520 THE BIOLOGY OF MARINE ANIMALS 



Although the pituitary gland, responding to stimulation through the 

 eyes, brings about darkening of the fish, it is still uncertain what mechanism 

 is responsible for blanching. One important, perhaps sole, factor is a 

 decrease in the concentration of intermedine in the circulation. Parker has 

 shown that when part of a dark young Mustelus is perfused with Ringer's 

 solution, the perfused area becomes lighter, due to loss of chromatophore- 

 expanding substance, intermedine, from the tissues. A further mechanism 

 has been proposed by Hogben and Waring, namely that the anterior lobe 

 of the pituitary produces a chromatophore-concentrating or W-substance, 

 which brings about blanching of the fish. When the anterior lobe of the 

 pituitary is removed from a dark specimen of Raja or Scyliorhinus, no 

 change in colour results, but when the same operation is carried out on a 

 pale fish, the animal darkens even when kept on a white background 

 (2, 53, 54, 69, 70). 



In addition to pituitary control of chromatophore responses, Parker 

 considers that in one species at least, Mustelus cam's, the chromatophores 

 are also regulated by concentrating nerve fibres. When a transverse cut is 

 made in the pectoral fin of this fish a pale band appears distal to the 

 incision, and a similar pale band can be produced by cutting or by electrical 

 stimulation of peripheral nerves. These effects are ascribed to excitation of 

 nerve fibres which elicit contraction of the chromatophores. Similar results 

 have not been obtained with species of Squalus, Raja or Scyliorhinus. Now, 

 the innervation of chromatophores in vertebrates is by way of the sympa- 

 thetic nervous system. In elasmobranchs, however, grey rami that would 

 normally carry such fibres to the spinal nerves, and thence to the skin, are 

 absent; such sympathetic fibres as do run to the periphery accompany 

 blood vessels, with which they are concerned (49, 51, 52, 53, 76, 78). 



In summary, physiological colour changes in selachians are restricted 

 and limited in extent and only certain species show background responses. 

 Colour changes in general are rather slow, and may require several 

 days to reach completion. The chromatophores are controlled by the 

 level of pituitary hormones in the blood. Intermedine, from the intermedi- 

 ate lobe, brings about chromatophore expansion and darkening of the fish. 

 Even in dark species like Torpedo torpedo, which show no responses to 

 changes in background, the pituitary is responsible for maintaining the 

 expanded condition of the chromatophores. 



Teleosts. The colour responses of many teleosts form very characteristic 

 features of their behaviour. The chromatophores responsible include 

 melanophores containing brown or black melanin pigments, xanthophores 

 and erythrophores containing yellow and red carotenoids, reddish allo- 

 phores, and guanophores containing guanine. The pigment cells are 

 located in the epidermis and dermis and, in particular, form two well- 

 defined layers at the upper and lower boundaries of the dermis (Fig. 12.9). 

 Their distribution and relative numbers vary from species to species; 

 frequently they are organized in grouped associations of melanophores 

 with iridosomes, xanthophores or erythrophores. In their fully contracted 



