496 



THE BIOLOGY OF MARINE ANIMALS 



rose red to olive green, in correlation with changes in its algal background 

 during migration from deeper to shallower water. 



The principle of obliterative shading, first clarified by Thayer, is widely 

 utilized by surface fish and other animals. In this kind of shading the 

 darker pigments occur on the back, and grade into lighter pigments on the 

 belly, thus counteracting the effects of superior lighting. Pelagic cetaceans 

 and fishes — for example, the tunny Thunnus thynnus and the blue shark 



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Fig. 1 1 .9. Patterns Assumed by Flounders (the Gulf Fluke Paralichthys 

 albiguttus) on Artificial Backgrounds of Different Patterns after One to 



Three Days 



(a) Individual on a black and white background consisting of 2 cm squares, after 

 having been adapted to white; (b) individual on black and white background, 5 mm 

 squares, after having been adapted to a white background ; (c) individual on a black and 

 white background consisting of circles 5 mm in diameter. The concealing pattern in this 

 fish was most remarkable, and the animal appeared to contain numerous holes, id) 

 Individual on a black and white background, consisting of 5 mm circles. (From photo- 

 graphs of Mast, 1914.) 



Carcharhinns glaucus — utilize countershading to maximal effect. Of equal 

 interest are examples of inverted countershading, as in the pelagic snail 

 Glaucus atlanticum which hangs belly uppermost from the surface film, 

 and of lack of countershading, as in the shark sucker or remora {Echeneis 

 naucrates) which attaches itself to larger fish with any side uppermost. 

 Superimposed frequently upon countershading is a pattern of disruptive 

 coloration in which the body form is broken up by irregular patches of 

 contrasting colours. This is well seen in the disruptive phases of many reef 



