68 BRUUN 



and to some extent the 9° C. isotherm, have also been indicated. It is 

 obvious that all catches of Spirula fall within the regions where the tem- 

 perature is about 10° C. or higher at a depth of 400 meters (Plate 2). 



I have not plotted the record by Okada ( 1933) because I do not know 

 the exact locality. It is probably from Japanese waters, an area where 

 one would expect Spirula to occur. In the South Atlantic off South 

 America, in the Indian Ocean off West Australia, and in the Pacific 

 off Central America there are places where one might also expect Spirula 

 to live. It seems quite unlikely, though, that the eastern parts of the 

 South Atlantic and the South Pacific would offer the temperature con- 

 ditions required. 



As indicated in the 1943 comparison of the eastern and western North 

 Atlantic, the supply of food does influence the population numbers of 

 Spirula, but I think the temperature may eventually turn out to be the 

 dominant factor. It would be very interesting to have attempts made to 

 catch Spirilla, especially close to the slopes of southeastern Japan and 

 Pacific Central America. Temperature observations at the depths of the 

 fishing should naturally also be made. 



In 1943 I gave 1750 meters as the lower depth limit for Spirula. I 

 am now inclined to think that this is too deep, even though a certain 

 number of specimens were found in nets which had worked at that depth. 

 Because the nets were open when being hauled the arguments given had 

 to be based on statistics. I shall continue to doubt that Spirula lives below 

 the thermocline, in the true psychrosphere, until the use of reliable clos- 

 ing nets disproves this supposition. 



6. SPIRULA AS A TYPE OF MESOPELAGIC ANIMAL 



While it is clear that Spirula does not normally live in the photic 

 layers of the oceans, and therefore is not epipelagic, I would like to have 

 it called mesopelagic, to avoid confusion with the term bathypelagic. I 

 suggest that the word bathypelagic be applied to species living in the 

 aphotic zone and in the psychrosphere between about 4° and 10° C. 

 Animals living at still lower temperatures in deep water can be called 

 abyssopelagic. Mesopelagic animals would accordingly be those found 

 in the aphotic zone of the thermosphere. In addition, several of the 

 pelagic fishes which were caught at Galathea Station 203 with Spirula 

 deserve a critical study as to ecological type. If Argyropelecus, Sternoptyx, 

 Polyipnus, Astronesthes and Stylephorus were found under conditions 

 quite normal for them, I would prefer to have them called mesopelagic 

 also, not bathypelagic. 



