IX. 9 (255) 



9. Colours of fishes 



The bony fishes show perhaps the most brilliant and varied colora- 

 tion of any animals, rivalling even the Lepidoptera and Cephalopoda 

 in this respect. The enormous range of colour and pattern provides 

 an excellent example of the detailed adjustment of the structure and 

 powers of animals to enable them to survive. A great difficulty is 

 introduced into the study of animal coloration by the fact that we are 

 usually ignorant of the capacity for visual discrimination possessed 

 by the animals likely to act as predators. Moreover, it is very difficult 

 for us to obtain this information. When we examine any two objects 

 we are able to say not merely that they are different but that one is 

 red and the other green. A person or animal that is colour-blind may 

 also be able to detect a difference, but yet remain unaware of any 

 distinction of colour; the objects appear to him only as differing in 

 brightness. In order to decide whether animals are able to distinguish 

 between light of two wavelengths we must present them with objects 

 of different colour but the same brightness. 



We are therefore faced with the possibility that some of the colours 

 that appear to us so brilliant are to other animals merely differences of 

 tone, and animals to us conspicuous because coloured, when seen in 

 monochrome, may be protected. Some of the colours of fishes may 

 be only a means of producing a pattern of protective greys, as seen 

 through the eyes of an attacker. However, there is no doubt that some 

 fishes are able to discriminate between illuminated bodies which 

 though of different wavelength reflect light of equal brightness. In 

 the subsequent description of fish coloration we shall not be able to 

 consider predators further, but shall describe the colours as they 

 appear to the eye of a normal man. 



The colour of fishes is produced by cells in the dermis, (a) the 

 chromatophores and (b) the reflecting cells or iridocytes (Fig. 152). 

 The chromatophores are branched cells containing pigment, which 

 may be either black (melanin) or red, orange, or yellow (carotenoids 

 or flavines). The iridocytes contain crystals of guanin, making them 

 opaque and able to reflect light so as to produce, where no chroma- 

 tophores are present, either a white or a silvery appearance. This 

 material is used in the manufacture of artificial pearls, the scales of the 

 cyprinoid Alburnus lucidus (the bleak) being used for the purpose. 

 The iridocytes may be either outside the scales, when they produce 

 an iridescent appearance, or inside them, giving a layer, the argenteum, 

 that produces a dead white or silvery colour. By a combination of the 



