tion can be made between the neritic (coastal) and oceanic (blue- 

 water) plankton. On a map, the loo-fathom line, which is also the 

 edge of the continental shelf, forms an approximate — not a sharp — 

 dividing line between the two types. 



No lines of demarcation are more important to the life of 

 plankton than those that mark changes in temperature. Even small 

 differences in heat and cold may spell the difference between life 

 and death for these tiny creatures. (They are also influenced by the 

 amount of salt in the water, and this varies perceptibly from one 

 part of the sea to another.) Since water temperature changes with 

 the depth, certain kinds of plankton live at one level in one part of 

 the ocean, at another level in another part. For instance, some 

 species that are found in both the Arctic and Antarctic oceans are 

 also found in temperate and tropical latitudes at depths where the 

 water temperatures are the same as in shallow polar waters. But 

 animals that are characteristic of the Gulf Stream cannot survive 

 in either deep water or in cold surface waters. Whenever, as some- 

 times happens, warm and cold currents change their course, the 

 effects on plankton can be disastrous. They can also be disastrous to 

 the sea creatures we know best — the fishes. 



Plankton, the basic food in the sea, is 

 composed of microscopic floating plants 

 and animals. Enlarged many times, this 

 photograph shows the translucent shells 

 of a velvet swimming crab larva (left), 

 a copepod (right) , a megalopa (top) , 

 and a squat lobster larva (bottom) . 



This beam trawl, used aboard the Challenger, 

 collected specimens from any desired depth. 

 Iron clamps kept the mouth of the bag 

 open. A small cotton bag lining the bottom 

 of the large net trapped minute organisms. 



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