25 



There are no genera common to the Pacific and Atlantic oceans, which 

 are not found in the Indian Ocean. 



To discuss the faunal independence of various regions, we can use 

 the index of commonahty of fauna proposed by Preston (1962a, b). This 

 is given as l-z, where z is the index of degree in the correlation equation 

 between the number of species inhabiting the two regions. The value of z 

 varies between and 1 . Preston observed that if 2 = 0, the faunas of the 

 two regions under consideration are indentical; if z = 1, these regions 

 have no common forms. At z = 0.27, both faunas are in equilibrium 

 30 and mutual exchange is fully possible. Hence if 2: > 0.27, there would 

 be some interaction between the faunas but this interaction would not be 

 complete and there would be some definite natural isolation. 



It seems that at the species level for the Physosomata fauna of the 

 Indian and Pacific oceans, z = 0.29, for the Indian and Atlantic oceans, 

 z = 0.26, and the Atlantic and Pacific oceans, z = 0.31. In other 

 words, the Indian Ocean has a total exchange of faunas with the Atlantic 

 Ocean, while faunas of the Pacific and Atlantic oceans are isolated to 

 some extent. 



The geographic distribution of the Physosomata is intimately linked 

 with their vertical distribution. 



The bulk of the species distributed only in the tropical region inhabit 

 the upper warmer water layers and many of them generally do not 

 descend to deep waters. Their distribution is characteristic for a vast 

 majority of surface tropical animals, particularly for numerous species 

 of the superfamily Platysceloidea. Together with them, several true deep- 

 water species aie also confined to the tropical zone, which inhabit the 

 deepest layers and do not rise over 1,000 m or even 2,000 m; such, 

 for example, are the Metalanceola chevreuxi, Lanceola loveni grossipes, 

 and others. Why these deepwater species are confined in distribution to 

 tropical regions, is difficult to say. 



Contraiily, a vast majority of the North Pacific and panoceanic 

 species are permanent inhabitants of the deeper layers, and only a small 

 number have a very wide range of vertical distribution, sometimes rising 

 to the surface, mostly at night {Lanceola sayana) and at a definite stage 

 of their life cycle (Mimonectes)} 



As noted earlier, the extensive panoceanic distribution of many 

 species of the Physosomata obviously depends primarily on their notable 

 eurybathic nature, examples of which have already been given. This 

 dependence, established for the benthic fauna by N.G. Vinogradova 

 (1958), finds extensive confirmation even among pelagic animals. 



^ Almost all reports of Mimonecetes at the surface relate to sexually mature, spawning 

 females. 



