236 MARINE ANIMALS 



from the same depth in the southern part of the North Atlantic are 

 much smaller than the northern ones. The sifting by size is similar 

 in the shrimp Acanthephyra. 15 Among the pelagic cephalopods, the 

 smaller forms are found only at the surface, while the larger inhabit 

 the oceanic depths. Pelagic dinoflagellates like Ceratium form longer 

 processes in warm than in cold water. Such feather-like processes as 

 are found in Calocalanus (Fig. 53g) occur in warm-water copepods 

 but not in polar forms. 15 This seems to be the governing factor in the 

 composition of the surface fauna of the tropical and subtropical Atlan- 

 tic, which contains siphonophores, medusae, salpas, and pyrosomas. 

 Among larger fishes, only specialized forms like Mola with its thick 

 layer of fat, or active swimmers, like the sharks, are found at the 

 surface; other fishes are represented mostly by juveniles. Such mechan- 

 ical relations with depth throw light upon the occurrence of surface 

 forms of the Norwegian seas, such as the pteropod Clione, the medusa 

 Aglantha, and the copepod Calanus, which occur also at depths of 

 750 to 1000 m. in the warmer part of the Atlantic, where the viscosity 

 is the same as that of the surface waters near Norway. 16 



The variation of viscosity with temperature may bear upon the 

 fact that many pelagic animals are smaller in warm seas than in cold. 

 This may be referable to direct action of the lower temperature, as it 

 applies also to the benthic animals, but it involves none the less a 

 coincident adaptation on the part of the pelagic fauna. The fishes 

 Cyclothone microdon and C. signata in the vicinity of the Canary 

 Islands are notably smaller than at the same depths in the north 

 Atlantic, 17 and the arrowworm Sagitta bipunctata reaches a length of 

 12 mm. in the Mediterranean and of 44 mm. in the Arctic. This matter 

 evidently requires consideration in connection with the size relations 

 discussed in Chapter X. 



Special means of securing food are required by pelagic animals, 

 since their food differs in a number of ways from that of the benthic 

 animals. There are no such great accumulations of detritus as are 

 present on the sea bottom, and there are none of the many-celled algae 

 and vascular plants. The basic food supply is the plant portion of the 

 plankton, that endless number of single-celled algae, such as are found 

 in the nannoplankton of the ocean. Lohmann found, in the Bay of 

 Kiel, that for every metazoan (such as an Ephyra or Sagitta) there 

 were 1000 protozoan and 7000 protophytan cells, and these figures are 

 low. 18 The devourers of these plankton algae are in part small animals, 

 such as radiolarians, Foraminifera, copepods, etc., but also include 

 larger ones which have solved the problem of securing large numbers 

 of the small forms. The larger plankton animals, small Metazoa up 



