THE CASPIAN SEA 



595 



they are not found at salinities above 4 to 5% . On the other hand, a series of 

 forms characteristic of brackish waters Brachionus mulleri, Synchaeta vorax, 

 S. neapolitana, and others) have been successfully distinguished. 



Cladocera are even more sharply divided into fresh- and brackish-water 

 forms. The first group includes the fairly numerous representatives of the 

 families Sididae, Daphnidae, Bosminidae and Chydoridae, which move far 

 out to sea only during the flooding of rivers. The second group consists of 

 genera of the Polyphemidae family (Polyphemus, Cercopagis, Apagis and 

 Evadne), which avoid places of considerably lowered salinity. Evadne trigona 



Fig. 282. Distribution of zooplankton biomass of Northern Caspian and the iso- 

 halines in September 1935 (Kusmorskaya). 



and Cercopagis gracillima are the most widely distributed and numerous 

 representatives of this group. Among the Copepoda the dominant form 

 for the whole Northern Caspian zooplankton is Calanipeda aquae dulcis — 

 an extremely eurybiotic and widely distributed species. From February to 

 November inclusive, Calanipeda comprises on the average 50 per cent of the 

 total zooplankton biomass. Halicyclops sarsi is also very numerous and widely 

 distributed. Heterocope caspia is much more stenohaline, avoiding both an 

 increase and decrease of salinity. The rest of the Copepoda are found much 

 less frequently and in smaller numbers. Among them too it is possible to dis- 

 tinguish a group of species connected with fresh water {Cyclops spp., Nitocra 

 incerta, Schizopera tenera, Nannopus palusths, Diaptomus gracilis, Eury femora 

 affinis and others) and the group of species connected with saline water 

 {Eurytemora grimmi, Idyaea brevicornis). 



Kusmorskaya gives the composition of zooplankton in the western half of 

 the Northern Caspian and in the Mangishlaksk area of the Caspian Sea in the 

 form shown in Table 245. 



