CHAPTER XIII 



THE BIOTIC DIVISIONS OF THE OCEAN (Continued): 



THE PELAGIAL 



As in the benthal, two vertical zones may be distinguished in the 

 pelagic communities, the lighted and the lightless, or abyssal zone. 

 They are not sharply distinguished, although an ill-defined intermedi- 

 ate stage can be recognized. The depth to which light penetrates 

 decreases from the tropics to the polar regions, and varies in each 

 locality according to the time of year, depending on the angle of 

 incidence. The lower limit, for practical purposes, lies between 200 to 

 600 m. Off Bermuda on "an exceedingly brilliant" day, Beebe 1 found 

 light still visible to the eye at 571 m. ; at 610 m. all daylight had van- 

 ished; earlier he had found no visibly detected light below 511 m. 



The general discussion of the pelagial may best be restricted prin- 

 cipally to the lighted zone, leaving the abyssal for treatment with the 

 abyssal benthal (Chapter XIV) . The common character of all pelagic 

 animals is their independence of the bottom, which means that they 

 have the ability to maintain themselves in the open water without 

 sinking. This capacity is variously developed. Some animals, like the 

 ctenophores, never sink to the bottom; many elements in the pelagic 

 fauna, however, are larvae of animals which spend their adult life in 

 the benthal. In some cases the pelagic period is so short that it appears 

 as the exception, and the animal must be reckoned as benthic (Torpedo, 

 Trachinus) . Even forms which are unquestionably pelagic, such as the 

 mackerels (Scomber scombrus), are thought to spend part of the year 

 resting on the bottom. 



Flotation mechanisms. — Living matter is heavier than sea water; 

 its specific gravity ranges from 1.02 to 1.06, averaging about 1.04. 2 

 Special adaptations are consequently required to prevent animals from 

 sinking. This distinguishes pelagic creatures from the animals of the 

 benthal and gives them certain features in common; these appear in 

 various groups by convergent evolution. 



The state of suspension may be regarded as a retarded sinking in 

 which the rate approaches zero as a limit. The rate of sinking depends 

 on various factors; it is reduced with reduction in the specific gravity, 



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