GENERAL CHARACTERISTICS AND GEOLOGICAL HISTORY 



373 



also migrated there at some unknown time. In recent decades the eastward 

 penetration of Mediterranean immigrants has continued, either with the 

 passive participation of man (Rhizosolenia, Mytilaster and two species of 

 shrimps Leander and two species of Balanus), or through measures being 

 taken for acclimatization (two species of the grey mullet Mugil, the poly- 

 chaete Nereis and the mollusc Syndesmya and some others). This enormous 

 activity of some forms of Mediterranean fauna, and their indubitable advant- 

 ages over the Caspian and Aral forms in the struggle for existence, is a clear 

 indication of the wide possibilities of acclimatization farther east of the 

 euryhaline Mediterranean fauna, inhabiting the Black Sea and the Sea of 

 Azov. The migration of the two brackish-water forms — the medusa Black- 

 fordia and the crab Rithropanopeus harrisi tridentatus — from the northwestern 

 part of the Atlantic Ocean to the Caspian Sea is also most curious. 



Impoverishment of the Mediterranean fauna 



For the reasons enumerated above the qualitative differentiation of the 

 Mediterranean fauna decreases with its movement to the east (Fig. 179). 



The Mediterranean flora and fauna become four times poorer by the time 

 they reach the Black Sea, while in the Sea of Azov only 2-5 per cent remains. 



Fig. 179. Qualitative abundance of Mediterranean flora and fauna and its impover- 

 ishment with its movement eastward. Total number of animal species is denoted by 

 the numerals (Zenkevitch). 1 Mediterranean fauna; 2 Caspian fauna; 3 Fresh- 

 water fauna ; 4 Arctic immigrants ; 5 Direction of migration. 



The qualitative impoverishment in the Black Sea affects primarily the 

 most stenohaline part of the population — Radiolaria, Siphonophora, Cteno- 

 phora, corals, many groups of crustaceans and especially decapods, all the 

 molluscs and especially cephalopods and gastropods, the echinoderms, the 

 tunicates and fish. 



In the Sea of Azov, of the 200* Mediterranean coelenterates only three 

 species are found, of the 1,457 species of molluscs only 12, of the 51 species of 

 crabs only one, of the 300 species of pelagic copepods only 8, and so on. 



Another characteristic factor in the qualitative impoverishment of the 



* The number of species of Mediterranean animals, according to Gr. Antipa (1941). 



