400 



JOHNSON AND BRINTON 



[CHAP. 18 



Interglacial warming of the oceans is of equal importance to oceanic cooling 

 in developing or maintaining antitropical distribution. The sub -surface equa- 

 torial region is characterized by a belt of cool water (Fig. 7) which extends 

 across the ocean. While species of the transition zone required cooling of the 

 eastern Pacific to accomplish equatorial transgressions, the central species 

 presumably crossed the tropics on the western side of the ocean during epochs 

 of general oceanic warming. Only then would temperatures in the sub-surface 

 equatorial belt become tolerable. E. mutica and Thysanopoda subaequalis 



Fig. 8. Composite track of portions of three expeditions on which the physical profiles 

 and species concentrations shown in Fig. 7 are based. 



transgress the equator in the western Pacific today. During epochs of oceanic 

 cooling, the central species would be withdrawn into the warm mid-parts of 

 the central gyrals. 



It is significant that the bisubtropical euphausiid species, including most of 

 the Euphausia species occupying the central water-masses of the oceans, are 

 also panoceanic, occurring in the Atlantic and Indian Oceans as well as the 

 Pacific. In contrast, subarctic euphausiid species (excepting the two most 

 northern species, Thysanoessa inermis and T. raschii, which appear to have 

 passed between the Atlantic and Pacific Oceans by way of the Arctic Ocean) 

 and the most tropical species, e.g. Euphausia americana of the Atlantic and 

 Nematoscelis gracilis of the Indo-Pacific equatorial water-mass, do not occur 

 in both the Atlantic and Indian-Pacific Oceans, but are endemic to one or the 

 other. Burkenroad (1936) adduced evidence from penaeid distributions that 

 only shallow water connected the Atlantic with the Pacific at Panama during 

 the Cenozoic. It appears probable that oceanic populations have not crossed 



