THE BLACK SEA 457 



number is greatly reduced as we pass into the more saline waters of the inlet. 

 ' Caspian ' crustaceans settle down farther up the stream of a river, moving up 

 the Dniester to its middle and upper parts. Twenty-four species of Gam- 

 maridae have been discovered in the Dniester and its inlet, seven of Coro- 

 phiidae and nine of Cumacea. In the 17 biocoenoses distinguished, the 'Cas- 

 pian ' species are predominant in 13, and the first among them are : Monodacna 

 pontica, Dreissensia polymorpha, Clessiniola variabilis, Micromelania lincta, 

 Pontogammarus maeoticus, Dikerogammarus villosus, Hypania invalida and 

 others. 



The Kuchurgan inlet of the same river system, but situated to the north 

 of the Dniester inlet and of a very low salinity (0-05 to 0-2% o by chlorine), has 

 a fauna characterized by the dominant role of its relict forms of Caspian 

 aspect (Markovsky, 1953) both in its plankton (Heterocope caspia), its 

 necto-benthos (Caspian mysids Paramysis, Mesomysis, Katamysis and 

 Limnomysis), and also in its benthos (Hypania, Hypaniola, Adacna, Mono- 

 dacna, Micromelania, Theodoxus, Dreissensia and others) (M. Yaroshenko, 

 1950). 



Some data on the fauna of the Danube delta may form substantial additions 

 to what has been said above. At one time the lower reaches of the Danube and 

 the estuary zones of the rivers of the northwestern Black Sea were occupied 

 by a wide arm of the sea, along the northwest side of which numerous inlets 

 were formed (Figs. 220 and 22 1). Some excellent research was carried out on the 

 fauna of the lower reaches of the Danube by Rumanian and Russian investi- 

 gators, in particular by Yu. Markovsky (1955) on the Killisk delta. In the 

 Killisk delta he recorded 1 40 species of invertebrates (without Protozoa) out 

 of the total number of 412 species known for all the Danube delta (among 

 them 58 molluscs and 186 crustaceans), including 36 species of molluscs and 

 43 species of crustaceans. In most cases half of the fauna, or more, consists of 

 'Caspian' forms (in Katlabug 62-8 per cent). The fresh- water aspect of the 

 ' Caspian ' species in the Danube and its delta is even more pronounced than 

 in the other inlets of the northwestern part of the Black Sea and the character- 

 istics of its plankton, plankton-benthos and benthos faunas are the same. 

 The aboriginal fresh-water species are greatly predominant in the plankton, 

 whereas in plankton-benthos the 'Caspian' species are just as predominant, 

 thanks to the mysids, and in the benthos half of the species are ' Caspian ' 

 forms. 



The fresh- water aspect of the Danube 'Caspian' forms which has been 

 acquired to a great degree has been used, and may be used later on a much 

 larger scale, for their acclimatization in bodies of fresh water of other river 

 systems and even in the Dnieper (F. Mordukhai-Boltovskoy, 1950, 1952; 

 Yu. Markovsky, 1952, 1954; P. Yuravel, 1950, 1952) where stable populations 

 increasing the valuable components of fish-food resources may be created. 



The greater fresh- water tendency of the Black-Azov Sea ' Caspian ' species, 

 as compared with that of the same community in the Caspian, and the 

 strengthening of the ' fresh- water ' aspect in the Black and Azov Seas from 

 east to west is difficult to explain. The easiest way would have been to assume 

 that the Pontic fauna remained in the Black and Azov Seas throughout the 



