Feeling blue is not so nice but easier
Blue is undoubtedly the most pervasive colour on our “Blue Planet”: the sky is blue (well, most of the time during the day), the sea is blue (unless polluted or full of green plankton), mountains and forests are (at least in the paintings of Higashiyama Kaii) and even horses can be blue (witness Franz Marc’s beautiful creations). However, when it comes to blue coloration in animals we have to admit that blue is quite a rare colour. As a child I often visiting the local zoo with my grandfather. I must have been really lucky, because I always stopped at the cage with a huge male mandrill Mandrillus sphinx, a monkey with a frighteningly bright red and blue nose as well as behind. The males of this African species belong to one of the few mammalian species to display bright red and blue coloration. And he does all that with a total lack of red and blue pigments, because blue pigments (with perhaps some very few exceptions like those described from so-called cyanophore cells in the skins of callionymid fishes) are unknown from vertebrates. By the way, do not believe when you find in the internet statements like, for example, the green colour in many frogs is created by yellow and blue pigments. There are no blue pigments. But if no blue pigments are there, but some vertebrates do display blue coloration, how come they are blue? And for what reason would any animal want to be blue in the first place? (That’s something for another blog).
Light from the sun is a small part of electromagnetic radiation, which we can perceive as light ranging from blue of around 400 nm to red of around 700 nm wavelengths. Chemicals absorbing some wavelengths of the impinging radiation are contained in cells known collectively as chromatophores. Of these the melanophores contain small brownish black granules, xanthophores tiny yellowish vesicles and erythrophores reddish pigments, which are common in fish and some amphibians. Guanophores, also termed iridophores, contain silvery crystalline reflective material and leucophores, finally, contain fat globules and are white. Blue-coloured feathers in birds, the skins of frogs, newts, many fishes and even the blue and red nose of the mandrill owe their existence to refractive and reflective properties of cells that overlie and cover the pigment-containing cells. These extra cells, not containing pigments themselves but only colourless submicroscopic structures, act as selective filters for incident and reflected radiation and are called “schemochromatic”. In combination with the chromatophores these schemochromatic cells are responsible for colorations that are not caused by pigments; colorations that may be blue, green or purplish with a metallic lustre that change with the angle you look at them. Colours such as these are referred to as “physical” in contrast to those that are due entirely to pigments in the chromatophores.
Amongst vertebrates blue compared with other colours especially red, black, white and green is relatively rare and most examples include fishes, amphibians (in particular tropical frogs), a few reptiles (lizards and snakes) and, of course birds with blue plumage. In invertebrates examples can be found amongst almost any phylum and certain jellyfish, the blue-ringed octopus, some starfish, various crabs and shrimps come to mind. Insects contain bluish species especially amongst some grasshoppers, damsel and dragonflies, butterflies, beetles and true flies (think of the greenishblue blue-bottle fly). A few species of butterflies, e.g. nymphalid species of the genus Nessaea, seem unique (unless additional research reveals more examples perhaps even amongst some fish and amphibians), as these butterflies are known to produce a true blue pigment in their wings. The chemical is pterobilin; a derivative of pteridine, which is also present in blue patches of Graphium sarpedon males in which together with Dr E. Eguchi in 1983 (Annot. Zool. Jap.) I could show that the blue patches strongly reflected UV. Assuming the blue colour to be purely “physical”, we did not chemically analyse the colour and thus perhaps missed a chance to be the first to have discovered a blue pigment. That’s something I could now feel a little blue about, isn’t it?
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2020.
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