FISHES 149 



which dictate fish colour changes are many and varied, 

 but an outstanding factor in their determination is in most 

 cases that of concealment. The inability of fishes inhabit- 

 ing the abyss to change colour may be explained by the fact 

 that it would serve them no useful purpose. The majority 

 are black or red, colours which in complete darkness 

 are effective cloaks of invisibility. 



But with fish that live in constant exposure to varying 

 lights and ever-changing surroundings, the case is very 

 different, and ability to harmonise quickly with whatever 

 scenic environment may offer becomes a necessity if foes 

 are to be successfully eluded. Good examples of the 

 latter are to be seen in the Pike tanks at the Zoo, where the 

 fish, caught in deep and dark lakes and originally dark 

 green in colour, have adapted themselves to the bright 

 sandy colour schemes of their new premises. 



In some of the marine tanks startling changes are to 

 be observed. At the Zoo Aquarium systematic experi- 

 ments have been made with Flatfishes — the most 

 accomplished of the quick-change artists of the under- 

 water world. These fishes if placed in tanks with various 

 coloured floors — even floors decorated with tessellated or 

 geometric patterns — at once accommodate their colours 

 and patterning to blend with their surroundings. It was 

 found that in the case of a Flounder that it copies red 

 backgrounds less accurately than it does blue and green, 

 though brown and yellow are assimilated almost instan- 

 taneously. 



It is amongst fishes living in varied surroundings that 

 changes of colour are the most notable. But though 

 such changes usually have concealment as their motive, 



