12 $ A BIOLOGY OK CRUSTACEA 



tion. If the distribution of the hadal isopods is studied it is found 

 that most of the genera occur in all the oceans, but only five per 

 cent of the species are found in more than one ocean. It is likely 

 of course that the hadal species will be less widely distributed than 

 ordinary abyssal species because the hadal zone occurs in the form 

 of trenches which are to a large extent isolated from each other, 

 and only form 1-2 per cent of the area of the oceans. It is worth 

 noting that the abyssal zone is by far the most extensive habitat in 

 the world; 76 per cent of the area of the oceans and over half the 

 surface of the whole globe is occupied by the abyssal depths of the 

 sea. 



There is great uniformity in the conditions on the abyssal floor. 

 The temperature is low, between 2 and 4 C, and the salinity is 

 practically constant everywhere. The main variable is pressure, 

 which increases directly with depth, so that in the hadal zone it 

 may be five times as great as at the beginning of the abyss. The 

 uniformity of temperature and salinity offers a partial explanation 

 of the cosmopolitan distribution of many abyssal forms; once a 

 creature has become adapted to such conditions there will be no 

 great barrier to its spreading. 



The only abyssal area which has been studied in sufficient detail 

 for its bottom dwelling fauna to be characterised is the Norwegian 

 Basin, lying between Norway and Greenland. The endemic Crus- 

 tacea of this region include the isopod Mesidotea megalura, which 

 is a close relative of the glacial relict M. entomon which we will 

 meet in the Caspian and the Baltic. Others, such as the amphipod 

 Bruzelia dentata, and the mysid Pseudomysis abyssi, may also be 

 regarded as characteristic of this area. 



The animals of the shelf are more distinctly divided into 

 geographical units than those of the deep sea. The various faunas 

 have been dealt with most admirably by Ekman (1953), who con- 

 siders all types of animals, not just the Crustacea. Ekman's main 

 divisions of the shelf fauna have been adopted in the account which 

 follows. 



There is a vast circumtropical area, with a great wealth of species, 

 lying between the water isotherms for 20 C. at the coldest part of 

 the year. A diluted version of this fauna extends to the regions with 

 a minimum water temperature of 1 6-1 8° C. Two very characteristic 

 associations are found on this warm-water shelf: coral reefs and 

 mangrove swamps, each with its own type of crustacean fauna. 



