50 The Medium 



In the ocean many more groups are influenced by the transporting 

 action of the water. This affects not only the varied permanent 

 planktonic population but also the planktonic larvae of the benthos 

 and of the nekton. Species having planktonic larvae are much more 

 numerous in the marine environment than in fresh water. The plank- 

 tonic life of marine larvae also tends to be longer, often with many 

 more stages. Certain euphausiid crustaceans, for example, have five 

 larval stages including about twenty moults before reaching the adult 

 condition. 



So characteristic are the planktonic forms of many water masses 

 that they are sometimes used in tracing the currents. Members of the 

 permanent plankton which live in specific water masses must go 

 wherever the water goes, like Mary's lamb. Such species are termed 

 current indicators. Russell ( 1939 ) has shown, for example, that the 

 "Channel water" ordinarily occupying the English Channel is popu- 

 lated by a plankton community of which the chaetognath, Sagitta 

 setosa, is a characteristic member. Occasionally, however, a mass of 

 "mixed oceanic and coastal water" moves into the Channel from the 

 region south of Ireland, and its presence is revealed by the abundance 

 of another species, Sagitta elegans. A third species, Sagitta serrato- 

 dentata, is an indicator for the "pure oceanic water" which is found 

 to the west of the water mass tagged by S. elegans. 



The Gulf weed Sargassum, which can be seen by any traveler 

 crossing the southern part of the North Atlantic ocean, is an indicator 

 for water of tropical origin. This plant begins life attached to the 

 bottom around certain of the West Indian Islands ( Parr, 1939 ) . It is 

 torn up by storms and drifts out to sea where it continues to grow 

 vegetatively for years. Masses of Sargassum clumped together gave 

 rise in the past to the legend of the Sargasso Sea where ships were 

 supposed to become hopelessly entangled in the seaweed. You will 

 remember that, in crossing the Atlantic, Columbus was encouraged 

 by the presence of drifting seaweed to keep on, believing that he had 

 seen an indisputable sign of the proximity of land. It is fortunate 

 that Columbus did not realize that Sargassum may be carried hun- 

 dreds or even thousands of miles from shore by the currents of the 

 ocean. 



The matter of transport by ocean currents should not be left without 

 mention of the classical example of the eel. For generations the 

 people of Europe wondered where eels came from. Young elvers 

 ascended their rivers in the spring, and mature eels left their rivers 

 in the fall after several years' life in fresh water. No one knew where 

 or when the eels spawned. Finally, it was discovered that a small. 



