30 



The Medium 



Green plants are confined to relatively shallow subsurface depths be- 

 cause of their need for light. Many types of animals, on the other 

 hand, display a very great vertical range in their distribution. 



Several species of invertebrates are found at depths extending from 

 near the surface in the littoral zone down to 4000 m or even 5000 m 

 in the abyssobenthic zone (Fig. 2.4). Although the individuals of 

 these bottom-living forms do not travel far, the species as a whole 

 have become adapted to this great vertical range over a period of 



Neritic 



Oceanic - 



50 s2 



200 



1000 



Benthic 



division DeeD-sea f Archlbenthic 

 ^,,J1„„ J zone 

 I ^y^'^"' Abyssobenthic 

 [ zone 



Fig. 2.4. The chief zones of the marine environment. The division between the 

 neritic and oceanic provinces occurs at the edge of the continental shelf where 

 depth is about 200 m. The lower limit of the archibenthic zone occurs between 

 800 and 1100 m. The Httoral zone forms the upper part of the neritic benthic 

 zone and usually receives strong wave and current action and sufficient light for 

 plant growth. The depth of its lower limit is variable but is often in the neighbor- 

 hood of 40 to 60 m. These divisions also apply in a general way to lakes. 

 ( Modified from The Oceans by Sverdrup et al., 1942, copyright Prentice-Hall, 



Inc., N. Y.) 



time. In certain other kinds of marine animals the individuals are 

 known to change level over considerable distances and hence are 

 able to withstand correspondingly great pressure changes. Some 

 species of fish move downward as much as 400 m during the day and 

 swim up again to their former level each night, thus subjecting them- 

 selves twice daily to a pressure change of 40 atmospheres. Certain 

 small planktonic Crustacea similarly carry out diurnal vertical migra- 

 tions of 200 m to possibly 600 m in amplitude (Waterman, Nunne- 

 macher,'Chace, and Clarke, 1939). 



The foregoing is not to imply that most species of marine organisms 

 do not have definite vertical limits to their distribution. Many species 



