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



due, as we have seen, to unfavourable oxygen conditions (often 0-5 to 1 cm 3 

 per litre) and under certain circumstances the appearance of hydrogen 

 sulphide from the bed, soft beds being very rich in hydrogen sulphide (for 

 instance the muds of the Ural Trench according to Fedosov). 



Therefore such oligo-oxybiotic groups as Oligochaeta and Chironomidae 

 find favourable conditions here for their existence. Areas of the bottom open 

 to continuous currents and well aerated, and therefore practically free of 

 smelts which are easily washed away by the currents, are thickly populated by 

 benthos, feeding mainly on detritus carried over the sea-bed. Ivanov has 

 shown that the waters of the Central Caspian, rich in plant food, move into 

 the southern part of the Northern Caspian, causing a luxuriant development 

 of plankton and benthos. The filter-feeding phenomenon is not as strongly 

 manifest in the bottom-living fauna of the Caspian Sea as in that of the Black 

 Sea ; this may be due to the absence of such powerful filter-feeders as the sea 

 mussel, the oysters and phaseolin. Dreissena, however, is also a filter-feeder 

 and the presence of large patches of shell-gravel silts on the bottom of the 

 Caspian Sea leads to the conclusion that they have a biogenic origin. 



Perhaps a certain deficiency in the representation in the Caspian of the 

 filter-feeding phenomena has conditioned such a luxuriant development in it 

 of a typical filter-feeder, the alien Mytilaster— a development which is not 

 characteristic of it in its native habitat, the Black and Azov Seas. 



The process of the accumulation of silt soils may possibly increase in the 

 areas of the dense settlements of Mytilaster in the Caspian Sea. 



During the last 25 years much qualitative and quantitative research has 

 been carried out on the bottom-living fauna of the Caspian Sea. The Northern 

 Caspian has been investigated in particular detail. 



On the average the benthos biomass of the Northern Caspian has remained 

 unaltered, except for its catastrophic drop in 1937-38, which was followed by 

 fairly slow regeneration over many years. A second, less violent drop was 

 recorded in 1946 and 1947 (Table 262). 



As shown by Table 262 the drop in biomass is in both cases controlled 

 mainly by the decrease in the number of molluscs and, to a lesser extent, by 

 that of the crustaceans in 1937 and 1938. Birstein suggests that at that time 

 some suffocation phenomena took place as a result of oxygen shortage. 

 Bivalves are markedly predominant in the Northern Caspian benthos ; among 

 other groups Nereis stands out sharply (Table 263). 



These data on the state of benthos in the Northern Caspian can be supple- 

 mented by those given by V. Osadchikh (1958) for 1954 and 1956. The total 

 benthos biomass increased during this period by 30 per cent, mainly owing to 

 worms (by 98 per cent) and crustaceans (by 70 per cent). Oligochaete biomass 

 increased by 193 per cent and that of Nereis by 59 per cent. The chironomid 

 biomass increased very greatly (by 355 per cent). The food available for adult 

 fish rose by 25 per cent and for young fish by 46 per cent. Considerable 

 patches of Syndesmya were formed (Table 264). 



It is evident from the data in Table 264 that the intrusion of Nereis has had 

 no harmful effect on local fauna ; neither the oligochaetes nor the chironomids 

 have been affected, as might first have been expected if this effect existed. 



