DISTRIBUTION 131 



Pacific has eight species of the genus. These facts, together 

 with information about the fossils in the two areas, indicate 

 that the waters of the N. Pacific have been temperate for a longer 

 period than those of the N. Atlantic. When, during the Eocene, 

 Britain had a tropical climate the warm water in the N. Atlantic 

 reached further North than did the warm water in the Pacific, so 

 that a temperate water fauna developed first in the N. Pacific, and 

 only later in the N. Atlantic. 



Separate faunas are more difficult to distinguish when we leave 

 the sea floor and examine the swimming and floating Crustacea. 

 These are usually sub-divided according to the depth at which 

 they are found. The Crustacea swimming in the upper 200 metres 

 are part of the epipelagic fauna in contrast to the bathypelagic and 

 abyssopelagic fauna swimming below this depth. 



Some of the epipelagic copepods are cosmopolitan; Oithona 

 similis is found from the Arctic Sea, through the Mediterranean 

 and Red Seas, and over the Indian and Pacific Oceans to the Ant- 

 arctic Seas. Some live all round the globe, but are confined to the 

 colder waters, or descend into the deeper cooler water in tropical 

 regions. It is thus possible for a species to be epipelagic in temperate 

 waters and bathypelagic nearer the equator. 



The only geographical division of the epipelagic Crustacea that 

 can reasonably be made at present is into a warm-water fauna and 

 a northern and a southern cold-water fauna. There are many 

 examples of circumglobal warm-water species, perhaps the most 

 interesting is the mysidacean Siriella thompsoni, for most other 

 Mvsidacea live only in the waters above the shelves. The Northern 

 cold-water epipelagic forms can be typified by the copepod 

 Pareucheta glacialis, while in Antarctic waters the Euphausiacea 

 are important, particularly Euphausia superba, which forms the 

 main food of the whalebone whales in this area. 



As one passes downwards in the sea, through the epipelagic and 

 into the bathypelagic zone, there is a change in the species of 

 Crustacea which one finds. Particular species swim at their own 

 specific depths. The prawn Systellaspis debilis swims between 150 

 and 500 metres from the surface, while Hymenodora glacialis swims 

 between 700 and 1,200 metres. Both these depths are the day-time 

 depths, for, like most other planktonic animals, these Crustacea 

 migrate upwards during the night. A further instance of variation 

 in the specific composition of the crustacean fauna with depth is 

 found when the numbers of species of copepods at various depths 



