578 



Nereis in large quantities in the intestines of sturgeon caught off Chechen' 

 Island ; this result was later obtained in other places in the Caspian. These 

 findings were a proof of the success of the acclimatization of Nereis in the 

 Caspian Sea. Syndesmya ovata was not found in the Caspian until 1955 

 (A. Saenkova, 1959). 



In a few years Nereis biomass in the Caspian Sea formed 100,000 tons. The 

 colonization of the Caspian Sea by shrimps was of about the same nature. 

 Barnacles had developed in exceptional numbers (G. Zevina, 1958). They have 

 spread all over the shallow parts of the Caspian Sea bottom, over hydro- 

 technical constructions and, in certain seasons of the year, on fishing gear, 

 which was covered by them. Undoubtedly their total mass is now an almost 

 solid layer of not less than some hundreds of tons. These factors indicate an 

 exclusive activity and vitality of many euryhaline and eurytopic members of 

 the Mediterranean fauna, which have also colonized the Sea of Azov. The 

 endemic fauna of the Caspian Sea is also probably wasting some of the life 

 resources of this Sea, and it may not be very powerful in its struggle with its 

 most active Mediterranean rivals. Such detritus-eating forms as the red mullet 

 and Nereis are certainly wasting some of the Sea resources. On the other hand, 

 Mytilaster is in close competition with the local Dreissensia as a filter feeder 

 and fouling organism; barnacles, which arrived later, are a closely related 

 biological form, and possibly also take part in the rivalry. Hence two accli- 

 matizations can be distinguished : that of intrusion, when the local forms 

 (Nereis, Mugil) remain undisturbed, and when they are dislodged (Myti- 

 laster, Rhizosolenia) (L. A. Zenkevitch, 1940). Shrimps, perhaps, have estab- 

 lished some relationship with local mysids. While some forms are undoubtedly 

 useful in the Caspian Sea (Nereis, Syndesmya, Mugil) the usefulness of others 

 is not clear (Leander, Mytilaster), and others still play a negative role (Rhizo- 

 solenia, Balanus and possibly Mytilaster). 



The unusual fate of many new immigrants into the Caspian Sea has empha- 

 sized the conceptions of potential habitat and of the acclimatization stock 

 (L. A. Zenkevitch, 1940). For most of the land and marine forms their actual 

 habitat is probably far from occupying all that part of the biosphere in which 

 these forms could live, and into which, for some reason, they cannot penetrate. 

 All these parts of the biosphere form potential habitats for them. On the 

 other hand, many species could live in areas where they are absent if they 

 were brought into them. Such forms belong to the acclimatization stock for 

 these areas. The most successful acclimatization of the Baltic herring Clupea 

 harengus membras in the Aral Sea can, from this point of view, serve as a 

 good example for the Soviet Seas. It is quite probable that, for the Caspian 



