of the new area occurs, the breeding rate of the intruder decreases 

 (Phase III of acclimatization). For some time the population of this 

 species remains at the same high level (IV). Then, a phase of slight 

 (V), and then more rapid (VI) population decrease occurs, due to the 

 appearance of competitors and natural enemies and a reduction of the 

 available natural food. Subsequently, the rate of decline slows (VII), 

 and, finally, the population of the species reaches a certain steady 

 level, subject to the fluctuations which are normal for all species 

 (Phase VIII). This detailed system of L. A. Zenkevitch, which reflects 

 the process of acclimatization quite well, can be reduced to five main 

 periods (see figure): l--the initial phase--the moment of introduction 

 (Phase I), 2--the increase in population or intensity of intrusion 

 (Phase II), 3--the period of high population (Phases III-IV), 4--the 

 period of decreasing population (Phases V-VII), and 5--the period of 

 naturalization of the introduced species and the stabilization of its 

 population (Phase VIII). 



2.2 Forms of Acclimatization 



Acclimatization of exota can significantly influence the 

 composition and productivity of the biota of the recipient body of 

 water. The hydrobionts introduced frequently replace or reduce the 

 population of local forms. The introduction of the Indian-Western 

 Pacific diatom Biddulfia sinensis , carried by the hulls of ships, into 

 the North Sea around 1903, led to its colossal multiplication, and in 

 some parts of the sea it became the dominant species of phytoplankton 

 (Hardy, 1956). A gastropod mollusk Crepidula fornicata was 

 unintentionally carried from the east coast of North America to the 

 coastal waters of western Europe, where it spread from Sweden to France 

 (Walne, 1956; Walford, Wicklund, 1973) and became a very numerous 

 species on the oyster bars, causing significant deterioration in the 

 conditions of existence of oysters and requiring that the oyster bars be 

 cleared with special dredges. The boring mollusk Ocenebra japonica , 

 unintentionally introduced along with the Pacific oysters brought in 

 from Japan, has done great harm to oysters along the west coast of North 

 America. Similar occurrences have been reported in the Black Sea, where 

 the predaceous mollusk Rapana venosa , accidentally brought in from the 

 Sea of Japan, has acclimatized and has virtually wiped out all of the 

 oyster bars in the eastern portion of the sea in a few years, greatly 

 reducing the biomass of other large bivalve mollusks. 



We must distinguish substitution acclimatization, such as the cases 

 we have just described, from interstitial acclimatization (a term used 

 by Zenkevitch, 1963), when the new species inserts itself into the 

 composition of biota without any apparent expulsion of any aboriginal 

 species. Examples of interstitial acclimatiztion are rather obvious in 

 cases of intrusion of species which differ in terms of ecology from the 

 components of the biota of the recipient body of water. For example, 

 the planned introduction of the polychaete worm Nereis di versicolor , the 

 bivalve mollusk Abra ( =Syndesmya ) ovata and the mullets ^Liza (= Mugil ) 

 aurata and _L. sal i ens from the Azov Sea to the Caspian Sea in 1938-1947 

 has led to a significant increase in the biomass of the benthos of the 

 Caspian Sea and to the formation of a Caspian population of mullets. 



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