332 



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



baltica, which is the largest mass form of the present-day Baltic Sea. In 

 various places, however, as a result of unfavourable saline, gaseous or bio- 

 coenotic environment, Macoma baltica disappears, or is replaced either partly 

 or completely by other forms. In the Arcona and Bornholm depressions such 

 forms are Astarte borealis and Macoma calcarea. 



In the depths of the central area of the Baltic Sea benthos biomass falls 

 almost to zero ; Macoma does not penetrate there, its place being taken by 

 polychaetes {Scoloplos armiger and Terebellides stromii) and crustaceans 

 (Pontoporeia femorata and Mesidothea entomori). Farther into the Gulfs of 

 Bothnia and of Finland, except for the actual coastal strip, benthos biomass 



Fig. 161. Zonal distribution of Baltic fauna (Zenkevitch). 1 Eriocheir sinensis; 

 2 Balanus improvisus ; 3 Fucus vesiculosus and Chorda /Hum ; 4 Mytilus edulis ; 

 5 Mesidothea entomon ; 6 Macoma baltica ; 7 Pontoporeia affinis and P. femorata ; 

 8 Nereis diver sicolor ; 9 Aurclia aurita ; 1 Priapidus caudatus ; 1 1 Pleuronectes flesus ; 

 12 Herring; 13 Sprat tits sprat t us balticus; 14 Cod. 



decreases markedly and, finally, at a salinity of about 3-5% , Macoma dis- 

 appears, while the Pontoporeia-Mesidothea community remains, acquiring a 

 considerable admixture of fresh-water forms and in the actual coastal zone 

 being replaced by oligochaetes and chironomid larvae. 



Macoma baltica, with its comparatively thin shell and high nutrient indices, 

 is devoured in huge quantities by various Baltic fishes. 



Like other bodies of water the Baltic Sea varies greatly in the numerical 

 content of individual benthos mass forms in different seasons of the year — 

 Nereis diversicolor, Cardium edule, Macoma baltica, Pontoporeia affinis, 

 Mesidothea entomon and Corophium volutator. The last-named, an original 

 member of the Amphipoda group which lives in U-shaped tubes in the bot- 

 tom, also provides an example of sharp fluctuations in numbers from year 

 to year. The observations of S. Siigerstrale of Twerminn (Finland), carried 



