484 THE BIOLOGY OF MARINE ANIMALS 



Structural Colours 



Among marine animals structural colours are produced by two physical 

 processes, namely diffraction and interference. In the former the colours 

 are due to diffraction of light by fine parallel ridges which form a closely 

 spaced grating. Interference colours result from the decomposition of 

 white light by interference from thin layers of plates of the same order of 

 thickness as the wave-length of light. The iridescence shown by many 

 marine animals is due to interference, and is characterized by its metallic 

 brilliance and by the shift in colours which occurs from red to blue as the 

 angle of incidence of the light is increased. 



The beautiful sheen of mollusc shells, of the pearl oyster Pinctada, 

 abalones and ormers Haliotus, and the pearly nautilus, is due largely to 

 iridescence, on which diffraction colours may be superimposed. In lamelli- 

 branchs the internal or nacreous layer of the valves is lamellated, and con- 

 sists of more or less regularly spaced plates of calcium carbonate separated 

 by thin layers of horny conchiolin. Where these plates abut against the 

 middle prismatic layer they form a diffraction grating and give rise to 

 diffraction colours. Pfund (69) found a periodicity in the spacing of succes- 

 sive laminae of 2,400 to 8,000 lines/cm in different regions of a specimen 

 of mother-of-pearl examined, the periodicity being remarkably constant 

 at any one locus. Iridescence is produced by reflexion from numerous 

 parallel laminae of about equal thickness (Fig. 11.5). In abalone shell the 

 thickness of laminae is 0-48//, equal to the wave-length of blue-green light 

 (4,800A). In shell lacking colour but displaying a pearly lustre, the laminae 

 vary greatly in thickness. Some shells show neither colour nor pearly 

 lustre, due to the irregular character of the chalky material in the shell, 

 which scatters the light diffusely. 



In some gorgonians from deep water the calcareous axes display brilliant 

 iridescent colours, for example Irido gorgia. The iridescence in these 

 animals is an accidental by-product of structure, and is normally not 

 seen, but there are many other animals which appear iridescent when alive. 

 In cuttlefish and teleosts there are abundant guanine crystals in the skin, 

 usually in special iridophores, which produce the shiny glistening effects 

 so often observed. The copepod Sapphirina gleams in the sunlight like a 

 flashing jewel, due to guanine crystals in the integument. 



The existence of a black pigmented screen behind the guanine layer 

 augments the intensity of iridescence by absorbing the penetrating longer 

 rays. Some colours are the result of both iridescent and pigmentary effects. 

 In the cuttlefish, for example, a combination of blue iridescence from under- 

 lying iridophores and yellow reflexion from expanded xanthophores gives 

 rise to a green colour. Blue and green pigments are rare in fishes, occurring 

 in wrasse and parrot fishes. Usually these colours are produced by inter- 

 ference and refraction from guanine microlamellae in conjunction with 

 melanin granules and xanthophores. The beautiful metallic appearance 

 of the iris of fishes is due to crystals of guanine. In the sapphirine gurnard 



