COLOUR CHANGES 



519 



becomes dark on a black background in two or three days, and blanches 

 on a white ground. Colour changes in elasmobranchs are also rather slow, 

 for example, the European dogfish Scyliorhinus canicula requires three to 

 four days to change from pale to dark, or to make the reverse change. 

 Certain dark species of selachians — Torpedo torpedo, Raja clavata, and 

 R. batis — show little or no capacity for colour changes when placed on 

 white backgrounds. The responses of the smooth hound Mustelus cams, 

 on the other hand, are fairly rapid, for the animals change from pale to 

 dark in two hours, and from dark to pale in two days. The chromatophores 

 involved in these activities are epidermal and dermal melanophores, dermal 

 xanthophores, guanophores and a third class of light-brown chromato- 



5 -; 



o 



•a 



£* 



to 



Q 



^ 3 



o 



c 



2- 



20 



80 



100 



40 60 



Time (hours) 



Fig. 12.8. Background Responses in the European Dogfish 



Scyliorhinus canicula 



Hollow circles, dermal melanophores; solid circles, epidermal melanophores. The 

 upper curve A shows the degree of dispersion undergone by melanophores on transfer- 

 ring a fish from a white to a black background. The lower curve, B, shows the melanin 

 concentration which occurs on changing from a black to a white background. (Redrawn 

 from Waring (69).) 



phores. Of these the dermal melanophores are chiefly responsible for 

 colour changes (Fig. 12.8). In some species, such as the nurse-hound 

 Scyliorhinus stellaris, the chromatophores are spatially organized to form 

 specific colour patterns (54). 



In general the chromatophores of selachian fish are under pituitary 

 control, as was first demonstrated by Lundstrom and Bard. Removal of 

 the pituitary gland in Mustelus cam's and Squalus acanthias is followed by 

 permanent blanching of the fish, and melanophore expansion is brought 

 about by subsequent injection of pituitary extract. The hormone respons- 

 ible for expansion of the melanophores and darkening of the skin in 

 Mustelus and other selachians is known as B-substance or intermedine. Its 

 production has been traced to the neuro-intermediate lobe of the pituitary 

 gland (2, 34, 48, 51, 52, 70, 71, 74). 



