AX AEROBIC HABITATS 29 



The oceanograpliic investigations of recent years have 

 clearly demonstrated that in all major oceans a minimum 

 concentration of oxygen occurs at mid-depth, but the 

 reason for this curious phenomenon is still a matter of 

 controversy. The degree of oxygen deficiency varies great- 

 ly in various oceans. In the western basin of the North 

 Atlantic, for example, (Seiwell, 1934) the oxygen values 

 are never very low and it seems questionable whether the 

 conditions prevailing there have any influence upon the 

 respiration of the local fauna. In parts of the South 

 Atlantic (Wattenberg, 1929), and especially in the East- 

 ern Pacific (Schmidt, 1925; Moberg, 1930 and 1930a; 

 Ito, 1930; Thomsen, 1931; Thompson, Thomas and 

 Barnes, 1934) extremely low values, frequently only 

 small fractions of a cubic centimeter per liter, have been 

 found. In the Western Pacific the conditions are less 

 extreme. The conclusion that such very low oxygen 

 values must have an influence upon the mode of energy 

 production of the fauna — and it is known that an 

 abundant plankton does occur in these laj^ers (Schmidt, 

 1925) — seems inescapable. Experiments upon the oxy- 

 gen requirements and anaerobic abilities of animals se- 

 cured from these regions have never been performed. 

 They should prove extremely interesting and it would not 

 be too surprising if future investigation would reveal the 

 oxygen-minimum layer as the most important of all 

 anaerobic habitats (because of its large area). One may 

 expect that the results of such experiments fall in line 

 with the observation of Xikitin and Malm (1928) who re- 

 ported (in a paper which unfortunately was not avail- 

 able) that the zooplanktou from the oxygen-poor deeper 

 layers of the Black Sea can be kept alive longer with less 

 oxygen (5-6%) than species from the upper strata. 



In Figure 3 are represented vertical sections of two 

 water basins illustrating cases of oxygen minima. In 

 one case (Gulf of California) larger animals should 

 hardly be able to get sufficient oxygen for a purely aerobic 



