange and yellow colours are often achieved by pigments (e.g. carotenoids), but blue pigments are rare, perhaps because they necessitate more complex chemistry.

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