THE PELAGIAL 239 



and Voluta, from whose large-yolked eggs the young snails appear in 

 a benthic juvenile stage. 



Variations in environmental conditions are less frequent and less 

 abrupt in the open sea than in the shore waters or on land. Neverthe- 

 less, the conditions are not uniform, and pelagic life is accordingly not 

 uniformly distributed. The short-lived and rapidly multiplying nanno- 

 plankton affords the best index to the existing conditions. Its distribu- 

 tion is neither fortuitous nor uniform, but ecologic, i.e., dependent on 

 environmental conditions. Each smallest portion of the ocean has at 

 all times a population of nannoplankton and microplankton which is 

 an exact reflection of the environmental conditions. 24 Long-lived ani- 

 mals can exist for periods of time in unfavorable localities, as in polar 

 seas, drawing upon their stored supplies. But as these are also ulti- 

 mately dependent on the plant plankton, they are abundant where 

 the plankton is at least periodically rich. The extremes of density of 

 life in the pelagial are shown in the poorest catch of plankton, 763 

 organisms per liter in the tropical Atlantic, as compared with a maxi- 

 mum of 76,915 in a liter in the cold water of the North Atlantic. 



The study of pelagic distribution on this basis has just begun. 

 Lohmann 25 divided the pelagial into rich and poor sections according 

 to the existence of more or less than 1000 nannoplanktonic organisms 

 per liter. In the rich areas the distribution is not uniform, as numerous 

 distinct aggregations of especial density of life appear. The rich domain 

 of pelagic plankton is the upper 100 m. of the sea water, since the 

 plant element in it is dependent on light, and the impoverishment 

 of the plankton begins below this level. The basis of study of plankton 

 distribution in this direction lies in the construction of lines of equal 

 plankton development or "isoplankts," especially for equal amounts 

 of the plankton as a whole and for certain outstanding species. 26 



Among longer-lived animals, aggregations appear which may be 

 due to instinctive gregariousness, as in the schools of herring, cods, 

 mackerels, and other fishes wandering to feeding grounds or breeding 

 places. Such aggregations also appear in less active forms, such as the 

 numbers of Salpa fusiformis which are developed annually in July 

 and August north of the Hebrides, and the more accidental accumula- 

 tions on the borders of currents and in bays. The former have been 

 distinguished by Apstein as "production," the latter as "swarm" aggre- 

 gations. The great numbers of the northern copepod Calanus fin- 

 marchicus (Fig. 59) may color wide stretches of the sea brown. The 

 siphonophore Velella forms swarms of enormous extent. Such an accu- 

 mulation observed by the Plankton expedition in the Atlantic extended 

 for 300 km. Salpas and pyrosomas, ctenophores and siphonophores, and 



