INTRODUCTORY 7 



time. For fixed or sedentary animals such as molluscs, this is determined 

 by the lowest salinity at springs during the season of maximal run-off. 

 Active and migratory animals such as shrimps, however, are able to 

 execute seasonal movements up and down the estuary, moving towards 

 the mouth in winter and upstream during the spring and summer (4, 23, 29, 

 44). 



The Sea Floor 



Below low-tide mark the sea floor slopes gently at first across the conti- 

 nental shelf to the continental edge at about 200 metres. The gradient then 

 increases and the floor falls off rapidly down to a depth of around 4,000 

 metres. This region of sharp descent is the continental slope below which 

 the floor tends to level out again as the abyssal plain. The continental 

 shelf has an average width of about 50 kilometres, but the actual extent 

 varies widely in different parts of the world. In certain areas, such as the 

 Grand Banks of Newfoundland and the North Sea, the shelf extends 

 several hundred kilometres offshore, whereas off steep coasts, such as 

 Spain and Chile, it is only a few kilometres wide. The abyssal plain is far 

 from even, and presents great depressions and ridges; the greatest depres- 

 sions, called deeps, extend down to 10,500 metres. 



Pelagic Zone 



The pelagic zone comprises the waters of the open ocean, and because 

 of its volume, expanse and the density of its population, it forms the major 

 oceanic environment which the biologist has to consider. It is sometimes 

 divided into three horizontal regions on the basis of light penetration in 

 the following manner. 



1 . An upper photosynthetic zone in which the light intensity is sufficient 

 to provide plants with energy for growth. This will vary in depth according 

 to the amount of light falling upon the water, and with the transparency 

 of the water. In clear tropical oceanic waters the photosynthetic zone will 

 extend down to some 100 metres, but will be less elsewhere. This region 

 is often rich in plankton and herbivorous animals. 



2. A twilight zone extending below the photosynthetic zone to the limit 

 of light penetration. 



3. An aphotic zone extending from the limit of light penetration to the 

 sea bottom. This region is dark, without living plant life and is populated 

 solely by carnivorous animals and detritus feeders. 



Planktonic Organisms. The most important members of the phyto- 

 plankton, upon which all pelagic animals are ultimately dependent for food, 

 are the diatoms, followed by dinoflagellates. These plants can flourish 

 only in the photosynthetic layer and, correspondingly, zooplankton is 

 densest near the surface and diminishes with depth. The character of the 

 plankton also changes qualitatively as well as quantitatively in hauls from 

 deeper waters, surface species giving way to mesopelagic and bathypelagic 

 forms. The quantity of zooplankton in surface waters also shows great 



