244 MARINE ANIMALS 



colithophorids, peridinians, and zooflagellates with the seasonal forms 

 in fresh-water animals. These relations require further investigation. 



The conditions are quite different for organisms in the non-circulat- 

 ing currents, which either carry warm water into a cold region as does 

 the Gulf Stream or the Japan Current, or cold water into warm, like 

 the Labrador Current or the Humboldt Current. The distance reached 

 depends on the season, so that index forms of cold water such as 

 Oikopleura labradoriensis and Frittilaria borealis reach the North Sea 

 in spring, while in the summer warm-water forms such as Physophora 

 hydrostatica are carried to the Lofoten Islands. 37 These foreigners 

 suffer in the new environment, are unable to breed, and ultimately die. 

 The contrasts are especially notable where a warm and cold current 

 meet. On many banks there may be rapid and extensive displacements 

 of water and consequent extreme fluctuations of temperature. Fewkes 38 

 noted a variation of 5.5° between two successive tides in Narragansett 

 Bay, caused by change in the relative predominance of the Gulf 

 Stream and Labrador currents. There may be a great mortality in the 

 plankton at such places, as on the New England Banks, the Agulhas 

 Bank, the Spitzbergen Bank, or the juncture of the Japan Current 

 and the cold Ojashio on the east coast of Japan. Murray showed that 

 the deposits of pelagic Foraminifera on the sea bottom were greatest 

 where currents of different temperature met. Unusual westward exten- 

 sion of the cold waters over the warmer southern New England banks 

 in 1882 caused extensive destruction of the tilefish, Lopholatilus 

 chamaeleonticeps, which did not reappear for many years, 39 and the 

 southern forms of plankton formerly abundant in that area on the 

 border of the Gulf Stream also disappeared. Such massive destruction 

 provides food for the bottom fauna, and by its disintegration for the 

 plant plankton, which in turn favors the redevelopment of pelagic 

 life. 



The plankton carried along by the current becomes mixed with 

 that from other sources, so that its composition changes with the 

 progress of the current. Such currents have a characteristic fauna, 

 varying with their origin, and the better defined, the more the current 

 differs in physical and chemical characters from the neighboring waters. 

 The Guinea Current, which flows from west to east near the equator, 

 is sharply distinguished from the adjacent north and south equatorial 

 currents by vast amounts of the schizophycean Trichodesmium and by 

 the appearance of neritic forms like the appendiculate Oikopleura 

 dioica and the peridinean Prosocentrum micans. The life of the 

 Labrador Current differs from that of the adjacent Gulf Stream. 

 An index form for the Polar Current north of Iceland is the copepod 



