612 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



Vertical distribution of zoo benthos. O. Grimm had already pointed out in 1877 

 the vertical zonality of the distribution of Caspian fauna and had estab- 

 lished three faunal zones covering the upper 300 m. A similar division of the 

 Sea was suggested by N. M. Knipovitch (1921), based on the distribution of a 

 series of hydrological and biological factors. He suggested four zones for the 

 Central Caspian and three for the Southern. The upper zone (100 to 200m), with 

 its seasonal temperature fluctuations and a larger oxygen content, was further 

 divided into sub-zones by Knipovitch. The second zone is characterized by a 

 lower oxygen content and a fairly constant temperature (down to 450 m). 

 According'to Knipovitch the third zone with a constant temperature and low 

 oxygen content extends in the Southern Caspian to the sea-bed and in 

 the Central to 750 m. Below it lies the fourth zone, characterized by the pre- 

 sence of hydrogen sulphide. According to Knipovitch the limit of bottom life 

 lies at 415 m in the Central Caspian and at 460 m in the Southern. The main 

 mass of benthos lives in the two upper zones. More recently S. P. Brujevitch 

 (1937) suggested a diagram for the vertical division of the Sea according to 

 chemical indices, of which mention has been made above. 



As has been shown by recent research the maximum depths for bottom- 

 living organisms are greater than those suggested by Grimm and Knipovitch, 

 the 400 to 500 m deep water column was found to contain some benthos, 

 although here it is poor both in number and variety (Fig. 287). Hypania in- 

 xalida was discovered down to maximum depths (960 m) ; Pseudolibrotus was 

 caught in plankton nets below 600 m. Some mysids were found at almost the 

 same depth. They can all, apparently, exist on very small amounts of oxygen. 



In the Central Caspian bottom fauna becomes very scarce at about 100 m. 

 Crustaceans of Arctic origin live here : Mesidothea entomon, Pseudolibrotus 

 platyceras, Ps. caspius, Pontoporeia affinis, Mysis caspia, M. microphthalma 

 and M. amblyops. AmathiUina spinosa, Pandorites podoceroides, Niphargoides 

 grimmi, Stenocuma diastvloides are found down to a depth of 150 m. An 

 almost complete absence of molluscs is characteristic ; only very rarely would 

 a grab bring up Dreissena grimmi, Dr. rostiformis, Micromelania spica, M. 

 caspia and M. elegantula. Deeper down (to 400 m) the Oligochaeta and 

 Hypania invalida are found. In the Southern Caspian, only Hypania invalida 

 and the Arctic mysids were found. 



The specific deep-water fauna is absent from the great depths of the Caspian 

 Sea. These are inhabited first by the forms of Arctic origin, adapted to low 

 temperature ; secondly by Caspian autochthonous forms, descendants of the 

 shallower fauna, which acquired a deep-water aspect. The fauna is much 

 richer above 100 m. Bivalves begin to play a dominant role here by their bio- 

 mass (up to 90 per cent). However, at a depth of 50 to 100 m, the greatest 

 mass forms are absent : MytiJaster lineatus, Dreissena polymorpha, Dr. caspia, 

 Didacna trigonoides, D. barbot-de-marnyi, D. crassa, all the Adacna species, 

 Cardium edule, Theodoxus pallasi, Hydrobia, all the species of the Ponto- 

 gammarus genus, almost all of the Pterocuma, Turbellaria and Cordylo- 

 phora caspia. Instead the original fauna of the large, higher crustaceans are 

 most developed here : AmathiUina spinosa, Dikerogammarus caspius, D. grimmi, 

 D. macrocephalus, Gammarus placidus, Paramysis eurylepis, Metamysis infiata 



