THE PELAGIAL 245 



Calanus hyperboreus, while in the Norwegian Sea it is C. finmarchicus. 

 A characteristic animal of the North Atlantic current, rarely found 

 outside of its limits, is the basking shark, Cetorhinus maximus. 



Great numbers of animals may accumulate at the borders of cur- 

 rents, just as debris is carried into quiet bays by flood water in streams. 

 A cloud of Salpa fiagellifera 100 m. long has been encountered on the 

 border of the Benguela current, 39, 4 " and certain stations on the borders 

 of great currents are especially rich in pteropods. 41 



Algae provided with floating bladders are carried off by the oceanic 

 currents and accumulate in the quiet eddies in great amounts. In the 

 eddy of the North Atlantic current the weed is Sargassum baccijerum ; 

 in the Indo-Pacific there are other species of Sargassum, and in the 

 South Pacific Macrocystis pyrijera. The weed is carried back and forth 

 by wind, and may reenter the current for a second circuit. Such accumu- 

 lations of seaweed form a special biotope in the pelagial. 



A Sargassum community. — The best-known example of such an 

 eddy is the Atlantic Sargasso Sea. 42 The Sargassum comes from the 

 coasts of the Caribbean islands, carried away by the strong Florida 

 Current, especially during the hurricane season. The masses of 

 Sargassum are separated out to the right of the current, and continue 

 to grow, without reproducing, until the bladder-like floats disintegrate 

 and the weed sinks to the bottom. The weed affords a base for a 

 curious nomadic fauna 43 of littoral origin, coming from the original 

 home of the Sargassimi, but enabled by it to live a littoral life on the 

 high seas. Various adaptations are necessary, such as attachment to 

 the weed, and in the more free-swimming forms, provision for the 

 young by nest-building, or care of the young. Some of these forms, 

 such as the fish Pterophryne histrio, are common on the Gulf Coast. 

 Others, like the small crab Planes minutus, are widespread on floating 

 bodies of every kind; the shrimp Leander tenuicornis is also found in 

 the Pacific. Others appear to have become specifically transformed 

 (such as the shrimp Latreutes ensiferus) and are confined to the 

 Sargasso Sea. 44 The change to this half pelagic existence being im- 

 possible for some of the original inhabitants of the seaweed, its fauna 

 in the Sargasso Sea is poor in species. Besides a few species of algae 

 there are 16 animals directly attached to the weed, 10 hydroids, 2 

 polychaetes, 2 bryozoans, 1 cirripede, 1 ascidian, 45 and in addition 

 small free-swimming forms live in or rest upon the weed, such as turbel- 

 larians, snails, and fishes. The number of individuals of the Sargasso 

 animals is fairly large. These animals are wholly dependent on the 

 weed and constitute a pseudobenthos (Fig. 62). The bryozoan Mem- 

 branipora is abundant, as are the spiral calcareous tubes of the 



